Re: [syzbot] [integrity] [overlayfs] general protection fault in d_path

[Casey Schaufler]

On 9/20/2023 5:10 PM, Stefan Berger wrote:
>
> On 9/20/23 18:09, Stefan Berger wrote:
>>
>> On 9/20/23 17:16, Jeff Layton wrote:
>>> On Wed, 2023-09-20 at 16:37 -0400, Stefan Berger wrote:
>>>> On 9/20/23 13:01, Stefan Berger wrote:
>>>>> On 9/17/23 20:04, syzbot wrote:
>>>>>> syzbot has bisected this issue to:
>>>>>>
>>>>>> commit db1d1e8b9867aae5c3e61ad7859abfcc4a6fd6c7
>>>>>> Author: Jeff Layton <jlayton@xxxxxxxxxx>
>>>>>> Date:   Mon Apr 17 16:55:51 2023 +0000
>>>>>>
>>>>>>       IMA: use vfs_getattr_nosec to get the i_version
>>>>>>
>>>>>> bisection log:
>>>>>> https://syzkaller.appspot.com/x/bisect.txt?x=106f7e54680000
>>>>>> start commit:   a747acc0b752 Merge tag
>>>>>> 'linux-kselftest-next-6.6-rc2'
>>>>>> of g..
>>>>>> git tree:       upstream
>>>>>> final oops:
>>>>>> https://syzkaller.appspot.com/x/report.txt?x=126f7e54680000
>>>>>> console output:
>>>>>> https://syzkaller.appspot.com/x/log.txt?x=146f7e54680000
>>>>>> kernel config:
>>>>>> https://syzkaller.appspot.com/x/.config?x=df91a3034fe3f122
>>>>>> dashboard link:
>>>>>> https://syzkaller.appspot.com/bug?extid=a67fc5321ffb4b311c98
>>>>>> syz repro:
>>>>>> https://syzkaller.appspot.com/x/repro.syz?x=1671b694680000
>>>>>> C reproducer:
>>>>>> https://syzkaller.appspot.com/x/repro.c?x=14ec94d8680000
>>>>>>
>>>>>> Reported-by: syzbot+a67fc5321ffb4b311c98@xxxxxxxxxxxxxxxxxxxxxxxxx
>>>>>> Fixes: db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the
>>>>>> i_version")
>>>>>>
>>>>>> For information about bisection process see:
>>>>>> https://goo.gl/tpsmEJ#bisection
>>>>> The final oops shows this here:
>>>>>
>>>>> BUG: kernel NULL pointer dereference, address: 0000000000000058
>>>>> #PF: supervisor read access in kernel mode
>>>>> #PF: error_code(0x0000) - not-present page
>>>>> PGD 0 P4D 0
>>>>> Oops: 0000 [#1] PREEMPT SMP
>>>>> CPU: 0 PID: 3192 Comm: syz-executor.0 Not tainted
>>>>> 6.4.0-rc2-syzkaller #0
>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine,
>>>>> BIOS Google 08/04/2023
>>>>> RIP: 0010:__lock_acquire+0x35/0x490 kernel/locking/lockdep.c:4946
>>>>> Code: 83 ec 18 65 4c 8b 35 aa 60 f4 7e 83 3d b7 11 e4 02 00 0f 84 05
>>>>> 02 00 00 4c 89 cb 89 cd 41 89 d5 49 89 ff 83 fe 01 77 0c 89 f0
>>>>> <49> 8b
>>>>> 44 c7 08 48 85 c0 75 1b 4c 89 ff 31 d2 45 89 c4 e8 74 f6 ff
>>>>> RSP: 0018:ffffc90002edb840 EFLAGS: 00010097
>>>>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000002
>>>>> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000050
>>>>> RBP: 0000000000000002 R08: 0000000000000001 R09: 0000000000000000
>>>>> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>>>>> R13: 0000000000000000 R14: ffff888102ea5340 R15: 0000000000000050
>>>>> FS:  0000000000000000(0000) GS:ffff88813bc00000(0000)
>>>>> knlGS:0000000000000000
>>>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>> CR2: 0000000000000058 CR3: 0000000003aa8000 CR4: 00000000003506f0
>>>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>> Call Trace:
>>>>>   <TASK>
>>>>>   lock_acquire+0xd8/0x1f0 kernel/locking/lockdep.c:5691
>>>>>   seqcount_lockdep_reader_access include/linux/seqlock.h:102 [inline]
>>>>>   get_fs_root_rcu fs/d_path.c:243 [inline]
>>>>>   d_path+0xd1/0x1f0 fs/d_path.c:285
>>>>>   audit_log_d_path+0x65/0x130 kernel/audit.c:2139
>>>>>   dump_common_audit_data security/lsm_audit.c:224 [inline]
>>>>>   common_lsm_audit+0x3b3/0x840 security/lsm_audit.c:458
>>>>>   smack_log+0xad/0x130 security/smack/smack_access.c:383
>>>>>   smk_tskacc+0xb1/0xd0 security/smack/smack_access.c:253
>>>>>   smack_inode_getattr+0x8a/0xb0 security/smack/smack_lsm.c:1187
