linux-kernel - Re: 5.9-rc7 null ptr deref in __i915_gem_userptr_get_pages

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Message-ID: <CAHmME9qBtUuOSEU3Cb9rL7SHaBAwk862VEPdmYcisnSHDERtvQ@mail.gmail.com>
Date:   Mon, 28 Sep 2020 12:17:58 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Peter Xu <peterx@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     intel-gfx@...ts.freedesktop.org,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: 5.9-rc7 null ptr deref in __i915_gem_userptr_get_pages_worker

Alright, the failing code seems to be in mm:

        if (flags & FOLL_PIN)
                atomic_set(&current->mm->has_pinned, 1);

Apparently you can't rely on current->mm being valid in this context;
it's null here, hence the +0x64 for has_pinned's offset.

This was added by 008cfe4418b3 ("mm: Introduce mm_struct.has_pinned"),
which is new for rc7 indeed.

The crash goes away when changing that to:

        if ((flags & FOLL_PIN) && current->mm)
                atomic_set(&current->mm->has_pinned, 1);

But I haven't really evaluated whether or not that's racy or if I need
to take locks to do such a thing.