[v3,28/29] riscv: Documentation for shadow stack on riscv

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+.. SPDX-License-Identifier: GPL-2.0
+
+:Author: Deepak Gupta <debug@rivosinc.com>
+:Date:   12 January 2024
+
+=========================================================
+Shadow stack to protect function returns on RISC-V Linux
+=========================================================
+
+This document briefly describes the interface provided to userspace by Linux
+to enable shadow stack for user mode applications on RISV-V
+
+1. Feature Overview
+--------------------
+
+Memory corruption issues usually result in to crashes, however when in hands of
+an adversary and if used creatively can result into variety security issues.
+
+One of those security issues can be code re-use attacks on program where adversary
+can use corrupt return addresses present on stack and chain them together to perform
+return oriented programming (ROP) and thus compromising control flow integrity (CFI)
+of the program.
+
+Return addresses live on stack and thus in read-write memory and thus are
+susceptible to corruption and allows an adversary to reach any program counter
+(PC) in address space. On RISC-V `zicfiss` extension provides an alternate stack
+`shadow stack` on which return addresses can be safely placed in prolog of the
+function and retrieved in epilog. `zicfiss` extension makes following changes
+
+	- PTE encodings for shadow stack virtual memory
+	  An earlier reserved encoding in first stage translation i.e.
+	  PTE.R=0, PTE.W=1, PTE.X=0  becomes PTE encoding for shadow stack pages.
+
+	- `sspush x1/x5` instruction pushes (stores) `x1/x5` to shadow stack.
+
+	- `sspopchk x1/x5` instruction pops (loads) from shadow stack and compares
+	  with `x1/x5` and if un-equal, CPU raises `software check exception` with
+	  `*tval = 3`
+
+Compiler toolchain makes sure that function prologs have `sspush x1/x5` to save return
+address on shadow stack in addition to regular stack. Similarly function epilogs have
+`ld x5, offset(x2)`; `sspopchk x5` to ensure that popped value from regular stack
+matches with popped value from shadow stack.
+
+2. Shadow stack protections and linux memory manager
+-----------------------------------------------------
+
+As mentioned earlier, shadow stack get new page table encodings and thus have some
+special properties assigned to them and instructions that operate on them as below
+
+	- Regular stores to shadow stack memory raises access store faults.
+	  This way shadow stack memory is protected from stray inadvertant
+	  writes
+
+	- Regular loads to shadow stack memory are allowed.
+	  This allows stack trace utilities or backtrace functions to read
+	  true callstack (not tampered)
+
+	- Only shadow stack instructions can generate shadow stack load or
+	  shadow stack store.
+
+	- Shadow stack load / shadow stack store on read-only memory raises
+	  AMO/store page fault. Thus both `sspush x1/x5` and `sspopchk x1/x5`
+	  will raise AMO/store page fault. This simplies COW handling in kernel
+	  During fork, kernel can convert shadow stack pages into read-only
+	  memory (as it does for regular read-write memory) and as soon as
+	  subsequent `sspush` or `sspopchk` in userspace is encountered, then
+	  kernel can perform COW.
+
+	- Shadow stack load / shadow stack store on read-write, read-write-
+	  execute memory raises an access fault. This is a fatal condition
+	  because shadow stack should never be operating on read-write, read-
+	  write-execute memory.
+
+3. ELF and psABI
+-----------------
+
+Toolchain sets up `GNU_PROPERTY_RISCV_FEATURE_1_BCFI` for property
+`GNU_PROPERTY_RISCV_FEATURE_1_AND` in notes section of the object file.
+
+4. Linux enabling
+------------------
+
+User space programs can have multiple shared objects loaded in its address space
+and it's a difficult task to make sure all the dependencies have been compiled
+with support of shadow stack. Thus it's left to dynamic loader to enable
+shadow stack for the program.
+
+5. prctl() enabling
+--------------------
+
+`PR_SET_SHADOW_STACK_STATUS` / `PR_GET_SHADOW_STACK_STATUS` /
+`PR_LOCK_SHADOW_STACK_STATUS` are three prctls added to manage shadow stack
+enabling for tasks. prctls are arch agnostic and returns -EINVAL on other arches.
