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SubjectRe: [PATCH 00/17] ARMv8.3 pointer authentication support
On Tue, Nov 13, 2018 at 10:17 AM, Kristina Martsenko
<kristina.martsenko@arm.com> wrote:
> When the PAC authentication fails, it doesn't actually generate an
> exception, it just flips a bit in the high-order bits of the pointer,
> making the pointer invalid. Then when the pointer is dereferenced (e.g.
> as a function return address), it generates the usual type of exception
> for an invalid address.

Ah! Okay, thanks. I missed that detail. :)

What area of memory ends up being addressable with such bit flips?
(i.e. is the kernel making sure nothing executable ends up there?)

> So when a function return fails in user mode, the exception is handled
> in __do_user_fault and a forced SIGSEGV is delivered to the task. When a
> function return fails in kernel mode, the exception is handled in
> __do_kernel_fault and the task is killed.
>
> This is different from stack protector as we don't panic the kernel, we
> just kill the task. It would be difficult to panic as we don't have a
> reliable way of knowing that the exception was caused by a PAC
> authentication failure (we just have an invalid pointer with a specific
> bit flipped). We also don't print out any PAC-related warning.

There are other "guesses" in __do_kernel_fault(), I think? Could a
"PAC mismatch?" warning be included in the Oops if execution fails in
the address range that PAC failures would resolve into?

-Kees

--
Kees Cook

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