Re: [PATCH v5 16/39] x86/mm: Check shadow stack page fault errors

[Kees Cook]

On Thu, Jan 19, 2023 at 01:22:54PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> The CPU performs "shadow stack accesses" when it expects to encounter
> shadow stack mappings. These accesses can be implicit (via CALL/RET
> instructions) or explicit (instructions like WRSS).
> 
> Shadow stack accesses to shadow-stack mappings can result in faults in
> normal, valid operation just like regular accesses to regular mappings.
> Shadow stacks need some of the same features like delayed allocation, swap
> and copy-on-write. The kernel needs to use faults to implement those
> features.
> 
> The architecture has concepts of both shadow stack reads and shadow stack
> writes. Any shadow stack access to non-shadow stack memory will generate
> a fault with the shadow stack error code bit set.
> 
> This means that, unlike normal write protection, the fault handler needs
> to create a type of memory that can be written to (with instructions that
> generate shadow stack writes), even to fulfill a read access. So in the
> case of COW memory, the COW needs to take place even with a shadow stack
> read. Otherwise the page will be left (shadow stack) writable in
> userspace. So to trigger the appropriate behavior, set FAULT_FLAG_WRITE
> for shadow stack accesses, even if the access was a shadow stack read.
> 
> For the purpose of making this clearer, consider the following example.
> If a process has a shadow stack, and forks, the shadow stack PTEs will
> become read-only due to COW. If the CPU in one process performs a shadow
> stack read access to the shadow stack, for example executing a RET and
> causing the CPU to read the shadow stack copy of the return address, then
> in order for the fault to be resolved the PTE will need to be set with
> shadow stack permissions. But then the memory would be changeable from
> userspace (from CALL, RET, WRSS, etc). So this scenario needs to trigger
> COW, otherwise the shared page would be changeable from both processes.
> 
> Shadow stack accesses can also result in errors, such as when a shadow
> stack overflows, or if a shadow stack access occurs to a non-shadow-stack
> mapping. Also, generate the errors for invalid shadow stack accesses.
> 
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook