Re: [PATCH v5 29/39] x86/shstk: Introduce routines modifying shstk

[Kees Cook]

On Thu, Jan 19, 2023 at 01:23:07PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> Shadow stacks are normally written to via CALL/RET or specific CET
> instructions like RSTORSSP/SAVEPREVSSP. However during some Linux
> operations the kernel will need to write to directly using the ring-0 only
> WRUSS instruction.
> 
> A shadow stack restore token marks a restore point of the shadow stack, and
> the address in a token must point directly above the token, which is within
> the same shadow stack. This is distinctively different from other pointers
> on the shadow stack, since those pointers point to executable code area.
> 
> Introduce token setup and verify routines. Also introduce WRUSS, which is
> a kernel-mode instruction but writes directly to user shadow stack.
> 
> In future patches that enable shadow stack to work with signals, the kernel
> will need something to denote the point in the stack where sigreturn may be
> called. This will prevent attackers calling sigreturn at arbitrary places
> in the stack, in order to help prevent SROP attacks.
> 
> To do this, something that can only be written by the kernel needs to be
> placed on the shadow stack. This can be accomplished by setting bit 63 in
> the frame written to the shadow stack. Userspace return addresses can't
> have this bit set as it is in the kernel range. It is also can't be a
> valid restore token.
> 
> 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