| Message ID | 20231114-clone3-shadow-stack-v2-2-b613f8681155@kernel.org |
|---|---|
| State | New |
| Headers | show |
| Series | fork: Support shadow stacks in clone3() | expand |
The 11/16/2023 00:52, Edgecombe, Rick P wrote: > On Wed, 2023-11-15 at 18:43 +0000, Mark Brown wrote: > > > > > otherwise 0 size would be fine: the child may not execute > > > a call instruction at all. > > It seems like a special case. Where should the SSP be for the new > thread? yes it is likely not an interesting case in practice so < 8 can be an error i think. > > > > > I think for CLONE_VM we should not require a non-zero size. > > > > > Speaking of > > > > > CLONE_VM we should probably be clear on what the expected > > > > > behavior is > > > > > for situations when a new shadow stack is not usually > > > > > allocated. > > > > > !CLONE_VM || CLONE_VFORK will use the existing shadow stack. > > > > > Should we > > > > > require shadow_stack_size be zero in this case, or just ignore > > > > > it? I'd > > > > > lean towards requiring it to be zero so userspace doesn't pass > > > > > garbage > > > > > in that we have to accommodate later. What we could possibly > > > > > need to do > > > > > around that though, I'm not sure. What do you think? > > > > > > Yes, requiring it to be zero in that case makes sense I think. > > > > > i think the condition is "no specified separate stack for > > > the child (stack==0 || stack==sp)". > > > > > CLONE_VFORK does not imply that the existing stack will be > > > used (a stack for the child can be specified, i think both > > > glibc and musl do this in posix_spawn). > > > > That also works as a check I think, though it requires the arch to > > check > > for the stack==sp case - I hadn't been aware of the posix_spawn() > > usage, > > the above checks Rick suggested just follow the handling for implicit > > allocation we have currently. > > I didn't realize it was passing its own stack either. I guess the > reason is to avoid stack overflows. But none of the specific reasons > listed in the comments seem to applicable to shadow stacks. while CLONE_VFORK allows the child to use the parent shadow stack (parent and child cannot execute at the same time and the child wont return to frames created by the parent), we want to enable precise size accounting of the shadow stack so requesting a new shadow stack should work if new stack is specified. but stack==0 can force shadow_stack_size==0. i guess the tricky case is stack!=0 && shadow_stack_size==0: the user may want a new shadow stack with default size logic, or (with !CLONE_VM || CLONE_VFORK) wants to use the existing shadow stack from the parent. > > What is the case for stack=sp bit of the logic? iirc it is not documented in the clone man page what stack=0 means and of course you don't want sp==0 in the vfork child so some targets sets stack to sp in vfork, others set it 0 and expect the kernel to do the right thing. this likely does not apply to clone3 where the size has to be specified so maybe stack==sp does not need special treatment. > I need to look into this more. My first thought is, passing in a stack > doesn't absolutely mean they want a new shadow stack allocated either. > We are changing one heuristic, for another. yes. > The other thought is, the new behavior in the !CLONE_VM case doesn't > seem optimal. fork has ->stack==0. Then we would be allocating a stack > in only the child's MM and changing the SSP there, and for what reason? > So I don't think we should fully move away from taking hints from the > CLONE flags. only stack!=0 case is tricky. stack==0 means existing shadow stack. > > Maybe an alternate could just be to lose the CLONE_VFORK specific stack > sharing logic. CLONE_VM always gets a new shadow stack. I don't think > it would disturb userspace functionally, but just involves more > mapping/unmapping. IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.
