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

[Stefan Berger]

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.

   Stefan