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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Sun, 4 Apr 2021 11:10:48 +0300
Message-ID: <CAOQ4uxjk4XYuwz5HCmN-Ge=Ld=tM1f7ZxVrd5U1AC2Wisc9MTA@mail.gmail.com>
Subject: Re: [syzbot] possible deadlock in ovl_maybe_copy_up
To:     syzbot <syzbot+c18f2f6a7b08c51e3025@syzkaller.appspotmail.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        Miklos Szeredi <miklos@szeredi.hu>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Goldwyn Rodrigues <rgoldwyn@suse.com>
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On Sat, Apr 3, 2021 at 10:18 PM syzbot
<syzbot+c18f2f6a7b08c51e3025@syzkaller.appspotmail.com> wrote:
>
> syzbot has found a reproducer for the following issue on:
>
> HEAD commit:    454c576c Add linux-next specific files for 20210401
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1616e07ed00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=920cc274cae812a5
> dashboard link: https://syzkaller.appspot.com/bug?extid=c18f2f6a7b08c51e3025
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13da365ed00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13ca9d16d00000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+c18f2f6a7b08c51e3025@syzkaller.appspotmail.com
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.12.0-rc5-next-20210401-syzkaller #0 Not tainted
> ------------------------------------------------------
> syz-executor144/9166 is trying to acquire lock:
> ffff888144cf0460 (sb_writers#5){.+.+}-{0:0}, at: ovl_maybe_copy_up+0x11f/0x190 fs/overlayfs/copy_up.c:995
>
> but task is already holding lock:
> ffff8880256d42c0 (&iint->mutex){+.+.}-{3:3}, at: process_measurement+0x3a8/0x17e0 security/integrity/ima/ima_main.c:253
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (&iint->mutex){+.+.}-{3:3}:
>        __mutex_lock_common kernel/locking/mutex.c:949 [inline]
>        __mutex_lock+0x139/0x1120 kernel/locking/mutex.c:1096
>        process_measurement+0x3a8/0x17e0 security/integrity/ima/ima_main.c:253
>        ima_file_check+0xb9/0x100 security/integrity/ima/ima_main.c:499
>        do_open fs/namei.c:3361 [inline]
>        path_openat+0x15b5/0x27e0 fs/namei.c:3492
>        do_filp_open+0x17e/0x3c0 fs/namei.c:3519
>        do_sys_openat2+0x16d/0x420 fs/open.c:1187
>        do_sys_open fs/open.c:1203 [inline]
>        __do_sys_open fs/open.c:1211 [inline]
>        __se_sys_open fs/open.c:1207 [inline]
>        __x64_sys_open+0x119/0x1c0 fs/open.c:1207
>        do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>        entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> -> #0 (sb_writers#5){.+.+}-{0:0}:
>        check_prev_add kernel/locking/lockdep.c:2938 [inline]
>        check_prevs_add kernel/locking/lockdep.c:3061 [inline]
>        validate_chain kernel/locking/lockdep.c:3676 [inline]
>        __lock_acquire+0x2a17/0x5230 kernel/locking/lockdep.c:4902
>        lock_acquire kernel/locking/lockdep.c:5512 [inline]
>        lock_acquire+0x1ab/0x740 kernel/locking/lockdep.c:5477
>        percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
>        __sb_start_write include/linux/fs.h:1758 [inline]
>        sb_start_write include/linux/fs.h:1828 [inline]
>        mnt_want_write+0x6e/0x3e0 fs/namespace.c:375
>        ovl_maybe_copy_up+0x11f/0x190 fs/overlayfs/copy_up.c:995
>        ovl_open+0xba/0x270 fs/overlayfs/file.c:149
>        do_dentry_open+0x4b9/0x11b0 fs/open.c:826
>        vfs_open fs/open.c:940 [inline]
>        dentry_open+0x132/0x1d0 fs/open.c:956
>        ima_calc_file_hash+0x2d2/0x4b0 security/integrity/ima/ima_crypto.c:557
>        ima_collect_measurement+0x4ca/0x570 security/integrity/ima/ima_api.c:252
>        process_measurement+0xd1c/0x17e0 security/integrity/ima/ima_main.c:330
>        ima_file_check+0xb9/0x100 security/integrity/ima/ima_main.c:499
>        do_open fs/namei.c:3361 [inline]
>        path_openat+0x15b5/0x27e0 fs/namei.c:3492
>        do_filp_open+0x17e/0x3c0 fs/namei.c:3519
>        do_sys_openat2+0x16d/0x420 fs/open.c:1187
>        do_sys_open fs/open.c:1203 [inline]
>        __do_sys_open fs/open.c:1211 [inline]
>        __se_sys_open fs/open.c:1207 [inline]
>        __x64_sys_open+0x119/0x1c0 fs/open.c:1207
>        do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>        entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> other info that might help us debug this:
>
>  Possible unsafe locking scenario:
>
>        CPU0                    CPU1
>        ----                    ----
>   lock(&iint->mutex);
>                                lock(sb_writers#5);
>                                lock(&iint->mutex);
>   lock(sb_writers#5);
>
>  *** DEADLOCK ***
>
> 1 lock held by syz-executor144/9166:
>  #0: ffff8880256d42c0 (&iint->mutex){+.+.}-{3:3}, at: process_measurement+0x3a8/0x17e0 security/integrity/ima/ima_main.c:253
>

It's a false positive lockdep warning due to missing annotation of
stacking layer on iint->mutex in IMA code.

To fix it properly, iint->mutex, which can be taken in any of the
stacking fs layers, should be annotated with stacking depth like
ovl_lockdep_annotate_inode_mutex_key()

I think it's the same root cause as:
https://syzkaller.appspot.com/bug?extid=18a1619cceea30ed45af
https://syzkaller.appspot.com/bug?extid=ae82084b07d0297e566b

I think both of the above were marked "fixed" by a paper over.
The latter was marked "fixed" by "ovl: detect overlapping layers"
but if you look at the repro, the fact that 'workdir' overlaps with
'lowerdir' has nothing to do with the lockdep warning, so said
"fix" just papered over the IMA lockdep warning.

Thanks,
Amir.