On Thu, Nov 16, 2023 at 10:32:06AM +0000, Szabolcs.Nagy@arm.com wrote: > The 11/16/2023 00:52, Edgecombe, Rick P wrote: > > On Wed, 2023-11-15 at 18:43 +0000, Mark Brown wrote: > while CLONE_VFORK allows the child to use the parent shadow > stack (parent and child cannot execute at the same time and > the child wont return to frames created by the parent), we > want to enable precise size accounting of the shadow stack > so requesting a new shadow stack should work if new stack > is specified. > but stack==0 can force shadow_stack_size==0. > i guess the tricky case is stack!=0 && shadow_stack_size==0: > the user may want a new shadow stack with default size logic, > or (with !CLONE_VM || CLONE_VFORK) wants to use the existing > shadow stack from the parent. If shadow_stack_size is 0 then we're into clone() behaviour and doing the default/implicit handling which is to do exactly what the above describes. > > What is the case for stack=sp bit of the logic? > iirc it is not documented in the clone man page what stack=0 > means and of course you don't want sp==0 in the vfork child > so some targets sets stack to sp in vfork, others set it 0 > and expect the kernel to do the right thing. The manual page explicitly says that not specifying a stack means to use the same stack area as the parent. > this likely does not apply to clone3 where the size has to be > specified so maybe stack==sp does not need special treatment. You'd have to be jumping through hoops to manage to get the same stack pointer while explicitly specifying a stack with clone3() on architectures where the stack grows down. I'm not sure there's a reasonable use case.
The 11/16/2023 12:33, Mark Brown wrote: > On Thu, Nov 16, 2023 at 10:32:06AM +0000, Szabolcs.Nagy@arm.com wrote: > > The 11/16/2023 00:52, Edgecombe, Rick P wrote: > > > On Wed, 2023-11-15 at 18:43 +0000, Mark Brown wrote: > > > while CLONE_VFORK allows the child to use the parent shadow > > stack (parent and child cannot execute at the same time and > > the child wont return to frames created by the parent), we > > want to enable precise size accounting of the shadow stack > > so requesting a new shadow stack should work if new stack > > is specified. > > > but stack==0 can force shadow_stack_size==0. > > > i guess the tricky case is stack!=0 && shadow_stack_size==0: > > the user may want a new shadow stack with default size logic, > > or (with !CLONE_VM || CLONE_VFORK) wants to use the existing > > shadow stack from the parent. > > If shadow_stack_size is 0 then we're into clone() behaviour and doing > the default/implicit handling which is to do exactly what the above > describes. sounds good. > > > What is the case for stack=sp bit of the logic? > > > iirc it is not documented in the clone man page what stack=0 > > means and of course you don't want sp==0 in the vfork child > > so some targets sets stack to sp in vfork, others set it 0 > > and expect the kernel to do the right thing. > > The manual page explicitly says that not specifying a stack means to use > the same stack area as the parent. not for clone. clone3 yes. > > this likely does not apply to clone3 where the size has to be > > specified so maybe stack==sp does not need special treatment. > > You'd have to be jumping through hoops to manage to get the same stack > pointer while explicitly specifying a stack with clone3() on > architectures where the stack grows down. I'm not sure there's a > reasonable use case. ok makes sense. IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.
The 11/16/2023 12:33, Mark Brown wrote: > On Thu, Nov 16, 2023 at 10:32:06AM +0000, Szabolcs.Nagy@arm.com wrote: > > i guess the tricky case is stack!=0 && shadow_stack_size==0: > > the user may want a new shadow stack with default size logic, > > or (with !CLONE_VM || CLONE_VFORK) wants to use the existing > > shadow stack from the parent. > > If shadow_stack_size is 0 then we're into clone() behaviour and doing > the default/implicit handling which is to do exactly what the above > describes. to be clear does clone with flags==CLONE_VM|CLONE_VFORK always use the parent shadow stack independently of the stack argument? IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.
On Thu, Nov 16, 2023 at 01:55:07PM +0000, Szabolcs.Nagy@arm.com wrote: > The 11/16/2023 12:33, Mark Brown wrote: > > On Thu, Nov 16, 2023 at 10:32:06AM +0000, Szabolcs.Nagy@arm.com wrote: > > > i guess the tricky case is stack!=0 && shadow_stack_size==0: > > > the user may want a new shadow stack with default size logic, > > > or (with !CLONE_VM || CLONE_VFORK) wants to use the existing > > > shadow stack from the parent. > > If shadow_stack_size is 0 then we're into clone() behaviour and doing > > the default/implicit handling which is to do exactly what the above > > describes. > to be clear does clone with flags==CLONE_VM|CLONE_VFORK always > use the parent shadow stack independently of the stack argument? !CLONE_VM rather than CLONE_VM but yes, that's what the clone() and hence current clone3() behaviour is here. > IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you. There are mechanisms for disabling this...