>>>>>   security_inode_getattr+0x32/0x50 security/security.c:2114
>>>>>   vfs_getattr+0x1b/0x40 fs/stat.c:167
>>>>>   ovl_getattr+0xa6/0x3e0 fs/overlayfs/inode.c:173
>>>>>   ima_check_last_writer security/integrity/ima/ima_main.c:171
>>>>> [inline]
>>>>>   ima_file_free+0xbd/0x130 security/integrity/ima/ima_main.c:203
>>>>>   __fput+0xc7/0x220 fs/file_table.c:315
>>>>>   task_work_run+0x7d/0xa0 kernel/task_work.c:179
>>>>>   exit_task_work include/linux/task_work.h:38 [inline]
>>>>>   do_exit+0x2c7/0xa80 kernel/exit.c:871 <-----------------------
>>>>>   do_group_exit+0x85/0xa0 kernel/exit.c:1021
>>>>>   get_signal+0x73c/0x7f0 kernel/signal.c:2874
>>>>>   arch_do_signal_or_restart+0x89/0x290 arch/x86/kernel/signal.c:306
>>>>>   exit_to_user_mode_loop+0x61/0xb0 kernel/entry/common.c:168
>>>>>   exit_to_user_mode_prepare+0x64/0xb0 kernel/entry/common.c:204
>>>>>   __syscall_exit_to_user_mode_work kernel/entry/common.c:286 [inline]
>>>>>   syscall_exit_to_user_mode+0x2b/0x1d0 kernel/entry/common.c:297
>>>>>   do_syscall_64+0x4d/0x90 arch/x86/entry/common.c:86
>>>>>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>>>>
>>>>>
>>>>> do_exit has called exit_fs(tsk) [
>>>>> https://elixir.bootlin.com/linux/v6.4-rc2/source/kernel/exit.c#L867 ]
>>>>>
>>>>> exit_fs(tsk) has set tsk->fs = NULL [
>>>>> https://elixir.bootlin.com/linux/v6.4-rc2/source/fs/fs_struct.c#L103
>>>>> ]
>>>>>
>>>>> I think this then bites in d_path() where it calls:
>>>>>
>>>>>      get_fs_root_rcu(current->fs, &root);   [
>>>>> https://elixir.bootlin.com/linux/v6.4-rc2/source/fs/d_path.c#L285 ]
>>>>>
>>>>> current->fs is likely NULL here.
>>>>>
>>>>> If this was correct it would have nothing to do with the actual
>>>>> patch,
>>>>> though, but rather with the fact that smack logs on process
>>>>> termination. I am not sure what the solution would be other than
>>>>> testing for current->fs == NULL in d_path before using it and
>>>>> returning an error that is not normally returned or trying to
>>>>> intercept this case in smack.
>>>> I have now been able to recreate the syzbot issue with the test
>>>> program
>>>> and the issue is exactly the one described here, current->fs == NULL.
>>>>
>>> Earlier in this thread, Amir had a diagnosis that IMA is
>>> inappropriately
>>> trying to use f_path directly instead of using the helpers that are
>>> friendly for stacking filesystems.
>>>
>>> https://lore.kernel.org/linux-fsdevel/CAOQ4uxgjnYyeQL-LbS5kQ7+C0d6sjzKqMDWAtZW8cAkPaed6=Q@xxxxxxxxxxxxxx/
>>>
>>>
>>> I'm not an IMA hacker so I'm not planning to roll a fix here. Perhaps
>>> someone on the IMA team could try this approach?
>>
>>
>> I have applied this patch here from Amir now and it does NOT resolve
>> the issue:
>>
>> https://lore.kernel.org/linux-integrity/296dae962a2a488bde682d3def074db91686e1c3.camel@xxxxxxxxxxxxx/T/#m4ebdb780bf6952e7f210c55e87950d0cfa1d5eb0
>>
>>
>
> This seems to resolve the issue:
>
> diff --git a/security/smack/smack_access.c
> b/security/smack/smack_access.c
> index 585e5e35710b..57afcea1e39b 100644
> --- a/security/smack/smack_access.c
> +++ b/security/smack/smack_access.c
> @@ -347,6 +347,9 @@ void smack_log(char *subject_label, char
> *object_label, int request,
>         struct smack_audit_data *sad;
>         struct common_audit_data *a = &ad->a;
>
> +       if (current->flags & PF_EXITING)
> +               return;
> +

Based on what I see here I can understand that this prevents the panic,
but it isn't so clear what changed that introduced the problem.

>         /* check if we have to log the current event */
>         if (result < 0 && (log_policy & SMACK_AUDIT_DENIED) == 0)
>                 return;
>
>