On Tue, Mar 30, 2021 at 07:04:30PM +0200, Paolo Bonzini wrote:

On 30/03/21 17:26, syzbot wrote:

quoted

Hello,

syzbot found the following issue on:

HEAD commit:    93129492 Add linux-next specific files for 20210326
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=169ab21ad00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=6f2f73285ea94c45
dashboard link: https://syzkaller.appspot.com/bug?extid=015dd7cdbbbc2c180c65
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=119b8d06d00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=112e978ad00000

The issue was bisected to:

commit d40b9fdee6dc819d8fc35f70c345cbe0394cde4c
Author: Daniel Vetter [off-list ref]
Date:   Tue Mar 16 15:33:01 2021 +0000

     mm: Add unsafe_follow_pfn

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=122d2016d00000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=112d2016d00000
console output: https://syzkaller.appspot.com/x/log.txt?x=162d2016d00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+015dd7cdbbbc2c180c65@syzkaller.appspotmail.com
Fixes: d40b9fdee6dc ("mm: Add unsafe_follow_pfn")

This is basically intentional because get_vaddr_frames is broken, isn't it?
I think it needs to be ignored in syzkaller.

What?

The bisect is wrong (because it's blaming the commit which added the
warning instead of the commit which added the buggy caller) but the
warning is correct.

Plus users are going to be seeing this as well.  According to the commit
message for 69bacee7f9ad ("mm: Add unsafe_follow_pfn") "Unfortunately
there's some users where this is not fixable (like v4l userptr of iomem
mappings)".  It sort of seems crazy to dump this giant splat and then
tell users to ignore it forever because it can't be fixed...  0_0

regards,
dan carpenter

Paolo

quoted

------------[ cut here ]------------
unsafe follow_pfn usage
WARNING: CPU: 1 PID: 8426 at mm/memory.c:4807 unsafe_follow_pfn+0x20f/0x260 mm/memory.c:4807
Modules linked in:
CPU: 0 PID: 8426 Comm: syz-executor677 Not tainted 5.12.0-rc4-next-20210326-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:unsafe_follow_pfn+0x20f/0x260 mm/memory.c:4807
Code: 8b 7c 24 20 49 89 6d 00 e8 6e 84 64 07 e9 30 ff ff ff e8 f4 19 cb ff 48 c7 c7 40 1f 76 89 c6 05 56 eb 09 0c 01 e8 34 1a 21 07 <0f> 0b e9 71 fe ff ff 41 bc ea ff ff ff e9 06 ff ff ff e8 1a 65 0f
RSP: 0018:ffffc9000161f660 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 1ffff920002c3ecc RCX: 0000000000000000
RDX: ffff88801954d580 RSI: ffffffff815c3fd5 RDI: fffff520002c3ebe
RBP: ffff888023d56948 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffff815bd77e R11: 0000000000000000 R12: 0000000021000000
R13: ffff8880143a4010 R14: 0000000000000000 R15: 0000000000000110
FS:  00000000005d1300(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f172c4cd6c0 CR3: 0000000011f70000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  get_vaddr_frames+0x337/0x600 drivers/media/common/videobuf2/frame_vector.c:72
  vb2_create_framevec+0x55/0xc0 drivers/media/common/videobuf2/videobuf2-memops.c:50
  vb2_vmalloc_get_userptr+0xce/0x4c0 drivers/media/common/videobuf2/videobuf2-vmalloc.c:90
  __prepare_userptr+0x342/0x15f0 drivers/media/common/videobuf2/videobuf2-core.c:1128
  __buf_prepare+0x635/0x7d0 drivers/media/common/videobuf2/videobuf2-core.c:1367
  vb2_core_qbuf+0xa9d/0x11c0 drivers/media/common/videobuf2/videobuf2-core.c:1658
  vb2_qbuf+0x135/0x1a0 drivers/media/common/videobuf2/videobuf2-v4l2.c:820
  vb2_ioctl_qbuf+0xfb/0x140 drivers/media/common/videobuf2/videobuf2-v4l2.c:1050
  v4l_qbuf drivers/media/v4l2-core/v4l2-ioctl.c:2027 [inline]
  v4l_qbuf+0x92/0xc0 drivers/media/v4l2-core/v4l2-ioctl.c:2021
  __video_do_ioctl+0xb94/0xe20 drivers/media/v4l2-core/v4l2-ioctl.c:2951
  video_usercopy+0x253/0x1300 drivers/media/v4l2-core/v4l2-ioctl.c:3297
  v4l2_ioctl+0x1b3/0x250 drivers/media/v4l2-core/v4l2-dev.c:366
  vfs_ioctl fs/ioctl.c:48 [inline]
  __do_sys_ioctl fs/ioctl.c:753 [inline]
  __se_sys_ioctl fs/ioctl.c:739 [inline]
  __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:739
  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
  entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x443639
Code: 28 c3 e8 2a 14 00 00 66 2e 0f 1f 84 00 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 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffee3065668 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000443639
RDX: 0000000020000140 RSI: 00000000c058560f RDI: 0000000000000004
RBP: 00000000004031e0 R08: 00000000004004a0 R09: 00000000004004a0
R10: 00236962762f7665 R11: 0000000000000246 R12: 0000000000403270
R13: 0000000000000000 R14: 00000000004b1018 R15: 00000000004004a0


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
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For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this issue, for details see:
https://goo.gl/tpsmEJ#testing-patches

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