Re: [PATCH v4 29/39] x86/shstk: Handle signals for shadow stack

Kees Cook
2022-12-03T02:46:40Z

On Fri, Dec 02, 2022 at 04:35:56PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> When a signal is handled normally the context is pushed to the stack
> before handling it. For shadow stacks, since the shadow stack only track's
> return addresses, there isn't any state that needs to be pushed. However,
> there are still a few things that need to be done. These things are
> userspace visible and which will be kernel ABI for shadow stacks.
>
> One is to make sure the restorer address is written to shadow stack, since
> the signal handler (if not changing ucontext) returns to the restorer, and
> the restorer calls sigreturn. So add the restorer on the shadow stack
> before handling the signal, so there is not a conflict when the signal
> handler returns to the restorer.
>
> The other thing to do is to place some type of checkable token on the
> thread's shadow stack before handling the signal and check it during
> sigreturn. This is an extra layer of protection to hamper attackers
> calling sigreturn manually as in SROP-like attacks.
>
> For this token we can use the shadow stack data format defined earlier.
> Have the data pushed be the previous SSP. In the future the sigreturn
> might want to return back to a different stack. Storing the SSP (instead
> of a restore offset or something) allows for future functionality that
> may want to restore to a different stack.
>
> So, when handling a signal push
> - the SSP pointing in the shadow stack data format
> - the restorer address below the restore token.
>
> In sigreturn, verify SSP is stored in the data format and pop the shadow
> stack.
>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook