[syzbot] KFENCE: use-after-free in kvm_fastop_exception

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: b9011c7e671d Add linux-next specific files for 20210816
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=15221f4e300000
kernel config: https://syzkaller.appspot.com/x/.config?x=a245d1aa4f055cc1
dashboard link: https://syzkaller.appspot.com/bug?extid=7b938780d5deeaaf938f
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7b9387...@syzkaller.appspotmail.com

==================================================================
BUG: KFENCE: use-after-free read in kvm_fastop_exception+0xf6a/0x1058

Use-after-free read at 0xffff88823bd2c020 (in kfence-#149):
kvm_fastop_exception+0xf6a/0x1058
d_lookup+0xd8/0x170 fs/dcache.c:2370
lookup_dcache+0x1e/0x130 fs/namei.c:1520
__lookup_hash+0x29/0x180 fs/namei.c:1543
kern_path_locked+0x17e/0x320 fs/namei.c:2567
handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
handle drivers/base/devtmpfs.c:382 [inline]
devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
kthread+0x3e5/0x4d0 kernel/kthread.c:319
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

kfence-#149: 0xffff88823bd2c000-0xffff88823bd2cfff, size=4096, cache=names_cache

allocated by task 22 on cpu 1 at 161.928971s:
getname_kernel+0x4e/0x370 fs/namei.c:226
kern_path_locked+0x71/0x320 fs/namei.c:2558
handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
handle drivers/base/devtmpfs.c:382 [inline]
devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
kthread+0x3e5/0x4d0 kernel/kthread.c:319
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

freed by task 22 on cpu 1 at 161.929182s:
putname.part.0+0xe1/0x120 fs/namei.c:270
putname include/linux/err.h:41 [inline]
filename_parentat fs/namei.c:2547 [inline]
kern_path_locked+0xc2/0x320 fs/namei.c:2558
handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
handle drivers/base/devtmpfs.c:382 [inline]
devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
kthread+0x3e5/0x4d0 kernel/kthread.c:319
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

CPU: 1 PID: 22 Comm: kdevtmpfs Not tainted 5.14.0-rc5-next-20210816-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:kvm_fastop_exception+0xf6a/0x1058
Code: d3 ed e9 f7 ae 6c f8 49 8d 0e 48 83 e1 f8 4c 8b 21 41 8d 0e 83 e1 07 c1 e1 03 49 d3 ec e9 4d bc 6c f8 49 8d 4d 00 48 83 e1 f8 <4c> 8b 21 41 8d 4d 00 83 e1 07 c1 e1 03 49 d3 ec e9 3d c6 6c f8 bd
RSP: 0018:ffffc90000dcfae8 EFLAGS: 00010282
RAX: 34317974746d7367 RBX: ffff88806efb4330 RCX: ffff88823bd2c020
RDX: ffffed100ddf686d RSI: 0000000000000008 RDI: 0000000000000007
RBP: 0000000000000008 R08: 0000000000000000 R09: ffff88806efb4360
R10: ffffed100ddf686c R11: 0000000000000000 R12: ffff88823bd2c020
R13: ffff88823bd2c020 R14: ffff88806efb4360 R15: dffffc0000000000
FS: 0000000000000000(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff88823bd2c020 CR3: 000000004683b000 CR4: 00000000001506e0
Call Trace:
d_lookup+0xd8/0x170 fs/dcache.c:2370
lookup_dcache+0x1e/0x130 fs/namei.c:1520
__lookup_hash+0x29/0x180 fs/namei.c:1543
kern_path_locked+0x17e/0x320 fs/namei.c:2567
handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
handle drivers/base/devtmpfs.c:382 [inline]
devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
kthread+0x3e5/0x4d0 kernel/kthread.c:319
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
==================================================================
----------------
Code disassembly (best guess):
0: d3 ed shr %cl,%ebp
2: e9 f7 ae 6c f8 jmpq 0xf86caefe
7: 49 8d 0e lea (%r14),%rcx
a: 48 83 e1 f8 and $0xfffffffffffffff8,%rcx
e: 4c 8b 21 mov (%rcx),%r12
11: 41 8d 0e lea (%r14),%ecx
14: 83 e1 07 and $0x7,%ecx
17: c1 e1 03 shl $0x3,%ecx
1a: 49 d3 ec shr %cl,%r12
1d: e9 4d bc 6c f8 jmpq 0xf86cbc6f
22: 49 8d 4d 00 lea 0x0(%r13),%rcx
26: 48 83 e1 f8 and $0xfffffffffffffff8,%rcx
2a: 4c 8b 21 mov (%rcx),%r12 <-- trapping instruction
2d: 41 8d 4d 00 lea 0x0(%r13),%ecx
31: 83 e1 07 and $0x7,%ecx
34: c1 e1 03 shl $0x3,%ecx
37: 49 d3 ec shr %cl,%r12
3a: e9 3d c6 6c f8 jmpq 0xf86cc67c
3f: bd .byte 0xbd


---
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 syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
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[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit: b9011c7e671d Add linux-next specific files for 20210816
git tree: linux-next

console output: https://syzkaller.appspot.com/x/log.txt?x=177d4006300000

kernel config: https://syzkaller.appspot.com/x/.config?x=a245d1aa4f055cc1
dashboard link: https://syzkaller.appspot.com/bug?extid=7b938780d5deeaaf938f
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=157a41ee300000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14f78ff9300000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7b9387...@syzkaller.appspotmail.com

==================================================================
BUG: KFENCE: use-after-free read in kvm_fastop_exception+0xf6a/0x1058

Use-after-free read at 0xffff88823bd7e020 (in kfence-#190):

kvm_fastop_exception+0xf6a/0x1058
d_lookup+0xd8/0x170 fs/dcache.c:2370
lookup_dcache+0x1e/0x130 fs/namei.c:1520
__lookup_hash+0x29/0x180 fs/namei.c:1543
kern_path_locked+0x17e/0x320 fs/namei.c:2567
handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
handle drivers/base/devtmpfs.c:382 [inline]
devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
kthread+0x3e5/0x4d0 kernel/kthread.c:319
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

kfence-#190: 0xffff88823bd7e000-0xffff88823bd7efff, size=4096, cache=names_cache

allocated by task 22 on cpu 0 at 75.061659s:

getname_kernel+0x4e/0x370 fs/namei.c:226
kern_path_locked+0x71/0x320 fs/namei.c:2558
handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
handle drivers/base/devtmpfs.c:382 [inline]
devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
kthread+0x3e5/0x4d0 kernel/kthread.c:319
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

freed by task 22 on cpu 0 at 75.061679s:

putname.part.0+0xe1/0x120 fs/namei.c:270
putname include/linux/err.h:41 [inline]
filename_parentat fs/namei.c:2547 [inline]
kern_path_locked+0xc2/0x320 fs/namei.c:2558
handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
handle drivers/base/devtmpfs.c:382 [inline]
devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
kthread+0x3e5/0x4d0 kernel/kthread.c:319
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

CPU: 0 PID: 22 Comm: kdevtmpfs Not tainted 5.14.0-rc5-next-20210816-syzkaller #0

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:kvm_fastop_exception+0xf6a/0x1058
Code: d3 ed e9 f7 ae 6c f8 49 8d 0e 48 83 e1 f8 4c 8b 21 41 8d 0e 83 e1 07 c1 e1 03 49 d3 ec e9 4d bc 6c f8 49 8d 4d 00 48 83 e1 f8 <4c> 8b 21 41 8d 4d 00 83 e1 07 c1 e1 03 49 d3 ec e9 3d c6 6c f8 bd
RSP: 0018:ffffc90000dcfae8 EFLAGS: 00010282

RAX: 30317974746d7367 RBX: ffff88806c915068 RCX: ffff88823bd7e020
RDX: ffffed100d922a14 RSI: 0000000000000008 RDI: 0000000000000007
RBP: 0000000000000008 R08: 0000000000000000 R09: ffff88806c915098
R10: ffffed100d922a13 R11: 0000000000000000 R12: ffff88823bd7e020
R13: ffff88823bd7e020 R14: ffff88806c915098 R15: dffffc0000000000
FS: 0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: ffff88823bd7e020 CR3: 000000006fc3b000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

[syzbot]

syzbot has bisected this issue to:

commit c4512c63b1193c73b3f09c598a6d0a7f88da1dd8
Author: Matthieu Baerts <matthie...@tessares.net>
Date: Fri Jun 25 21:25:22 2021 +0000

mptcp: fix 'masking a bool' warning

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=122b0655300000
start commit: b9011c7e671d Add linux-next specific files for 20210816
git tree: linux-next
final oops: https://syzkaller.appspot.com/x/report.txt?x=112b0655300000
console output: https://syzkaller.appspot.com/x/log.txt?x=162b0655300000

kernel config: https://syzkaller.appspot.com/x/.config?x=a245d1aa4f055cc1
dashboard link: https://syzkaller.appspot.com/bug?extid=7b938780d5deeaaf938f

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=157a41ee300000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14f78ff9300000

Reported-by: syzbot+7b9387...@syzkaller.appspotmail.com
Fixes: c4512c63b119 ("mptcp: fix 'masking a bool' warning")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

[Matthieu Baerts]

Hello,

On 18/08/2021 00:21, syzbot wrote:
> syzbot has bisected this issue to:
>
> commit c4512c63b1193c73b3f09c598a6d0a7f88da1dd8
> Author: Matthieu Baerts <matthie...@tessares.net>
> Date: Fri Jun 25 21:25:22 2021 +0000
>
> mptcp: fix 'masking a bool' warning
>
> bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=122b0655300000
> start commit: b9011c7e671d Add linux-next specific files for 20210816
> git tree: linux-next
> final oops: https://syzkaller.appspot.com/x/report.txt?x=112b0655300000
> console output: https://syzkaller.appspot.com/x/log.txt?x=162b0655300000
> kernel config: https://syzkaller.appspot.com/x/.config?x=a245d1aa4f055cc1
> dashboard link: https://syzkaller.appspot.com/bug?extid=7b938780d5deeaaf938f
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=157a41ee300000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14f78ff9300000

I'm pretty sure the commit c4512c63b119 ("mptcp: fix 'masking a bool'
warning") doesn't introduce the reported bug. This minor fix is specific
to MPTCP which doesn't seem to be used here.

