Re: [syzbot] [mm?] kernel BUG in vma_replace_policy

[Suren Baghdasaryan]

On Tue, Sep 12, 2023 at 7:55 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Tue, Sep 12, 2023 at 06:30:46AM +0100, Matthew Wilcox wrote:
> > On Tue, Sep 05, 2023 at 06:03:49PM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    a47fc304d2b6 Add linux-next specific files for 20230831
> > > git tree:       linux-next
> > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=16502ddba80000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=6ecd2a74f20953b9
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=b591856e0f0139f83023
> > > compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=120e7d70680000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1523f9c0680000
> > >
> > > Downloadable assets:
> > > disk image: https://storage.googleapis.com/syzbot-assets/b2e8f4217527/disk-a47fc304.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/ed6cdcc09339/vmlinux-a47fc304.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/bd9b2475bf5a/bzImage-a47fc304.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+b591856e0f0139f83023@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > #syz test
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 42b5567e3773..90ad5fe60824 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -1342,6 +1342,7 @@ static long do_mbind(unsigned long start, unsigned long len,
> >       vma_iter_init(&vmi, mm, start);
> >       prev = vma_prev(&vmi);
> >       for_each_vma_range(vmi, vma, end) {
> > +             vma_start_write(vma);
> >               err = mbind_range(&vmi, vma, &prev, start, end, new);
> >               if (err)
> >                       break;
>
> Suren, can you take a look at this?  The VMA should be locked by the
> call to queue_pages_range(), but by the time we get to here, the VMA
> isn't locked.  I don't see anywhere that we cycle the mmap_lock (which
> would unlock the VMA), but I could have missed something.  The two
> VMA walks should walk over the same set of VMAs.  Certainly the VMA
> being dumped should have been locked by the pagewalk:

Sure, I'll look into this today. Somehow this report slipped by me
unnoticed. Thanks!

>
>  vma ffff888077381a00 start 0000000020c2a000 end 0000000021000000 mm ffff8880258a8980
>  prot 25 anon_vma 0000000000000000 vm_ops 0000000000000000
>  pgoff 20c2a file 0000000000000000 private_data 0000000000000000
>  flags: 0x8100077(read|write|exec|mayread|maywrite|mayexec|account|softdirty)
>
>   syscall(__NR_mbind, /*addr=*/0x20400000ul, /*len=*/0xc00000ul, /*mode=*/4ul,
>           /*nodemask=*/0ul, /*maxnode=*/0ul, /*flags=*/3ul);
>
> 20400000 + c00000 should overlap 20c2a000-21000000