Re: [syzbot] BUG: Bad page map (5)

[Yang Shi <shy828301@gmail.com>]

On Tue, Sep 13, 2022 at 11:39 AM Zach O'Keefe <zokeefe@google.com> wrote:
>
> On Sep 13 09:14, Yang Shi wrote:
> > On Mon, Sep 12, 2022 at 2:47 PM Yang Shi <shy828301@gmail.com> wrote:
> > >
> > > On Sun, Sep 11, 2022 at 9:27 PM syzbot
> > > <syzbot+915f3e317adb0e85835f@syzkaller.appspotmail.com> wrote:
> > > >
> > > > syzbot has found a reproducer for the following issue on:
> > > >
> > > > HEAD commit:    e47eb90a0a9a Add linux-next specific files for 20220901
> > > > git tree:       linux-next
> > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=17330430880000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=7933882276523081
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=915f3e317adb0e85835f
> > > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13397b77080000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1793564f080000
> > > >
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+915f3e317adb0e85835f@syzkaller.appspotmail.com
> > > >
> > > > BUG: Bad page map in process syz-executor198  pte:8000000071c00227 pmd:74b30067
> > > > addr:0000000020563000 vm_flags:08100077 anon_vma:ffff8880547d2200 mapping:0000000000000000 index:20563
> > > > file:(null) fault:0x0 mmap:0x0 read_folio:0x0
> > > > CPU: 1 PID: 3614 Comm: syz-executor198 Not tainted 6.0.0-rc3-next-20220901-syzkaller #0
> > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2022
> > > > Call Trace:
> > > >  <TASK>
> > > >  __dump_stack lib/dump_stack.c:88 [inline]
> > > >  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
> > > >  print_bad_pte.cold+0x2a7/0x2d0 mm/memory.c:565
> > > >  vm_normal_page+0x10c/0x2a0 mm/memory.c:636
> > > >  hpage_collapse_scan_pmd+0x729/0x1da0 mm/khugepaged.c:1199
> > > >  madvise_collapse+0x481/0x910 mm/khugepaged.c:2433
> > > >  madvise_vma_behavior+0xd0a/0x1cc0 mm/madvise.c:1062
> > > >  madvise_walk_vmas+0x1c7/0x2b0 mm/madvise.c:1236
> > > >  do_madvise.part.0+0x24a/0x340 mm/madvise.c:1415
> > > >  do_madvise mm/madvise.c:1428 [inline]
> > > >  __do_sys_madvise mm/madvise.c:1428 [inline]
> > > >  __se_sys_madvise mm/madvise.c:1426 [inline]
> > > >  __x64_sys_madvise+0x113/0x150 mm/madvise.c:1426
> > > >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > > >  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > > > RIP: 0033:0x7f770ba87929
> > > > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> > > > RSP: 002b:00007f770ba18308 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
> > > > RAX: ffffffffffffffda RBX: 00007f770bb0f3f8 RCX: 00007f770ba87929
> > > > RDX: 0000000000000019 RSI: 0000000000600003 RDI: 0000000020000000
> > > > RBP: 00007f770bb0f3f0 R08: 00007f770ba18700 R09: 0000000000000000
> > > > R10: 00007f770ba18700 R11: 0000000000000246 R12: 00007f770bb0f3fc
> > > > R13: 00007ffc2d8b62ef R14: 00007f770ba18400 R15: 0000000000022000
> > > >  </TASK>
> > >
> > > I think I figured out the problem. The reproducer actually triggered
> > > the below race in madvise_collapse():
> > >
> > >              CPU A
> > >         CPU B
> > > mmap 0x20000000 - 0x21000000 as anon
> > >
> > >        madvise_collapse is called on this area
> > >
> > >            Retrieve start and end address from the vma (NEVER updated
> > > later!)
> > >
> > >            Collapsed the first 2M area and dropped mmap_lock
> > > Acquire mmap_lock
> > > mmap io_uring file at 0x20563000
> > > Release mmap_lock
> > >
> > >             Reacquire mmap_lock
> > >
> > >             revalidate vma pass since 0x20200000 + 0x200000 >
> > > 0x20563000
> > >
> > >             scan the next 2M (0x20200000 - 0x20400000), but due to
> > > whatever reason it didn't release mmap_lock
> > >
> > >             scan the 3rd 2M area (start from 0x20400000)
> > >
> > >               actually scan the new vma created by io_uring since the
> > > end was never updated
> > >
> > > The below patch should be able to fix the problem (untested):
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 5f7c60b8b269..e708c5d62325 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -2441,8 +2441,10 @@ int madvise_collapse(struct vm_area_struct
> > > *vma, struct vm_area_struct **prev,
> > >                 memset(cc->node_load, 0, sizeof(cc->node_load));
> > >                 result = hpage_collapse_scan_pmd(mm, vma, addr, &mmap_locked,
> > >                                                  cc);
> > > -               if (!mmap_locked)
> > > +               if (!mmap_locked) {
> > >                         *prev = NULL;  /* Tell caller we dropped mmap_lock */
> > > +                       hend = vma->end & HPAGE_PMD_MASK;
> > > +               }
> >
> > This is wrong. We should refetch the vma end after
> > hugepage_vma_revalidate() otherwise the vma is still the old one.
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index a3acd3e5e0f3..1860be232a26 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2592,6 +2592,8 @@ int madvise_collapse(struct vm_area_struct *vma,
> > struct vm_area_struct **prev,
> >                                 last_fail = result;
> >                                 goto out_nolock;
> >                         }
> > +
> > +                       hend = vma->vm_end & HPAGE_PMD_MASK;
> >                 }
> >                 mmap_assert_locked(mm);
> >                 memset(cc->node_load, 0, sizeof(cc->node_load));
> >
> >
> > >
> > >                 switch (result) {
> > >                 case SCAN_SUCCEED:
> > >
> > >
>
> Hey Yang,
>
> Thanks for triaging this, and apologies for intro'ing this bug.
>
> Also thank you for the repro explanation - I believe you are correct here.
>
> Generalizing the issue of:
>
>         1) hugepage_vma_revalidate() pmd X
>         2) collapse of pmd X doesn't drop mmap_lock
>         3) don't revalidate pmd X+1
>         4) attempt collapse of pmd X+1
>
> I think the only problem is that
>
>         hugepage_vma_revalidate()
>                 transhuge_vma_suitable()
>
> only checks if a single hugepage-sized/aligned region properly fits / is aligned

