Re: [PATCH] x86/cet: Fix shskt manipulation error with BUGFRAME_{warn,run_fn}

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Aug 12, 2021 at 06:03:50PM +0100, Andrew Cooper wrote:
> This was a clear oversight in the original CET work.  The BUGFRAME_run_fn and
> BUGFRAME_warn paths update regs->rip without an equivlenet adjustment to the
> shadow stack, causes IRET to suffer #CP due to the mismatch.
> 
> One subtle, and therefore fragile, aspect of extable_shstk_fixup() was that it
> required regs->rip to have its old value as a cross-check that the correct
> word in the shadow stack was being adjusted.
> 
> Rework extable_shstk_fixup() into fixup_exception_return() which takes
> ownership of the update to both the regular and shadow stacks, ensuring that
> the regs->rip update is ordered suitably.
> 
> Use the new fixup_exception_return() for BUGFRAME_run_fn and BUGFRAME_warn to
> ensure that the shadow stack is updated too.
> 
> Fixes: 209fb9919b50 ("x86/extable: Adjust extable handling to be shadow stack 
> compatible")
> Reported-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

With this path, I don't observe the crash anymore. Thanks!

Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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