On Thu, 2023-11-16 at 15:35 +0000, Mark Brown wrote: > On Thu, Nov 16, 2023 at 01:55:07PM +0000, > Szabolcs.Nagy@arm.com wrote: > > The 11/16/2023 12:33, Mark Brown wrote: > > > On Thu, Nov 16, 2023 at 10:32:06AM +0000, > > > Szabolcs.Nagy@arm.com wrote: > > > > > i guess the tricky case is stack!=0 && shadow_stack_size==0: > > > > the user may want a new shadow stack with default size logic, > > > > or (with !CLONE_VM || CLONE_VFORK) wants to use the existing > > > > shadow stack from the parent. > > > > If shadow_stack_size is 0 then we're into clone() behaviour and > > > doing > > > the default/implicit handling which is to do exactly what the > > > above > > > describes. > > > to be clear does clone with flags==CLONE_VM|CLONE_VFORK always > > use the parent shadow stack independently of the stack argument? > > !CLONE_VM rather than CLONE_VM but yes, that's what the clone() and > hence current clone3() behaviour is here. "flags & CLONE_VM" gets a new shadow stack, unless also "flags & CLONE_VFORK". Other flags in there are not consulted for the logic of whether to create a new shadow stack. So CLONE_VM|CLONE_VFORK will use the parent shadow stack. !CLONE_VM will also sort of use the same shadow stack, but it's a COW one. Now that I've thought about it more, removing the CLONE_VFORK part of the logic has several downsides. It is a little extra work to create and unmap a shadow stack for an operation that is supposed to be this limited fast thing. It also will change the SSP(let me know if anyone has a general term we can use) for the child. So if you have like: ssp = _get_ssp() if (!vfork()) { foo = *ssp; ... } ...it's awkward edge. In the vfork man page it points to fork which has the text: "The child process is an exact duplicate of the parent process except for the following points", which obviously doesn't include SSP. Lastly, there are already cases where the x86 glibc implementation stays on the shadow stack when it switches regular stacks (i.e. sigaltstack()). vfork children are not supposed to return, so it should normally work to be on the same shadow stack. So it's not a special situation unless we can resolve those other situations, which are limited by the stack lifetime issues. What about a CLONE_NEW_SHSTK for clone3 that forces a new shadow stack? So keep the existing logic, but the new flag can override the logic for !CLONE_VM and CLONE_VFORK if the caller wants. The behavior of shadow_stack_size is then simple. 0 means use default size, !0 means use the passed size. No need to overload and tie up args->stack. In the other direction though... CLONE_VFORK can be used to stay on the existing shadow stack and possibly corrupt it. This connects with earlier discussions around signals dropping a token before being handled and the overflow use case, and trying to guarantee one thread per shadow stack at a time, etc. So if there is any inclination towards trying to get that, it might actually be useful for another reason. It will close one method for getting two threads on the same shadow stack at the same time (one is sleeping yes, but it's the same problem in effect).
On Thu, Nov 16, 2023 at 12:52:09AM +0000, Edgecombe, Rick P wrote: > On Wed, 2023-11-15 at 18:43 +0000, Mark Brown wrote: > > > end marker token (0) needs it i guess. > > x86 doesn't currently have end markers. Actually, that's a point - > > should we add a flag for specifying the use of end markers here? > > There's code in my map_shadow_stack() implementation for arm64 which > > does that. > Hmm, I guess there isn't a way to pass a flag for the initial exec > stack? So probably it should just mirror that behavior. Unless you > think a lot of people would like to skip the default behavior. I don't really know that anyone would particularly want to use a flag on arm64, I was more thinking for the benefit of x86 where any termination record would be a change. It's certainly easier to not have flags so I'm more than happy to leave things as they are, there's nothing stopping further extensions of the ABI if we decide we want them later. > And of course we don't have a marker on x86 (TODO with alt shadow > stacks). We could still check for size < 8 if we want it to be a > universal thing. It does seem simpler, size < 8 is all edge case.
