Re: [PATCH v23 23/28] x86/cet/shstk: Handle signals for shadow stack

Borislav Petkov
2021-03-19T12:51:39Z

On Tue, Mar 16, 2021 at 08:10:49AM -0700, Yu-cheng Yu wrote:
> To deliver a signal, create a shadow stack restore token and put the token
> and the signal restorer address on the shadow stack. For sigreturn, verify
> the token and restore from it the shadow stack pointer.
>
> 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; those pointers point to executable code area.
>
> In sigreturn, restoring from a token ensures the target address is the
> location pointed by the token.
>
> Introduce WRUSS, which is a kernel-mode instruction but writes directly to
> user shadow stack. It is used to construct the user signal stack as
> described above.
>
> Currently there is no systematic facility for extending a signal context.
> Introduce a signal context extension 'struct sc_ext', which is used to save
> shadow stack restore token address and WAIT_ENDBR status. WAIT_ENDBR will
> be introduced later in the Indirect Branch Tracking (IBT) series, but add
> that into sc_ext now to keep the struct stable in case the IBT series is
> applied later.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
> arch/x86/ia32/ia32_signal.c | 17 +++
> arch/x86/include/asm/cet.h | 8 ++
> arch/x86/include/asm/fpu/internal.h | 10 ++
> arch/x86/include/asm/special_insns.h | 32 ++++++
> arch/x86/include/uapi/asm/sigcontext.h | 9 ++
> arch/x86/kernel/cet.c | 152 +++++++++++++++++++++++++
> arch/x86/kernel/fpu/signal.c | 100 ++++++++++++++++
> arch/x86/kernel/signal.c | 10 ++
> 8 files changed, 338 insertions(+)

The commit message lacks structure in explaining what the
problem/missing functionality is and why this is solved the way it is,
with stack tokens. Here's a good example how to structure it properly:

https://git.kernel.org/tip/323950a8a98b492ac2fa168e8e4c0becfb4554dd

Also, this patch does a couple of things at once and it needs splitting
for easier review.

Thx.

--
Regards/Gruss,
Boris.