Re: [PATCH] amdgpu_dm: fix nonblocking atomic commit use-after-free

Mazin Rezk
2020-07-25T05:21:00Z

On Saturday, July 25, 2020 12:59 AM, Duncan <1i5t5.duncan@cox.net> wrote:

> On Sat, 25 Jul 2020 03:03:52 +0000
> Mazin Rezk mnrzk@protonmail.com wrote:
>
> > > Am 24.07.20 um 19:33 schrieb Kees Cook:
> > >
> > > > There was a fix to disable the async path for this driver that
> > > > worked around the bug too, yes? That seems like a safer and more
> > > > focused change that doesn't revert the SLUB defense for all
> > > > users, and would actually provide a complete, I think, workaround
> >
> > That said, I haven't seen the async disabling patch. If you could
> > link to it, I'd be glad to test it out and perhaps we can use that
> > instead.
>
> I'm confused. Not to put words in Kees' mouth; /I/ am confused (which
> admittedly could well be just because I make no claims to be a
> coder and am simply reading the bug and thread, but I'd appreciate some
> "unconfusing" anyway).
>
> My interpretation of the "async disabling" reference was that it was to
> comment #30 on the bug:
>
> https://bugzilla.kernel.org/show_bug.cgi?id=207383#c30
>
> ... which (if I'm not confused on this point too) appears to be yours.
> There it was stated...
>
> > > > >
>
> I've also found that this bug exclusively occurs when commit_work is on
> the workqueue. After forcing drm_atomic_helper_commit to run all of the
> commits without adding to the workqueue and running the OS, the issue
> seems to have disappeared.
> <<<<
>
> Would not forcing all commits to run directly, without placing them on
> the workqueue, be "async disabling"? That's what I /thought/ he was
> referencing.

Oh, I thought he was referring to a different patch. Kees, could I get
your confirmation on this?

The change I made actually affected all of the DRM code, although this could
easily be changed to be specific to amdgpu. (By forcing blocking on
amdgpu_dm's non-blocking commit code)

That said, I'd still need to test further because I only did test it for a
couple of hours then. Although it should work in theory.

>
> OTOH your base/context swap idea sounds like a possibly "less
> disturbance" workaround, if it works, and given the point in the
> commit cycle... (But if it's out Sunday it's likely too late to test
> and get it in now anyway; if it's another week, tho...)

The base/context swap idea should make the use-after-free behave how it
did in 5.6. Since the bug doesn't cause an issue in 5.6, it's less of a
"less disturbance" workaround and more of a "no disturbance" workaround.

Thanks,
Mazin Rezk

>
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> "Every nonfree program has a lord, a master --
> and if you use the program, he is your master." Richard Stallman