On Thu, 2023-11-16 at 18:14 +0000, Mark Brown wrote: > On Thu, Nov 16, 2023 at 12:52:09AM +0000, Edgecombe, Rick P wrote: > > On Wed, 2023-11-15 at 18:43 +0000, Mark Brown wrote: > > > > end marker token (0) needs it i guess. > > > > x86 doesn't currently have end markers. Actually, that's a point > > > - > > > should we add a flag for specifying the use of end markers here? > > > There's code in my map_shadow_stack() implementation for arm64 > > > which > > > does that. > > > Hmm, I guess there isn't a way to pass a flag for the initial exec > > stack? So probably it should just mirror that behavior. Unless you > > think a lot of people would like to skip the default behavior. > > I don't really know that anyone would particularly want to use a flag > on > arm64, I was more thinking for the benefit of x86 where any > termination > record would be a change. It's certainly easier to not have flags so > I'm more than happy to leave things as they are, there's nothing > stopping further extensions of the ABI if we decide we want them > later. I'm hoping that shifting the shadow stack start by one frame for thread stacks (where there is no token to find) will not disturb anything. But for x86, we will need a new elf bit to be fully safe in implementing alt shadow stack. When we do that we will have a chance to add an end of stack marker without compatibility issues on x86. So just doing default behavior seems fine. For map_shadow_stack, the end of stack marker will shift the token, which userspace needs to find, so that is why I wanted a flag for that. Appreciate the consideration.
On Thu, Nov 16, 2023 at 06:11:17PM +0000, Edgecombe, Rick P wrote: > Now that I've thought about it more, removing the CLONE_VFORK part of > the logic has several downsides. It is a little extra work to create > and unmap a shadow stack for an operation that is supposed to be this > limited fast thing. It does rather feel like it's defeating the point of the thing. > It also will change the SSP(let me know if anyone has a general term we > can use) for the child. So if you have like: SSP seems fine, we're already using shadow stack here. > What about a CLONE_NEW_SHSTK for clone3 that forces a new shadow stack? > So keep the existing logic, but the new flag can override the logic for > !CLONE_VM and CLONE_VFORK if the caller wants. The behavior of > shadow_stack_size is then simple. 0 means use default size, !0 means > use the passed size. No need to overload and tie up args->stack. That does seem like it cuts through the ambiguous cases. If we go for that it feels like we should require the flag when specifying a size, just to be sure that everything is clear. Though having said that we could just always allocate a shadow stack if a size is specified regardless of the flags, requiring people who want non-default behaviour to have some idea what stack size they want. I don't think I have strong opinons between having the new flag or always allocating a stack if a size is specified.
On Tue, Nov 14, 2023 at 08:05:55PM +0000, Mark Brown wrote: >Unlike with the normal stack there is no API for configuring the the shadow >stack for a new thread, instead the kernel will dynamically allocate a new >shadow stack with the same size as the normal stack. This appears to be due >to the shadow stack series having been in development since before the more >extensible clone3() was added rather than anything more deliberate. > >Add parameters to clone3() specifying the address and size of a shadow >stack for the newly created process, Probably should update commit message in next version. Address is not specified anymore. >we validate that the range specified >is accessible to userspace but do not validate that it has been mapped >appropriately for use as a shadow stack (normally via map_shadow_stack()). >If the shadow stack is specified in this way then the caller is responsible >for freeing the memory as with the main stack. If no shadow stack is >specified then the existing implicit allocation and freeing behaviour is >maintained. > >If the architecture does not support shadow stacks the shadow stack >parameters must be zero, architectures that do support the feature are >expected to have the same requirement on individual systems that lack >shadow stack support. > >Update the existing x86 implementation to pay attention to the newly added >arguments, in order to maintain compatibility we use the existing behaviour >if no shadow stack is specified. Minimal validation is done of the supplied >parameters, detailed enforcement is left to when the thread is executed. >Since we are now using four fields from the kernel_clone_args we pass that >into the shadow stack code rather than individual fields. > >Signed-off-by: Mark Brown <broonie@kernel.org> >--- > arch/x86/include/asm/shstk.h | 11 +++++++---- > arch/x86/kernel/process.c | 2 +- > arch/x86/kernel/shstk.c | 30 +++++++++++++++++++++++++----- > include/linux/sched/task.h | 2 ++ > include/uapi/linux/sched.h | 4 ++++ > kernel/fork.c | 24 ++++++++++++++++++++++-- > 6 files changed, 61 insertions(+), 12 deletions(-) > >diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h >index 42fee8959df7..8be7b0a909c3 100644 >--- a/arch/x86/include/asm/shstk.h >+++ b/arch/x86/include/asm/shstk.h >@@ -6,6 +6,7 @@ > #include <linux/types.h> > > struct task_struct; >+struct kernel_clone_args; > struct ksignal; > > #ifdef CONFIG_X86_USER_SHADOW_STACK >@@ -16,8 +17,8 @@ struct thread_shstk { > > long shstk_prctl(struct task_struct *task, int option, unsigned long arg2); > void reset_thread_features(void); >-unsigned long shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags, >- unsigned long stack_size); >+unsigned long shstk_alloc_thread_stack(struct task_struct *p, >+ const struct kernel_clone_args *args); > void shstk_free(struct task_struct *p); > int setup_signal_shadow_stack(struct ksignal *ksig); > int restore_signal_shadow_stack(void); >@@ -26,8 +27,10 @@ static inline long shstk_prctl(struct task_struct *task, int option, > unsigned long arg2) { return -EINVAL; } > static inline void reset_thread_features(void) {} > static inline unsigned long shstk_alloc_thread_stack(struct task_struct *p, >- unsigned long clone_flags, >- unsigned long stack_size) { return 0; } >+ const struct kernel_clone_args *args) >+{ >+ return 0; >+} > static inline void shstk_free(struct task_struct *p) {} > static inline int setup_signal_shadow_stack(struct ksignal *ksig) { return 0; } > static inline int restore_signal_shadow_stack(void) { return 0; } >diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c >index b6f4e8399fca..a9ca80ea5056 100644 >--- a/arch/x86/kernel/process.c >+++ b/arch/x86/kernel/process.c >@@ -207,7 +207,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) > * is disabled, new_ssp will remain 0, and fpu_clone() will know not to > * update it. > */ >- new_ssp = shstk_alloc_thread_stack(p, clone_flags, args->stack_size); >+ new_ssp = shstk_alloc_thread_stack(p, args); > if (IS_ERR_VALUE(new_ssp)) > return PTR_ERR((void *)new_ssp); > >diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c >index 59e15dd8d0f8..7ffe90010587 100644 >--- a/arch/x86/kernel/shstk.c >+++ b/arch/x86/kernel/shstk.c >@@ -191,18 +191,38 @@ void reset_thread_features(void) > current->thread.features_locked = 0; > } > >-unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags, >- unsigned long stack_size) >+unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, >+ const struct kernel_clone_args *args) > { > struct thread_shstk *shstk = &tsk->thread.shstk; >+ unsigned long clone_flags = args->flags; > unsigned long addr, size; > > /* > * If shadow stack is not enabled on the new thread, skip any >- * switch to a new shadow stack. >+ * implicit switch to a new shadow stack and reject attempts to >+ * explciitly specify one. > */ >- if (!features_enabled(ARCH_SHSTK_SHSTK)) >+ if (!features_enabled(ARCH_SHSTK_SHSTK)) { >+ if (args->shadow_stack) >+ return (unsigned long)ERR_PTR(-EINVAL); >+ > return 0; >+ } >+ >+ /* >+ * If the user specified a shadow stack then do some basic >+ * validation and use it. The caller is responsible for >+ * freeing the shadow stack. >+ */ >+ if (args->shadow_stack_size) { >+ size = args->shadow_stack_size; >+ >+ if (size < 8) >+ return (unsigned long)ERR_PTR(-EINVAL); >+ } else { >+ size = args->stack_size; >+ } > > /* > * For CLONE_VFORK the child will share the parents shadow stack. >@@ -222,7 +242,7 @@ unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long cl > if (!(clone_flags & CLONE_VM)) > return 0; > >- size = adjust_shstk_size(stack_size); >+ size = adjust_shstk_size(size); > addr = alloc_shstk(0, size, 0, false); > if (IS_ERR_VALUE(addr)) > return addr; >diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h >index a23af225c898..94e7cf62be51 100644 >--- a/include/linux/sched/task.h >+++ b/include/linux/sched/task.h >@@ -41,6 +41,8 @@ struct kernel_clone_args { > void *fn_arg; > struct cgroup *cgrp; > struct css_set *cset; >+ unsigned long shadow_stack; >+ unsigned long shadow_stack_size; > }; > > /* >diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h >index 3bac0a8ceab2..a998b6d0c897 100644 >--- a/include/uapi/linux/sched.h >+++ b/include/uapi/linux/sched.h >@@ -84,6 +84,8 @@ > * kernel's limit of nested PID namespaces. > * @cgroup: If CLONE_INTO_CGROUP is specified set this to > * a file descriptor for the cgroup. >+ * @shadow_stack_size: Specify the size of the shadow stack to allocate >+ * for the child process. > * > * The structure is versioned by size and thus extensible. > * New struct members must go at the end of the struct and >@@ -101,12 +103,14 @@ struct clone_args { > __aligned_u64 set_tid; > __aligned_u64 set_tid_size; > __aligned_u64 cgroup; >+ __aligned_u64 shadow_stack_size; > }; > #endif > > #define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */ > #define CLONE_ARGS_SIZE_VER1 80 /* sizeof second published struct */ > #define CLONE_ARGS_SIZE_VER2 88 /* sizeof third published struct */ >+#define CLONE_ARGS_SIZE_VER3 96 /* sizeof fourth published struct */ > > /* > * Scheduling policies >diff --git a/kernel/fork.c b/kernel/fork.c >index 10917c3e1f03..b0df69c8185e 100644 >--- a/kernel/fork.c >+++ b/kernel/fork.c >@@ -3067,7 +3067,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs, > CLONE_ARGS_SIZE_VER1); > BUILD_BUG_ON(offsetofend(struct clone_args, cgroup) != > CLONE_ARGS_SIZE_VER2); >- BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER2); >+ BUILD_BUG_ON(offsetofend(struct clone_args, shadow_stack_size) != >+ CLONE_ARGS_SIZE_VER3); >+ BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER3); > > if (unlikely(usize > PAGE_SIZE)) > return -E2BIG; >@@ -3110,6 +3112,7 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs, > .tls = args.tls, > .set_tid_size = args.set_tid_size, > .cgroup = args.cgroup, >+ .shadow_stack_size = args.shadow_stack_size, > }; > > if (args.set_tid && >@@ -3150,6 +3153,23 @@ static inline bool clone3_stack_valid(struct kernel_clone_args *kargs) > return true; > } > >+/** >+ * clone3_shadow_stack_valid - check and prepare shadow stack >+ * @kargs: kernel clone args >+ * >+ * Verify that shadow stacks are only enabled if supported. >+ */ >+static inline bool clone3_shadow_stack_valid(struct kernel_clone_args *kargs) >+{ >+#ifdef CONFIG_ARCH_HAS_USER_SHADOW_STACK >+ /* The architecture must check support on the specific machine */ >+ return true; >+#else >+ /* The architecture does not support shadow stacks */ >+ return !kargs->shadow_stack_size; >+#endif >+} >+ > static bool clone3_args_valid(struct kernel_clone_args *kargs) > { > /* Verify that no unknown flags are passed along. */ >@@ -3172,7 +3192,7 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs) > kargs->exit_signal) > return false; > >- if (!clone3_stack_valid(kargs)) >+ if (!clone3_stack_valid(kargs) || !clone3_shadow_stack_valid(kargs)) > return false; > > return true; > >-- >2.30.2 >
diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h index 42fee8959df7..8be7b0a909c3 100644 --- a/arch/x86/include/asm/shstk.h +++ b/arch/x86/include/asm/shstk.h @@ -6,6 +6,7 @@ #include <linux/types.h> struct task_struct; +struct kernel_clone_args; struct ksignal; #ifdef CONFIG_X86_USER_SHADOW_STACK @@ -16,8 +17,8 @@ struct thread_shstk { long shstk_prctl(struct task_struct *task, int option, unsigned long arg2); void reset_thread_features(void); -unsigned long shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags, - unsigned long stack_size); +unsigned long shstk_alloc_thread_stack(struct task_struct *p, + const struct kernel_clone_args *args); void shstk_free(struct task_struct *p); int setup_signal_shadow_stack(struct ksignal *ksig); int restore_signal_shadow_stack(void); @@ -26,8 +27,10 @@ static inline long shstk_prctl(struct task_struct *task, int option, unsigned long arg2) { return -EINVAL; } static inline void reset_thread_features(void) {} static inline unsigned long shstk_alloc_thread_stack(struct task_struct *p, - unsigned long clone_flags, - unsigned long stack_size) { return 0; } + const struct kernel_clone_args *args) +{ + return 0; +} static inline void shstk_free(struct task_struct *p) {} static inline int setup_signal_shadow_stack(struct ksignal *ksig) { return 0; } static inline int restore_signal_shadow_stack(void) { return 0; } diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c index b6f4e8399fca..a9ca80ea5056 100644 --- a/arch/x86/kernel/process.c +++ b/arch/x86/kernel/process.c @@ -207,7 +207,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) * is disabled, new_ssp will remain 0, and fpu_clone() will know not to * update it. */ - new_ssp = shstk_alloc_thread_stack(p, clone_flags, args->stack_size); + new_ssp = shstk_alloc_thread_stack(p, args); if (IS_ERR_VALUE(new_ssp)) return PTR_ERR((void *)new_ssp); diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index 59e15dd8d0f8..7ffe90010587 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -191,18 +191,38 @@ void reset_thread_features(void) current->thread.features_locked = 0; } -unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags, - unsigned long stack_size) +unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, + const struct kernel_clone_args *args) { struct thread_shstk *shstk = &tsk->thread.shstk; + unsigned long clone_flags = args->flags; unsigned long addr, size; /* * If shadow stack is not enabled on the new thread, skip any - * switch to a new shadow stack. + * implicit switch to a new shadow stack and reject attempts to + * explciitly specify one. */ - if (!features_enabled(ARCH_SHSTK_SHSTK)) + if (!features_enabled(ARCH_SHSTK_SHSTK)) { + if (args->shadow_stack) + return (unsigned long)ERR_PTR(-EINVAL); + return 0; + } + + /* + * If the user specified a shadow stack then do some basic + * validation and use it. The caller is responsible for + * freeing the shadow stack. + */ + if (args->shadow_stack_size) { + size = args->shadow_stack_size; + + if (size < 8) + return (unsigned long)ERR_PTR(-EINVAL); + } else { + size = args->stack_size; + } /* * For CLONE_VFORK the child will share the parents shadow stack. @@ -222,7 +242,7 @@ unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long cl if (!(clone_flags & CLONE_VM)) return 0; - size = adjust_shstk_size(stack_size); + size = adjust_shstk_size(size); addr = alloc_shstk(0, size, 0, false); if (IS_ERR_VALUE(addr)) return addr; diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h index a23af225c898..94e7cf62be51 100644 --- a/include/linux/sched/task.h +++ b/include/linux/sched/task.h @@ -41,6 +41,8 @@ struct kernel_clone_args { void *fn_arg; struct cgroup *cgrp; struct css_set *cset; + unsigned long shadow_stack; + unsigned long shadow_stack_size; }; /* diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h index 3bac0a8ceab2..a998b6d0c897 100644 --- a/include/uapi/linux/sched.h +++ b/include/uapi/linux/sched.h @@ -84,6 +84,8 @@ * kernel's limit of nested PID namespaces. * @cgroup: If CLONE_INTO_CGROUP is specified set this to * a file descriptor for the cgroup. + * @shadow_stack_size: Specify the size of the shadow stack to allocate + * for the child process. * * The structure is versioned by size and thus extensible. * New struct members must go at the end of the struct and @@ -101,12 +103,14 @@ struct