* 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. 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
