| Message ID | 20221110124400.zgymc2lnwqjukgfh@techsingularity.net |
|---|---|
| State | New |
| Headers | show |
| Series | x86: Drop fpregs lock before inheriting FPU permissions | expand |
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c index 3b28c5b25e12..d00db56a8868 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/arch/x86/kernel/fpu/core.c @@ -605,9 +605,9 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal) if (test_thread_flag(TIF_NEED_FPU_LOAD)) fpregs_restore_userregs(); save_fpregs_to_fpstate(dst_fpu); + fpregs_unlock(); if (!(clone_flags & CLONE_THREAD)) fpu_inherit_perms(dst_fpu); - fpregs_unlock(); /* * Children never inherit PASID state.
Mike Galbraith reported the following against an old fork of preempt-rt but the same issue also applies to the current preempt-rt tree. BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:46 in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: systemd preempt_count: 1, expected: 0 RCU nest depth: 0, expected: 0 Preemption disabled at: fpu_clone+0xfa/0x480 CPU: 6 PID: 1 Comm: systemd Tainted: G E (unreleased) Call Trace: <TASK> dump_stack_lvl+0x45/0x5b ? fpu_clone+0xfa/0x480 __might_resched+0x165/0x200 rt_spin_lock+0x2d/0x70 fpu_clone+0x32a/0x480 ? copy_thread+0xef/0x270 ? copy_process+0xd2c/0x1c00 ? shmem_alloc_inode+0x16/0x30 ? kmem_cache_alloc+0x120/0x2a0 ? kernel_clone+0x9b/0x460 ? __do_sys_clone+0x72/0xa0 ? do_syscall_64+0x58/0x80 ? __x64_sys_rt_sigprocmask+0x93/0xd0 ? syscall_exit_to_user_mode+0x18/0x40 ? do_syscall_64+0x67/0x80 ? syscall_exit_to_user_mode+0x18/0x40 ? do_syscall_64+0x67/0x80 ? syscall_exit_to_user_mode+0x18/0x40 ? do_syscall_64+0x67/0x80 ? exc_page_fault+0x6a/0x190 ? entry_SYSCALL_64_after_hwframe+0x61/0xcb </TASK> The splat comes from fpu_inherit_perms() being called under fpregs_lock(), and us reaching the spin_lock_irq() therein due to fpu_state_size_dynamic() returning true despite static key __fpu_state_size_dynamic having never been enabled. Mike's assessment looks correct. fpregs_lock on a PREEMPT_RT kernel disables preemption so calling spin_lock_irq() in fpu_inherit_perms is unsafe. This problem exists since commit 9e798e9aa14c ("x86/fpu: Prepare fpu_clone() for dynamically enabled features"). Even though the original bug report should not have enabled the paths at all, the bug still exists. fpregs_lock is necessary when editing the FPU registers or a task's FP state but it is not necessary for fpu_inherit_perms. The only write of any FP state in fpu_inherit_perms is for the new child which is not running yet and cannot context switch or be borrowed by a kernel thread yet. Hence, fpregs_lock is not protecting anything in the new child until clone() completes and can be dropped earlier. The siglock still needs to be acquired by fpu_inherit_perms as the read of the parents permissions has to be serialised. Reported-by: Mike Galbraith <efault@gmx.de> Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> --- arch/x86/kernel/fpu/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)