clone_args { __aligned_u64 set_tid; __aligned_u64 set_tid_size; __aligned_u64 cgroup; + __aligned_u64 shadow_stack_size; }; #endif #define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */ #define CLONE_ARGS_SIZE_VER1 80 /* sizeof second published struct */ #define CLONE_ARGS_SIZE_VER2 88 /* sizeof third published struct */ +#define CLONE_ARGS_SIZE_VER3 96 /* sizeof fourth published struct */ /* * Scheduling policies diff --git a/kernel/fork.c b/kernel/fork.c index 10917c3e1f03..b0df69c8185e 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -3067,7 +3067,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs, CLONE_ARGS_SIZE_VER1); BUILD_BUG_ON(offsetofend(struct clone_args, cgroup) != CLONE_ARGS_SIZE_VER2); - BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER2); + BUILD_BUG_ON(offsetofend(struct clone_args, shadow_stack_size) != + CLONE_ARGS_SIZE_VER3); + BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER3); if (unlikely(usize > PAGE_SIZE)) return -E2BIG; @@ -3110,6 +3112,7 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs, .tls = args.tls, .set_tid_size = args.set_tid_size, .cgroup = args.cgroup, + .shadow_stack_size = args.shadow_stack_size, }; if (args.set_tid && @@ -3150,6 +3153,23 @@ static inline bool clone3_stack_valid(struct kernel_clone_args *kargs) return true; } +/** + * clone3_shadow_stack_valid - check and prepare shadow stack + * @kargs: kernel clone args + * + * Verify that shadow stacks are only enabled if supported. + */ +static inline bool clone3_shadow_stack_valid(struct kernel_clone_args *kargs) +{ +#ifdef CONFIG_ARCH_HAS_USER_SHADOW_STACK + /* The architecture must check support on the specific machine */ + return true; +#else + /* The architecture does not support shadow stacks */ + return !kargs->shadow_stack_size; +#endif +} + static bool clone3_args_valid(struct kernel_clone_args *kargs) { /* Verify that no unknown flags are passed along. */ @@ -3172,7 +3192,7 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs) kargs->exit_signal) return false; - if (!clone3_stack_valid(kargs)) + if (!clone3_stack_valid(kargs) || !clone3_shadow_stack_valid(kargs)) return false; return true;
Unlike with the normal stack there is no API for configuring the the shadow stack for a new thread, instead the kernel will dynamically allocate a new shadow stack with the same size as the normal stack. This appears to be due to the shadow stack series having been in development since before the more extensible clone3() was added rather than anything more deliberate. Add parameters to clone3() specifying the address and size of a shadow stack for the newly created process, we validate that the range specified is accessible to userspace but do not validate that it has been mapped appropriately for use as a shadow stack (normally via map_shadow_stack()). If the shadow stack is specified in this way then the caller is responsible for freeing the memory as with the main stack. If no shadow stack is specified then the existing implicit allocation and freeing behaviour is maintained. If the architecture does not support shadow stacks the shadow stack parameters must be zero, architectures that do support the feature are expected to have the same requirement on individual systems that lack shadow stack support. Update the existing x86 implementation to pay attention to the newly added arguments, in order to maintain compatibility we use the existing behaviour if no shadow stack is specified. Minimal validation is done of the supplied parameters, detailed enforcement is left to when the thread is executed. Since we are now using four fields from the kernel_clone_args we pass that into the shadow stack code rather than individual fields. Signed-off-by: Mark Brown <broonie@kernel.org> --- arch/x86/include/asm/shstk.h | 11 +++++++---- arch/x86/kernel/process.c | 2 +- arch/x86/kernel/shstk.c | 30 +++++++++++++++++++++++++----- include/linux/sched/task.h | 2 ++ include/uapi/linux/sched.h | 4 ++++ kernel/fork.c | 24 ++++++++++++++++++++++-- 6 files changed, 61 insertions(+), 12 deletions(-)