Date:   Mon, 13 Mar 2023 14:11:42 -0700
From:   Zach O'Keefe <zokeefe@google.com>
To:     linux-mm@kvack.org
Cc:     akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com,
        Matthew Wilcox <willy@infradead.org>, stevensd@chromium.org
Subject: Re: [syzbot] [mm?] kernel BUG in hpage_collapse_scan_file
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On Mar 13 12:38, Zach O'Keefe wrote:
> On Mon, Mar 13, 2023 at 12:16 PM Zach O'Keefe <zokeefe@google.com> wrote:
> >
> > On Mar 10 17:02, Zach O'Keefe wrote:
> > > On Fri, Mar 10, 2023 at 4:52 PM syzbot
> > > <syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com> wrote:
> > > >
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit:    857f1268a591 Merge tag 'objtool-core-2023-03-02' of git://..
> > > > git tree:       upstream
> > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=168e1032c80000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=f763d89e26d3d4c4
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=9578faa5475acb35fa50
> > > > compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=179e4e12c80000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=119cce98c80000
> > > >
> > > > Downloadable assets:
> > > > disk image: https://storage.googleapis.com/syzbot-assets/b3b7a7e333f1/disk-857f1268.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/5940be1cf171/vmlinux-857f1268.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/986015398e4a/bzImage-857f1268.xz
> > > >
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
> > > >
> > > > ------------[ cut here ]------------
> > > > 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
> > > > RIP: 0033:0x7fdbae65dc39
> > > > 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:00007fdbae5ee2f8 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
> > > > RAX: ffffffffffffffda RBX: 00007fdbae6e64b8 RCX: 00007fdbae65dc39
> > > > RDX: 0000000000000019 RSI: 000000000060005f RDI: 0000000020000000
> > > > RBP: 00007fdbae6e64b0 R08: 0000000000000001 R09: 0000000000000033
> > > > R10: 0000000000000000 R11: 0000000000000246 R12: 00007fdbae5ee300
> > > > R13: 0000000000000001 R14: 00007fdbae5ee400 R15: 0000000000022000
> > > >  </TASK>
> > > > Modules linked in:
> > > > ---[ end trace 0000000000000000 ]---
> > > > 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
> > > >
> > > >
> > > > ---
> > > > This report is generated by a bot. It may contain errors.
> > > > See https://goo.gl/tpsmEJ for more information about syzbot.
> > > > syzbot engineers can be reached at syzkaller@googlegroups.com.
> > > >
> > > > syzbot will keep track of this issue. See:
> > > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > > > syzbot can test patches for this issue, for details see:
> > > > https://goo.gl/tpsmEJ#testing-patches
> > >
> > > I had a look at this, and the issue is stemming from failed (due to
> > > error injection here) xas_store() in collapse_file() (in this report,
> > > specifically was picking on shmem after MADV_REMOVE punch). This puts
> > > the xa_state into an error state (-ENOMEM) and the subsequent
> > > xas_next() will (a) not increment xas->xa_index (which trips the
> > > VM_BUG_ON), and (b) returns NULL (which is confusing, since AFAIU,
> > > that's a "valid" entry for a truncated page cache entry, but also
> > > being used to indicate error).
> > >
> > > I think the right thing to do is to check xas_invalid() at the top of
> > > the loop, or checking return value of all those xas_store()'s and
> > > taking appropriate action. There is also the possibility this never
> > > occurs in practice due to the "Ensure we have slots for all the pages
> > > in the range" check at the top of the function, and that we are only
> > > able to trip this from error injection.
> >
> > Right, so looking a bit more into this this morning, my last question about
> > whether the xas_create_range() check at the top of collapse_file() guaranteeing
> > us the needed slots (and that syzbot was only able to trip this due to error
> > injection) is plainly false: we are actually attempting to allocate memory here,
> > so clearly the slots weren't already available - duh.
> >
> > Now, why isn't that well-intending pre-reservation enough? Well, we are dropping
> > the xarray lock ~ every iteration of the for-loop, then relocking it to store
> > the hugpage at the current index. While the lock is dropped, there isn't
> > anything protecting us from racing with page_cache_delete() -- here, from
> >
> >         __filemap_remove_folio()
> >         truncate_inode_folio()
> >         shmem_undo_range()
> >         shmem_truncate_range()
> >         vfs_fallocate()
> >         madvise_remove()
> >
> > which can then remove slots out from under us:
> >
> >         xas_delete_node()
> >         update_node()
> >         xas_store()
> >         page_cache_delete()
> >
> > So, I think this code needs to be guarded against concurrent slot removal.
> >
> > I think just giving up is the best (i.e. simplest) route (vs taking some
> > additional measures to serialize vs concurrent removal). One concern is that if
> > we've encountered ENOMEM situation where xas_store() is failing, then the
> > rollback code also won't work correctly. However, rollback xas_store() will
> > either replace the current hpage entry with the previous entry, or replace it
> > will a NULL entry (had it been a hole previously) -- neither of which will
> > involve any additional allocations -- so we're safe.
> >
> > Patch to fix this should be following in the next day or so.
> >
> >
> 
> Also, to be clear, the concurrent removal isn't actually a problem on
> its own, it's only concurrent removal + subsequent inability to
> allocate missing xarray slot that is the issue.

Expectedly, this area clashes significantly with David Stevens' "mm/khugepaged:
refactor collapse_file control flow" on mm-unstable.

Looking at the updated code now, while we do serialize on immap_lock at some
points in the new flow, there are still fallible xas_store() callsites, and
xas_next() calls that assume !xas_next() implies NULL-page entry (as opposed to
error).

IIUC, we are still waiting on a v5 of that series? If so, it seems like this fix
would be a great inclusion there. Reviewing this series is part of my pending
TODO work (apologies I haven't gotten to it sooner). As such, I'll hold off on
sending out a fix to this issue.

Best,
Zach