Re: [PATCH v2] mm: khugepaged: Fix kernel BUG in hpage_collapse_scan_file

[Yang Shi <shy828301@gmail.com>]

On Thu, Mar 30, 2023 at 6:33 PM Zach O'Keefe <zokeefe@google.com> wrote:
>
> On Mar 30 19:53, Ivan Orlov wrote:
> > Syzkaller reported the following issue:
> >
> > kernel BUG at mm/khugepaged.c:1823!
> > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > CPU: 1 PID: 5097 Comm: syz-executor220 Not tainted 6.2.0-syzkaller-13154-g857f1268a591 #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/16/2023
> > RIP: 0010:collapse_file mm/khugepaged.c:1823 [inline]
> > RIP: 0010:hpage_collapse_scan_file+0x67c8/0x7580 mm/khugepaged.c:2233
> > Code: 00 00 89 de e8 c9 66 a3 ff 31 ff 89 de e8 c0 66 a3 ff 45 84 f6 0f 85 28 0d 00 00 e8 22 64 a3 ff e9 dc f7 ff ff e8 18 64 a3 ff <0f> 0b f3 0f 1e fa e8 0d 64 a3 ff e9 93 f6 ff ff f3 0f 1e fa 4c 89
> > RSP: 0018:ffffc90003dff4e0 EFLAGS: 00010093
> > RAX: ffffffff81e95988 RBX: 00000000000001c1 RCX: ffff8880205b3a80
> > RDX: 0000000000000000 RSI: 00000000000001c0 RDI: 00000000000001c1
> > RBP: ffffc90003dff830 R08: ffffffff81e90e67 R09: fffffbfff1a433c3
> > R10: 0000000000000000 R11: dffffc0000000001 R12: 0000000000000000
> > R13: ffffc90003dff6c0 R14: 00000000000001c0 R15: 0000000000000000
> > FS:  00007fdbae5ee700(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007fdbae6901e0 CR3: 000000007b2dd000 CR4: 00000000003506e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> >  <TASK>
> >  madvise_collapse+0x721/0xf50 mm/khugepaged.c:2693
> >  madvise_vma_behavior mm/madvise.c:1086 [inline]
> >  madvise_walk_vmas mm/madvise.c:1260 [inline]
> >  do_madvise+0x9e5/0x4680 mm/madvise.c:1439
> >  __do_sys_madvise mm/madvise.c:1452 [inline]
> >  __se_sys_madvise mm/madvise.c:1450 [inline]
> >  __x64_sys_madvise+0xa5/0xb0 mm/madvise.c:1450
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> >
>
> Thanks, Ivan.
>
> In the process of reviewing this, I starting thinking if the !shmem case was
> also susceptible to a similar race, and I *think* it might be. Unfortunately, my
> time has ran out, and I haven't been able to validate ; I'm less familiar with
> the file-side of things.
>
> The underlying problem is race with truncation/hole-punch  under OOM condition.
> The nice do-while loop near the top of collapse_file() attempts to avoid this
> scenario by making sure enough slots are available. However, when we drop xarray
> lock, we open ourselves up to concurrent removal + slot deletion. Those slots
> then need to be allocated again -- which under OOM condition is failable.
>
> The syzbot reproducer picks on shmem, but I think this can occur for file as
> well. If we find a hole, we unlock the xarray and call
> page_cache_sync_readahead(), which if it succeeds, IIUC, will have allocated a
> new slot in our mapping pointing to the new page. We *then* locks the page. Only
> after the page is locked are we protected from concurrent removal (Note: this is
> what provides us protection in many of the xas_store() cases ; we've held the
> slot's contained page-lock since verifying the slot exists, protecting us from
> removal / reallocation races).

IIUC, the find_lock_page() should be able to handle the race. It does
check whether the page is truncated or not after locking the page.