I'm not sure how I can tell syzbot this is a false positive.

Cheers,
Matt
--
Tessares | Belgium | Hybrid Access Solutions
www.tessares.net

[Pavel Skripkin]

looks like it's fs/namei bug. Similar reports:

https://syzkaller.appspot.com/bug?id=517fa734b92b7db404c409b924cf5c997640e324

https://syzkaller.appspot.com/bug?id=484483daf3652b40dae18531923aa9175d392a4d


It's not false positive. I've suggested the fix here:
https://groups.google.com/g/syzkaller-bugs/c/HE3c2fP5nic/m/1Yk17GBeAwAJ
I am waiting for author comments about the fix :)

But, yes, syzbot bisection is often wrong, so don't rely on it much :)


With regards,
Pavel Skripkin

[Matthieu Baerts]

Hi Pavel,

Thank you for having checked!
Should we mark them as "#syz dup" if you think they have the same root
cause?

> It's not false positive. I've suggested the fix here:
> https://groups.google.com/g/syzkaller-bugs/c/HE3c2fP5nic/m/1Yk17GBeAwAJ
> I am waiting for author comments about the fix :)
>
> But, yes, syzbot bisection is often wrong, so don't rely on it much :)

Yes sorry, I wanted to say the bisection picked a wrong commit :)

All good then if syzbot often blames the wrong modification :)

[Pavel Skripkin]

On 8/18/21 11:21 AM, Matthieu Baerts wrote:
> Hi Pavel,
>
[snip]

>>>
>>> I'm pretty sure the commit c4512c63b119 ("mptcp: fix 'masking a bool'
>>> warning") doesn't introduce the reported bug. This minor fix is specific
>>> to MPTCP which doesn't seem to be used here.
>>>
>>> I'm not sure how I can tell syzbot this is a false positive.
>>>
>>
>>
>> looks like it's fs/namei bug. Similar reports:
>>
>> https://syzkaller.appspot.com/bug?id=517fa734b92b7db404c409b924cf5c997640e324
>>
>>
>> https://syzkaller.appspot.com/bug?id=484483daf3652b40dae18531923aa9175d392a4d
>
> Thank you for having checked!
> Should we mark them as "#syz dup" if you think they have the same root
> cause?
>

I think, yes, but I want to receive feedback from fs people about this
bug. There were huge updates last month, and, maybe, I am missing some
details. Alloc/free calltrace is the same, but anyway, I want some
confirmation to not close different bugs by mistake :)

If these bugs really have same root case I will close them manually
after fix posted.

>> It's not false positive. I've suggested the fix here:
>> https://groups.google.com/g/syzkaller-bugs/c/HE3c2fP5nic/m/1Yk17GBeAwAJ
>> I am waiting for author comments about the fix :)
>>
>> But, yes, syzbot bisection is often wrong, so don't rely on it much :)
>
> Yes sorry, I wanted to say the bisection picked a wrong commit :)
>
> All good then if syzbot often blames the wrong modification :)
>

With regards,
Pavel Skripkin

[Matthieu Baerts]

On 18/08/2021 10:55, Pavel Skripkin wrote:
> On 8/18/21 11:21 AM, Matthieu Baerts wrote:
>> Hi Pavel,
>>
> [snip]
>>>>
>>>> I'm pretty sure the commit c4512c63b119 ("mptcp: fix 'masking a bool'
>>>> warning") doesn't introduce the reported bug. This minor fix is
>>>> specific
>>>> to MPTCP which doesn't seem to be used here.
>>>>
>>>> I'm not sure how I can tell syzbot this is a false positive.
>>>>
>>>
>>>
>>> looks like it's fs/namei bug. Similar reports:
>>>
>>> https://syzkaller.appspot.com/bug?id=517fa734b92b7db404c409b924cf5c997640e324
>>>
>>>
>>>
>>> https://syzkaller.appspot.com/bug?id=484483daf3652b40dae18531923aa9175d392a4d
>>>
>>
>> Thank you for having checked!
>> Should we mark them as "#syz dup" if you think they have the same root
>> cause?
>>
>
> I think, yes, but I want to receive feedback from fs people about this
> bug. There were huge updates last month, and, maybe, I am missing some
> details. Alloc/free calltrace is the same, but anyway, I want some
> confirmation to not close different bugs by mistake :)
>
> If these bugs really have same root case I will close them manually
> after fix posted.

Thank you for the explanation. Sounds good to me!