lkml.org 
[lkml]   [2018]   [Nov]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 00/17] ARMv8.3 pointer authentication support
On Tue, Nov 13, 2018 at 05:09:00PM -0600, Kees Cook wrote:
> 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?

I'd personally prefer that we didn't try to guess if a fault is due to a failed
AUT*, even for logging.

Presently, it's not possible to distinguish between a fault resulting from a
failed AUT* and a fault which happens to have hte same bits/clear, so there are
false positives. The architecture may also change the precise details of the
faulting address, and we'd have false negatives in that case.

Given that, I think suggesting that a fault is due to a failed AUT* is liable
to make things more confusing.

Thanks,
Mark.

\
 
 \ /
  Last update: 2018-11-14 22:47    [W:0.120 / U:2.648 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site