[syzbot] [mm?] kernel BUG in move_pages

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e2425464bc87 Add linux-next specific files for 20240105
git tree: linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=14941cdee80000
kernel config: https://syzkaller.appspot.com/x/.config?x=4056b9349f3da8c9
dashboard link: https://syzkaller.appspot.com/bug?extid=705209281e36404998f6
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=125d0a09e80000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15bc7331e80000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/2f738185e2cf/disk-e2425464.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b248fcf4ea46/vmlinux-e2425464.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a9945c8223f4/bzImage-e2425464.xz

The issue was bisected to:

commit adef440691bab824e39c1b17382322d195e1fab0
Author: Andrea Arcangeli <aarc...@redhat.com>
Date: Wed Dec 6 10:36:56 2023 +0000

userfaultfd: UFFDIO_MOVE uABI

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=11cb6ea9e80000
final oops: https://syzkaller.appspot.com/x/report.txt?x=13cb6ea9e80000
console output: https://syzkaller.appspot.com/x/log.txt?x=15cb6ea9e80000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+705209...@syzkaller.appspotmail.com
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")

do_one_initcall+0x128/0x680 init/main.c:1237
do_initcall_level init/main.c:1299 [inline]
do_initcalls init/main.c:1315 [inline]
do_basic_setup init/main.c:1334 [inline]
kernel_init_freeable+0x692/0xc30 init/main.c:1552
kernel_init+0x1c/0x2a0 init/main.c:1442
ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:242
------------[ cut here ]------------
kernel BUG at include/linux/page-flags.h:1035!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 5068 Comm: syz-executor191 Not tainted 6.7.0-rc8-next-20240105-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023
RIP: 0010:PageAnonExclusive include/linux/page-flags.h:1035 [inline]
RIP: 0010:move_pages+0x1697/0x3d40 mm/userfaultfd.c:1402
Code: 00 00 48 c1 e8 0c 48 21 d0 48 c1 e0 06 48 01 c3 e9 b6 f7 ff ff e8 79 c6 9c ff 48 c7 c6 e0 7e dc 8a 48 89 df e8 0a 20 dc ff 90 <0f> 0b e8 62 c6 9c ff 48 89 da b8 ff ff 37 00 48 c1 ea 03 48 c1 e0
RSP: 0018:ffffc90003aefa98 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffffea0001e40000 RCX: ffffffff81687599
RDX: ffff88802a155940 RSI: ffffffff81eb5d46 RDI: 0000000000000000
RBP: ffff88802abab810 R08: 0000000000000000 R09: fffffbfff1e75fda
R10: ffffffff8f3afed7 R11: 0000000000000001 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000020518000 R15: 0000000000000000
FS: 00005555562cf380(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000204f8000 CR3: 000000006a725000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
userfaultfd_move fs/userfaultfd.c:2047 [inline]
userfaultfd_ioctl+0x683/0x6420 fs/userfaultfd.c:2169
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl fs/ioctl.c:857 [inline]
__x64_sys_ioctl+0x18f/0x210 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xd0/0x250 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x62/0x6a
RIP: 0033:0x7f4bada9b3e9
Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 00 00 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:00007fff2c1d6998 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fff2c1d6b68 RCX: 00007f4bada9b3e9
RDX: 00000000200000c0 RSI: 00000000c028aa05 RDI: 0000000000000003
RBP: 00007f4badb0e610 R08: 00007fff2c1d6b68 R09: 00007fff2c1d6b68
R10: 00007fff2c1d6b68 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fff2c1d6b58 R14: 0000000000000001 R15: 0000000000000001
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:PageAnonExclusive include/linux/page-flags.h:1035 [inline]
RIP: 0010:move_pages+0x1697/0x3d40 mm/userfaultfd.c:1402
Code: 00 00 48 c1 e8 0c 48 21 d0 48 c1 e0 06 48 01 c3 e9 b6 f7 ff ff e8 79 c6 9c ff 48 c7 c6 e0 7e dc 8a 48 89 df e8 0a 20 dc ff 90 <0f> 0b e8 62 c6 9c ff 48 89 da b8 ff ff 37 00 48 c1 ea 03 48 c1 e0
RSP: 0018:ffffc90003aefa98 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffffea0001e40000 RCX: ffffffff81687599
RDX: ffff88802a155940 RSI: ffffffff81eb5d46 RDI: 0000000000000000
RBP: ffff88802abab810 R08: 0000000000000000 R09: fffffbfff1e75fda
R10: ffffffff8f3afed7 R11: 0000000000000001 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000020518000 R15: 0000000000000000
FS: 00005555562cf380(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000204f8000 CR3: 000000006a725000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


---
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[Suren Baghdasaryan]

From a quick look, I think the new ioctl is being used against a
file-backed page and that's why PageAnonExclusive() throws this error.
I'll confirm if this is indeed the case and will add checks for that
case. Thanks!

[Suren Baghdasaryan]

Hmm. Looking at the reproducer it does not look like a file-backed
memory... Anyways, I'm on it.

[Suren Baghdasaryan]

Looks like the test is trying to move the huge_zero_page. Wonder how
we should handle this. Just fail or do something else? Adding David
and Peter for feedback.

[David Hildenbrand]

You'll need some special-casing to handle that. But it should be fairly
easy.

--
Cheers,

David / dhildenb

[Suren Baghdasaryan]

Ok, so should we treat zeropage the same as PAE and map destination
PTE/PMD to zeropage while clearing source PTE/PMD?

[David Hildenbrand]

Likely yes. So it's transparent for user space what we are moving. (this
sounds like an easy case to not require a prior write access just to
move it)

[Suren Baghdasaryan]

Ok, working on it. split_huge_pmd() already knows how to split
huge_zero_page but I think I'll need special handling in both
move_pages_pte() and move_pages_huge_pmd().

[David Hildenbrand]

A PTE-mapped huge zeropage is just a page table populated with the
ordinary shared zeropage. Are you moving the ordinary shared zeropage as
well? If not, you should do so for consistency (or not do either :) ).

[Suren Baghdasaryan]

Yes, I think I should move ordinary zeropages as well.

[Suren Baghdasaryan]

I have a version that seems to work but I want to test it more and
it's too heavy to be considered a quick fix for linux-next. I'll post
a simple one-line fix which takes care of this crash and keeps the
behavior for zeropages the same (ioctl returns -EBUSY). Later will
post a separate patch to move huge and ordinary zeropages.

[Suren Baghdasaryan]

I posted a quick fix for this issue here:
https://lore.kernel.org/all/20240112013935....@google.com/
It cleanly applies over linux-next, mm-stable and mm-unstable. Andrew,
Stephen, could you please pull the fix into your branches?
Thanks,
Suren.

[Stephen Rothwell]

Hi all,

On Thu, 11 Jan 2024 17:44:57 -0800 Suren Baghdasaryan <sur...@google.com> wrote:
>
> I posted a quick fix for this issue here:
> https://lore.kernel.org/all/20240112013935....@google.com/
> It cleanly applies over linux-next, mm-stable and mm-unstable. Andrew,
> Stephen, could you please pull the fix into your branches?

Since I will be away for a few days, I have applied that to linux-next today.

--
Cheers,
Stephen Rothwell