On Thu, Jul 27, 2023 at 7:22 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > On Thu, Jul 27, 2023 at 9:48 AM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > > > * syzbot <syzbot+8645fe63c4d22c8d27b8@xxxxxxxxxxxxxxxxxxxxxxxxx> [230726 02:57]: > > > syzbot has bisected this issue to: > > > > > > commit a52f58b34afe095ebc5823684eb264404dad6f7b > > > Author: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > > 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@xxxxxxxxxx/ > > > > > > 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 > >