I think it is what transhuge_vma_suitable() is designed for. As long
as one hugepage fits, it is suitable.

> in the VMA (i.e. the issue you found here).  All other checks should be
> intrinsic to the VMA itself and should be safe to skip if mmap_lock isn't
> dropped since last hugepage_vma_revalidate().
>
> As for the fix, I think your fix will work.  If a VMA's size changes inside the
> main for-loop of madvise_collapse, then at some point we will lock mmap_lock and
> call hugepage_vma_revalidate(), which might fail itself if the next
> hugepage-aligned/sized region is now not contained in the VMA.  By updating
> "hend" as you propose (i.e. using vma->m_end of the just-found VMA), we also
> ensure that for "addr" < "hend", the hugepage-aligned/sized region at "addr"
> will fit into the VMA.  Note that we don't need to worry about the VMA being
> shrank from the other direction, so updating "hend" should be enough.

Yeah, we don't have to worry about the other direction. The
hugepage_vma_revalidate() could handle it correctly. Either no valid
vma is found or the vma doesn't fit anymore.

>
> I think the fix is fine as-is.  I briefly thought a comment would be nice, but I
> think the code is self evident.  The alternative is introing another
> transhuge_vma_suitable() call in the "if (!mmap_locked) { .. } else { .. }"
> failure path, but I think your approach is easier to read.
>
> Thanks again for taking the time to debug this, and hopefully I can be more
> careful in the future.

It is fine.

>
> Best,
> Zach
>
> Reviewed-by: Zach O'Keefe <zokeefe@google.com>

Thanks.

>