PING^1: [PATCH] x86: Check corrupted return address when unwinding stack

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Sep 21, 2022 at 1:42 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> If shadow stack is enabled, when unwinding stack, we count how many stack
> frames we pop to reach the landing pad and adjust shadow stack by the same
> amount.  When counting the stack frame, we compare the return address on
> normal stack against the return address on shadow stack.  If they don't
> match, return _URC_FATAL_PHASE2_ERROR for the corrupted return address on
> normal stack.  Don't check the return address for
>
> 1. Non-catchable exception where exception_class == 0.  Process will be
> terminated.
> 2. Zero return address which marks the outermost stack frame.
> 3. Signal stack frame since kernel puts a restore token on shadow stack.
>
>         * unwind-generic.h (_Unwind_Frames_Increment): Add the EXC
>         argument.
>         * unwind.inc (_Unwind_RaiseException_Phase2): Pass EXC to
>         _Unwind_Frames_Increment.
>         (_Unwind_ForcedUnwind_Phase2): Likewise.
>         * config/i386/shadow-stack-unwind.h (_Unwind_Frames_Increment):
>         Take the EXC argument.  Return _URC_FATAL_PHASE2_ERROR if the
>         return address on normal stack doesn't match the return address
>         on shadow stack.
> ---
>  libgcc/config/i386/shadow-stack-unwind.h | 51 ++++++++++++++++++++++--
>  libgcc/unwind-generic.h                  |  2 +-
>  libgcc/unwind.inc                        |  4 +-
>  3 files changed, 50 insertions(+), 7 deletions(-)
>
> diff --git a/libgcc/config/i386/shadow-stack-unwind.h b/libgcc/config/i386/shadow-stack-unwind.h
> index 2b02682bdae..89d44165000 100644
> --- a/libgcc/config/i386/shadow-stack-unwind.h
> +++ b/libgcc/config/i386/shadow-stack-unwind.h
> @@ -54,10 +54,39 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>     aligned.  If the original shadow stack is 8 byte aligned, we just
>     need to pop 2 slots, one restore token, from shadow stack.  Otherwise,
>     we need to pop 3 slots, one restore token + 4 byte padding, from
> -   shadow stack.  */
> -#ifndef __x86_64__
> +   shadow stack.
> +
> +   When popping a stack frame, we compare the return address on normal
> +   stack against the return address on shadow stack.  If they don't match,
> +   return _URC_FATAL_PHASE2_ERROR for the corrupted return address on
> +   normal stack.  Don't check the return address for
> +   1. Non-catchable exception where exception_class == 0.  Process will
> +      be terminated.
> +   2. Zero return address which marks the outermost stack frame.
> +   3. Signal stack frame since kernel puts a restore token on shadow
> +      stack.
> + */
>  #undef _Unwind_Frames_Increment
> -#define _Unwind_Frames_Increment(context, frames)      \
> +#ifdef __x86_64__
> +#define _Unwind_Frames_Increment(exc, context, frames) \
> +    {                                                  \
> +      frames++;                                                \
> +      if (exc->exception_class != 0                    \
> +         && _Unwind_GetIP (context) != 0               \
> +         && !_Unwind_IsSignalFrame (context))          \
> +       {                                               \
> +         _Unwind_Word ssp = _get_ssp ();               \
> +         if (ssp != 0)                                 \
> +           {                                           \
> +             ssp += 8 * frames;                        \
> +             _Unwind_Word ra = *(_Unwind_Word *) ssp;  \
> +             if (ra != _Unwind_GetIP (context))        \
> +               return _URC_FATAL_PHASE2_ERROR;         \
> +           }                                           \
> +       }                                               \
> +    }
> +#else
> +#define _Unwind_Frames_Increment(exc, context, frames) \
>    if (_Unwind_IsSignalFrame (context))                 \
>      do                                                 \
>        {                                                        \
> @@ -83,5 +112,19 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>        }                                                        \
>      while (0);                                         \
>    else                                                 \
> -    frames++;
> +    {                                                  \
> +      frames++;                                                \
> +      if (exc->exception_class != 0                    \
> +         && _Unwind_GetIP (context) != 0)              \
> +       {                                               \
> +         _Unwind_Word ssp = _get_ssp ();               \
> +         if (ssp != 0)                                 \
> +           {                                           \
> +             ssp += 4 * frames;                        \
> +             _Unwind_Word ra = *(_Unwind_Word *) ssp;  \
> +             if (ra != _Unwind_GetIP (context))        \
> +               return _URC_FATAL_PHASE2_ERROR;         \
> +           }                                           \
> +       }                                               \
> +    }
>  #endif
> diff --git a/libgcc/unwind-generic.h b/libgcc/unwind-generic.h
> index a87c9b3ccf6..bf721282d03 100644
> --- a/libgcc/unwind-generic.h
> +++ b/libgcc/unwind-generic.h
> @@ -292,6 +292,6 @@ EXCEPTION_DISPOSITION _GCC_specific_handler (PEXCEPTION_RECORD, void *,
>  #define _Unwind_Frames_Extra(frames)
>
>  /* Increment frame count.  */
> -#define _Unwind_Frames_Increment(context, frames) frames++
> +#define _Unwind_Frames_Increment(exc, context, frames) frames++
>
>  #endif /* unwind.h */
> diff --git a/libgcc/unwind.inc b/libgcc/unwind.inc
> index 5efd8af1b15..a7111a7b3a8 100644
> --- a/libgcc/unwind.inc
> +++ b/libgcc/unwind.inc
> @@ -73,7 +73,7 @@ _Unwind_RaiseException_Phase2(struct _Unwind_Exception *exc,
>        gcc_assert (!match_handler);
>
>        uw_update_context (context, &fs);
> -      _Unwind_Frames_Increment (context, frames);
> +      _Unwind_Frames_Increment (exc, context, frames);
>      }
>
>    *frames_p = frames;
> @@ -191,7 +191,7 @@ _Unwind_ForcedUnwind_Phase2 (struct _Unwind_Exception *exc,
>        /* Update cur_context to describe the same frame as fs, and discard
>          the previous context if necessary.  */
>        uw_advance_context (context, &fs);
> -      _Unwind_Frames_Increment (context, frames);
> +      _Unwind_Frames_Increment (exc, context, frames);
>      }
>
>    *frames_p = frames;
> --
> 2.37.3
>

PING.  Jeff has approved the generic changes.

Thanks.

--
H.J.