Re: [PATCH] x86/fpu: verify xstate buffer size according with requested features

[Andrei Vagin]

On Thu, Jan 18, 2024 at 2:11 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jan 18 2024 at 14:02, Dave Hansen wrote:
> > On 1/18/24 11:54, Thomas Gleixner wrote:
> >> On Thu, Jan 18 2024 at 10:27, Dave Hansen wrote:
> >>> If we have nice, reliable fault handling and then decide that we've got
> >>> XRSTOR's running amok reading random memory all over the place that need
> >>> a nicer error message, then we can add that code to predict the future.
> >>> If our "predict the future" code goes wrong, then we lose an error
> >>> message -- not a big deal.
> >> After staring more at it, it's arguable to pass fpstate->user_size to
> >> fault_in_readable() and ignore fx_sw->xstate_size completely.
> >>
> >> That's a guaranteed to be reliable size which prevents endless loops
> >> because arguably that's the maximum size which can be touched by XRSTOR,
> >> no?

fpstate->user_size isn't constant.  It can be modified from the XFD #NM
handler. For example, it happens when a process invokes one of amx
instructions for the first time. It means we have to be able to restore
an fpu state from signal frames generated with a smaller
fpstate->user_size. Can it trigger any issues?

> >
> > I like it.  It takes fx_sw completely out of the picture, which was the
> > root of the problem in the first place.
>
> Correct.
>
> I really don't care about the esoteric case where this might
> theoretically result in a unjustified application abort.
>
> You really need to twist your brain around 6 corners and then squint
> twice to construct that case. Of course syzcaller might trigger it, but
> fuzzing the sigreturn frame is a #GP, #PF and whatever lottery anyway.

In my case, the bug was triggered by gVisor (it is like the user-mode
Linux).

Thanks,
Andrei