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Message-ID: <8a65f5eb-2b59-9903-c6b8-84971f8765ae@linux.ibm.com>
Date: Wed, 20 Sep 2023 16:37:49 -0400
From: Stefan Berger <stefanb@...ux.ibm.com>
To: syzbot <syzbot+a67fc5321ffb4b311c98@...kaller.appspotmail.com>,
amir73il@...il.com, brauner@...nel.org, jlayton@...nel.org,
linux-fsdevel@...r.kernel.org, linux-integrity@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-unionfs@...r.kernel.org,
miklos@...redi.hu, syzkaller-bugs@...glegroups.com,
zohar@...ux.ibm.com, casey@...aufler-ca.com
Subject: Re: [syzbot] [integrity] [overlayfs] general protection fault in
d_path
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@...nel.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@...kaller.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.
Stefan
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