Re: [syzbot] [mm?] WARNING: suspicious RCU usage in mas_walk (2)

[Jann Horn <jannh@google.com>]

On Thu, Jul 27, 2023 at 7:22 PM Suren Baghdasaryan <surenb@google.com> wrote:
> On Thu, Jul 27, 2023 at 9:48 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > * syzbot <syzbot+8645fe63c4d22c8d27b8@syzkaller.appspotmail.com> [230726 02:57]:
> > > syzbot has bisected this issue to:
> > >
> > > commit a52f58b34afe095ebc5823684eb264404dad6f7b
> > > Author: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > Date:   Mon Jul 24 18:54:10 2023 +0000
> > >
> > >     mm: handle faults that merely update the accessed bit under the VMA lock
> > >
> > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1783585ea80000
> > > start commit:   [unknown]
> > > git tree:       linux-next
> > > final oops:     https://syzkaller.appspot.com/x/report.txt?x=1443585ea80000
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1043585ea80000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=f481ab36ce878b84
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=8645fe63c4d22c8d27b8
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1697cec9a80000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1566986ea80000
> > >
> > > Reported-by: syzbot+8645fe63c4d22c8d27b8@syzkaller.appspotmail.com
> > > Fixes: a52f58b34afe ("mm: handle faults that merely update the accessed bit under the VMA lock")
> > >
> > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> >
> > This is caused by walking the maple tree without holding the mmap or rcu
> > read lock when per-vma locking is used for the page fault.
> >
> > We could wrap the find_mergeable_anon_vma() walk with an rcu read lock,
> > but I am unsure if that's the correct way to handle this as the anon_vma
> > lock is taken later in __anon_vma_prepare().  Note that the anon_vma
> > lock is per-anon_vma, so we cannot just relocate that lock.
>
> Hmm. lock_vma_under_rcu() specifically checks for vma->anon_vma==NULL
> condition (see [1]) to avoid going into find_mergeable_anon_vma() (a
> check inside anon_vma_prepare() should prevent that). So, it should
> fall back to mmap_lock'ing.

This syzkaller report applies to a tree with Willy's in-progress patch
series, where lock_vma_under_rcu() only checks for vma->anon_vma if
vma_is_anonymous() is true - it permits private non-anonymous VMAs
(which require an anon_vma for handling write faults)  even if they
don't have an anon_vma.

The commit bisected by syzkaller
(https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=a52f58b34afe095ebc5823684eb264404dad6f7b)
removes the vma_is_anonymous() check in handle_pte_fault(), so it lets
us reach do_wp_page() with a non-anonymous private VMA without
anon_vma, even though that requires allocation of an anon_vma.

So I think this is pretty clearly an issue with Willy's in-progress
patch series that syzkaller blamed correctly.



> Jann Horn is fixing an issue with this check in [2] which happens
> before we take the vma lock. So, it's possible that this race is
> causing a call to find_mergeable_anon_vma() while holding per-VMA
> lock. Another possibility is that the recent addition of vma_is_tcp()
> is messing things up here... Either way, find_mergeable_anon_vma()
> should never be called under per-VMA locks because it relies on
> neighboring VMAs to be stable and we do not lock those.
>
> [1] https://elixir.bootlin.com/linux/v6.5-rc3/source/mm/memory.c#L5396
> [2] https://lore.kernel.org/all/20230726214103.3261108-3-jannh@google.com/
>
>
> >
> > I'm wondering if we need find_mergeable_anon_vma() to take a read lock
> > on the VMA which contains the anon_vma to ensure it doesn't go away?
> > Maybe a find_and_lock_mergeable_anon_vma() and return a locked anon_vma?
> > Basically lock_vma_under_rcu(), anon_vma_lock_write(), vma_end_read().
> >
> > Thoughts?
> >
> > Thanks,
> > Liam
> >