x86: account for shadow stack in exception-from-stub recovery

x86: account for shadow stack in exception-from-stub recovery

Dealing with exceptions raised from within emulation stubs involves
discarding return address (replaced by exception related information).
Such discarding of course also requires removing the corresponding entry
from the shadow stack.

Also amend the comment in fixup_exception_return(), to further clarify
why use of ptr[1] can't be an out-of-bounds access.

This is CVE-2023-46841 / XSA-451.

Fixes: 209fb9919b50 ("x86/extable: Adjust extable handling to be shadow stack compatible")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 91f5f7a9154919a765c3933521760acffeddbf28
master date: 2024-02-27 13:49:22 +0100