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 <redacted>
Reviewed-by: Kees Cook <redacted>
---
 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.

https://people.kernel.org/tglx/notes-about-netiquette