Re: [syzbot] [mm?] kernel BUG in mfill_atomic_copy

[Axel Rasmussen]

On Fri, Jul 14, 2023 at 8:53 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 13 Jul 2023 20:34:45 -0700 syzbot <syzbot+42309678e0bc7b32f8e9@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    fe57d0d86f03 Add linux-next specific files for 20230710
> > git tree:       linux-next
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=14d46ed8a80000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=eaa6217eed71d333
> > dashboard link: https://syzkaller.appspot.com/bug?extid=42309678e0bc7b32f8e9
> > 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=14b63e18a80000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13784d64a80000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/9e7627fb1623/disk-fe57d0d8.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/848a690045db/vmlinux-fe57d0d8.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/36e5c3ae635e/bzImage-fe57d0d8.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+42309678e0bc7b32f8e9@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > ------------[ cut here ]------------
> > kernel BUG at mm/userfaultfd.c:573!
>
> Thanks.  cc's added.

Yeah, the problem is my commit "mm: userfaultfd: check for start + len
overflow in validate_range"

userfaultfd_copy *only* calls validate_range() on the dst range, not
the src. So an invalid src range (e.g. a zero sized one) isn't noticed
until further down in the call stack where we BUG.

I think the correct fix is just to call validate_range() on both
ranges. I'll send a commit to that effect.

>
> > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > CPU: 1 PID: 5031 Comm: syz-executor410 Not tainted 6.5.0-rc1-next-20230710-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/03/2023
> > RIP: 0010:mfill_atomic mm/userfaultfd.c:573 [inline]
> > RIP: 0010:mfill_atomic_copy+0x9ed/0x17e0 mm/userfaultfd.c:725
> > Code: 8d 8c 24 a8 00 00 00 4c 89 e7 e8 be 54 d9 ff 4c 63 f0 e9 71 fc ff ff e8 51 eb a1 ff 0f 0b e8 4a eb a1 ff 0f 0b e8 43 eb a1 ff <0f> 0b e8 3c eb a1 ff 0f 0b 49 c7 c6 ef ff ff ff e9 54 fd ff ff e8
> > RSP: 0018:ffffc9000395fb48 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: dffffc0000000000 RCX: 0000000000000000
> > RDX: ffff88801f67bb80 RSI: ffffffff81e32bad RDI: 0000000000000006
> > RBP: 0000000000000000 R08: 0000000000000006 R09: ffffffffffffffff
> > R10: 0000000005ffffff R11: 0000000000000001 R12: 0000200000000000
> > R13: 00005ffffffff001 R14: ffffffffffffffff R15: 0000000005ffffff
> > FS:  00005555571e03c0(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000000020000060 CR3: 000000002ba7a000 CR4: 00000000003506e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> >  <TASK>
> >  userfaultfd_copy fs/userfaultfd.c:1766 [inline]
> >  userfaultfd_ioctl+0xe43/0x4c40 fs/userfaultfd.c:2098
> >  vfs_ioctl fs/ioctl.c:51 [inline]
> >  __do_sys_ioctl fs/ioctl.c:870 [inline]
> >  __se_sys_ioctl fs/ioctl.c:856 [inline]
> >  __x64_sys_ioctl+0x19d/0x210 fs/ioctl.c:856
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > RIP: 0033:0x7f2e41c4b4b9
> > 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:00007fff13a95188 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> > RAX: ffffffffffffffda RBX: 00007fff13a95198 RCX: 00007f2e41c4b4b9
> > RDX: 0000000020000040 RSI: 00000000c028aa03 RDI: 0000000000000003
> > RBP: 00007fff13a95190 R08: 00007fff13a95190 R09: 00007f2e41c0e380
> > R10: 00007fff13a95190 R11: 0000000000000246 R12: 0000000000000000
> > R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> >  </TASK>
> > Modules linked in:
> > ---[ end trace 0000000000000000 ]---
> > RIP: 0010:mfill_atomic mm/userfaultfd.c:573 [inline]
> > RIP: 0010:mfill_atomic_copy+0x9ed/0x17e0 mm/userfaultfd.c:725
> > Code: 8d 8c 24 a8 00 00 00 4c 89 e7 e8 be 54 d9 ff 4c 63 f0 e9 71 fc ff ff e8 51 eb a1 ff 0f 0b e8 4a eb a1 ff 0f 0b e8 43 eb a1 ff <0f> 0b e8 3c eb a1 ff 0f 0b 49 c7 c6 ef ff ff ff e9 54 fd ff ff e8
> > RSP: 0018:ffffc9000395fb48 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: dffffc0000000000 RCX: 0000000000000000
> > RDX: ffff88801f67bb80 RSI: ffffffff81e32bad RDI: 0000000000000006
> > RBP: 0000000000000000 R08: 0000000000000006 R09: ffffffffffffffff
> > R10: 0000000005ffffff R11: 0000000000000001 R12: 0000200000000000
> > R13: 00005ffffffff001 R14: ffffffffffffffff R15: 0000000005ffffff
> > FS:  00005555571e03c0(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000000020000060 CR3: 000000002ba7a000 CR4: 00000000003506e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >
> >
> > ---
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> >
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> >
> > If the bug is already fixed, let syzbot know by replying with:
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> >
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> >
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> >
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> >
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