+
+`PR_SET_SHADOW_STACK_STATUS`: If arg1 `PR_SHADOW_STACK_ENABLE` and if CPU supports
+`zicfiss` then kernel will enable shadow stack for the task. Dynamic loader can
+issue this `prctl` once it has determined that all the objects loaded in address
+space have support for shadow stack. Additionally if there is a `dlopen` to an
+object which wasn't compiled with `zicfiss`, dynamic loader can issue this prctl
+with arg1 set to 0 (i.e. `PR_SHADOW_STACK_ENABLE` being clear)
+
+`PR_GET_SHADOW_STACK_STATUS`: Returns current status of indirect branch tracking.
+If enabled it'll return `PR_SHADOW_STACK_ENABLE`
+
+`PR_LOCK_SHADOW_STACK_STATUS`: Locks current status of shadow stack enabling on the
+task. User space may want to run with strict security posture and wouldn't want
+loading of objects without `zicfiss` support in it and thus would want to disallow
+disabling of shadow stack on current task. In that case user space can use this prctl
+to lock current settings.
+
+5. violations related to returns with shadow stack enabled
+-----------------------------------------------------------
+
+Pertaining to shadow stack, CPU raises software check exception in following
+condition
+
+	- On execution of `sspopchk x1/x5`, x1/x5 didn't match top of shadow stack.
+	  If mismatch happens then cpu does `*tval = 3` and raise software check
+	  exception
+
+Linux kernel will treat this as `SIGSEV`` with code = `SEGV_CPERR` and follow
+normal course of signal delivery.
+
+6. Shadow stack tokens
+-----------------------
+Regular stores on shadow stacks are not allowed and thus can't be tampered with via
+arbitrary stray writes due to bugs. Method of pivoting / switching to shadow stack
+is simply writing to csr `CSR_SSP` changes active shadow stack. This can be problematic
+because usually value to be written to `CSR_SSP` will be loaded somewhere in writeable
+memory and thus allows an adversary to corruption bug in software to pivot to an any
+address in shadow stack range. Shadow stack tokens can help mitigate this problem by
+making sure that:
+
+ - When software is switching away from a shadow stack, shadow stack pointer should be
+   saved on shadow stack itself and call it `shadow stack token`
+
+ - When software is switching to a shadow stack, it should read the `shadow stack token`
+   from shadow stack pointer and verify that `shadow stack token` itself is pointer to
+   shadow stack itself.
+
+ - Once the token verification is done, software can perform the write to `CSR_SSP` to
+   switch shadow stack.
+
+Here software can be user mode task runtime itself which is managing various contexts
+as part of single thread. Software can be kernel as well when kernel has to deliver a
+signal to user task and must save shadow stack pointer. Kernel can perform similar
+procedure by saving a token on user shadow stack itself. This way whenever sigreturn
+happens, kernel can read the token and verify the token and then switch to shadow stack.
+Using this mechanism, kernel helps user task so that any corruption issue in user task
+is not exploited by adversary by arbitrarily using `sigreturn`. Adversary will have to
+make sure that there is a `shadow stack token` in addition to invoking `sigreturn`
+
+7. Signal shadow stack
+-----------------------
+Following structure has been added to sigcontext for RISC-V. `rsvd` field has been kept
+in case we need some extra information in future for landing pads / indirect branch
+tracking. It has been kept today in order to allow backward compatibility in future.
+
+struct __sc_riscv_cfi_state {
+	unsigned long ss_ptr;
+	unsigned long rsvd;
+};
+
+As part of signal delivery, shadow stack token is saved on current shadow stack itself and
+updated pointer is saved away in `ss_ptr` field in `__sc_riscv_cfi_state` under `sigcontext`
+Existing shadow stack allocation is used for signal delivery. During `sigreturn`, kernel will
+obtain `ss_ptr` from `sigcontext` and verify the saved token on shadow stack itself and switch
+shadow stack.