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

[Stefan Berger <stefanb@linux.ibm.com>]

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.

    Stefan