>
> Maybe I'm just low on caffeine at the end of the day, and am missing something,
> but if I had more time, I'd be looking into the file-side some more to verify.
> Apologies that hasn't occurred to me until now ; I was looking at one of your
> comments and double-checked why I *thought* we were safe.
>
> Anyways, irrespective of that looming issues, some more notes to follow:
>
> > The 'xas_store' call during page cache scanning can potentially
> > translate 'xas' into the error state (with the reproducer provided
> > by the syzkaller the error code is -ENOMEM). However, there are no
> > further checks after the 'xas_store', and the next call of 'xas_next'
> > at the start of the scanning cycle doesn't increase the xa_index,
> > and the issue occurs.
> >
> > This patch will add the xarray state error checking after the
> > 'xas_store' and the corresponding result error code. It will
> > also add xarray state error checking via WARN_ON_ONCE macros,
> > to be sure that ENOMEM or other possible errors don't occur
> > at the places they shouldn't.
>
> Thanks for the additions here. I think it's worthwhile providing even more
> details about the specifics of the race we are fixing and/or guarding against to
> help ppl understand how that -ENOMEM comes about if the do-while loop has
> "Ensured" we have slots available (additionally, I think that comment can be
> augmented).
>
> > Tested via syzbot.
> >
> > Reported-by: syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
> > Link: https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc
> > Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
> > ---
> > V1 -> V2: Add WARN_ON_ONCE error checking and comments
> >
> >  mm/khugepaged.c | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 92e6f56a932d..8b6580b13339 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -55,6 +55,7 @@ enum scan_result {
> >       SCAN_CGROUP_CHARGE_FAIL,
> >       SCAN_TRUNCATED,
> >       SCAN_PAGE_HAS_PRIVATE,
> > +     SCAN_STORE_FAILED,
> >  };
>
> I'm still reluctant to add a new error code for this as this seems like quite a
> rare race that requires OOM to trigger. I'd be happier just reusing SCAN_FAIL,
> or, something we might get some millage out of later: SCAN_OOM.
>
> Also, a reminder to update include/trace/events/huge_memory.h, if you go that
> route.
>
> >
> >  #define CREATE_TRACE_POINTS
> > @@ -1840,6 +1841,15 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >                                       goto xa_locked;
> >                               }
> >                               xas_store(&xas, hpage);
> > +                             if (xas_error(&xas)) {
> > +                                     /* revert shmem_charge performed
> > +                                      * in the previous condition
> > +                                      */
>
> Nit: Here, and following, I think standard convention for multiline comment is
> to have an empty first and last line, eg:
>
>  +                                      /*
>  +                                       * revert shmem_charge performed
>  +                                       * in the previous condition
>  +                                       */
>
> Though, checkpatch.pl --strict didn't seem to care.
>
> > +                                     mapping->nrpages--;
> > +                                     shmem_uncharge(mapping->host, 1);
> > +                                     result = SCAN_STORE_FAILED;
> > +                                     goto xa_locked;
> > +                             }
> >                               nr_none++;
> >                               continue;
> >                       }
> > @@ -1992,6 +2002,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >
> >               /* Finally, replace with the new page. */
> >               xas_store(&xas, hpage);
> > +             /* We can't get an ENOMEM here (because the allocation happened before)
> > +              * but let's check for errors (XArray implementation can be
> > +              * changed in the future)
> > +              */
> > +             WARN_ON_ONCE(xas_error(&xas));
>
> Nit: it's not just that allocation happened before -- need some guarantee we've
> been protected from concurrent removal. This is what made me look at the file
> side.
>
> >               continue;
> >  out_unlock:
> >               unlock_page(page);
> > @@ -2029,6 +2044,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >       /* Join all the small entries into a single multi-index entry */
> >       xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> >       xas_store(&xas, hpage);
> > +     /* Here we can't get an ENOMEM (because entries were
> > +      * previously allocated) But let's check for errors
> > +      * (XArray implementation can be changed in the future)
> > +      */
> > +     WARN_ON_ONCE(xas_error(&xas));
>
> Ditto.
>
> Apologies I won't be around to see this change through -- I'm just out of time,
> and will be shutting my computer down tomorrow for 3 months.  Sorry for the poor
> timing, for raising issues, then disappearing. Hopefully I'm wrong and the
> file-side isn't a concern.
>
> Best,
> Zach
>
> >  xa_locked:
> >       xas_unlock_irq(&xas);
> >  xa_unlocked:
> > --
> > 2.34.1
> >