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Message-ID: <00dbd1e7-dfc8-86bc-536f-264a929ebb35@linux.ibm.com>
Date:   Wed, 20 Sep 2023 20:10:45 -0400
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Subject: Re: [syzbot] [integrity] [overlayfs] general protection fault in
 d_path
Content-Language: en-US
From:   Stefan Berger <stefanb@linux.ibm.com>
To:     Jeff Layton <jlayton@kernel.org>,
        syzbot <syzbot+a67fc5321ffb4b311c98@syzkaller.appspotmail.com>,
        amir73il@gmail.com, brauner@kernel.org,
        linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-unionfs@vger.kernel.org,
        miklos@szeredi.hu, syzkaller-bugs@googlegroups.com,
        zohar@linux.ibm.com, casey@schaufler-ca.com
References: <000000000000259bd8060596e33f@google.com>
 <bed99e92-cb7c-868d-94f3-ddf53e2b262a@linux.ibm.com>
 <8a65f5eb-2b59-9903-c6b8-84971f8765ae@linux.ibm.com>
 <ab7df5e93b5493de5fa379ccab48859fe953d7ae.camel@kernel.org>
 <b16550ac-f589-c5d7-e139-d585e8771cfd@linux.ibm.com>
In-Reply-To: <b16550ac-f589-c5d7-e139-d585e8771cfd@linux.ibm.com>
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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@kernel.org>
>>>>> 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@syzkaller.appspotmail.com
>>>>> 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@mail.gmail.com/ 
>>
>>
>> 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@linux.ibm.com/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;
+
         /* check if we have to log the current event */
         if (result < 0 && (log_policy & SMACK_AUDIT_DENIED) == 0)
                 return;