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Subject: Re: [syzbot] [reiserfs?] INFO: task hung in flush_old_commits
From:   Roberto Sassu <roberto.sassu@huaweicloud.com>
To:     Paul Moore <paul@paul-moore.com>
Cc:     linux-security-module@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        reiserfs-devel@vger.kernel.org, roberto.sassu@huawei.com,
        syzkaller-bugs@googlegroups.com,
        syzbot <syzbot+0a684c061589dcc30e51@syzkaller.appspotmail.com>,
        Jan Kara <jack@suse.cz>, Jeff Mahoney <jeffm@suse.com>
Date:   Fri, 26 May 2023 11:45:57 +0200
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On Wed, 2023-05-24 at 17:57 -0400, Paul Moore wrote:
> On Wed, May 24, 2023 at 11:50 AM Roberto Sassu
> <roberto.sassu@huaweicloud.com> wrote:
> > On Wed, 2023-05-24 at 11:11 -0400, Paul Moore wrote:
> > > On Wed, May 24, 2023 at 5:59 AM syzbot
> > > <syzbot+0a684c061589dcc30e51@syzkaller.appspotmail.com> wrote:
> > > > syzbot has bisected this issue to:
> > > > 
> > > > commit d82dcd9e21b77d338dc4875f3d4111f0db314a7c
> > > > Author: Roberto Sassu <roberto.sassu@huawei.com>
> > > > Date:   Fri Mar 31 12:32:18 2023 +0000
> > > > 
> > > >     reiserfs: Add security prefix to xattr name in reiserfs_security_write()
> > > > 
> > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11c39639280000
> > > > start commit:   421ca22e3138 Merge tag 'nfs-for-6.4-2' of git://git.linux-..
> > > > git tree:       upstream
> > > > final oops:     https://syzkaller.appspot.com/x/report.txt?x=13c39639280000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=15c39639280000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=7d8067683055e3f5
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=0a684c061589dcc30e51
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14312791280000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12da8605280000
> > > > 
> > > > Reported-by: syzbot+0a684c061589dcc30e51@syzkaller.appspotmail.com
> > > > Fixes: d82dcd9e21b7 ("reiserfs: Add security prefix to xattr name in reiserfs_security_write()")
> > > > 
> > > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> > > 
> > > Roberto, I think we need to resolve this somehow.  As I mentioned
> > > earlier, I don't believe this to be a fault in your patch, rather that
> > > patch simply triggered a situation that had not been present before,
> > > likely because the reiserfs code always failed when writing LSM
> > > xattrs.  Regardless, we still need to fix the deadlocks that sysbot
> > > has been reporting.
> > 
> > Hi Paul
> > 
> > ok, I will try.
> 
> Thanks Roberto.  If it gets to be too challenging, let us know and we
> can look into safely disabling the LSM xattrs for reiserfs, I'll be
> shocked if anyone is successfully using LSM xattrs on reiserfs.

Ok, at least I know what happens...

+ Jan, Jeff

I'm focusing on this reproducer, which works 100% of the times:

https://syzkaller.appspot.com/text?tag=ReproSyz&x=163079f9280000

This is the last lock, before things go wrong:

Thread 5 hit Breakpoint 2, reiserfs_write_lock (s=s@entry=0xffff888066e28000) at fs/reiserfs/lock.c:24
24	{
(gdb) bt
#0  reiserfs_write_lock (s=s@entry=0xffff888066e28000) at fs/reiserfs/lock.c:24
#1  0xffffffff821a559a in reiserfs_get_block (inode=inode@entry=0xffff888069fd0190, block=block@entry=15, bh_result=bh_result@entry=0xffff888075940000, create=create@entry=1) at fs/reiserfs/inode.c:680
#2  0xffffffff81f50254 in __block_write_begin_int (folio=0xffffea00019a9180, pos=pos@entry=61440, len=len@entry=1, get_block=get_block@entry=0xffffffff821a5390 <reiserfs_get_block>, iomap=iomap@entry=0x0 <fixed_percpu_data>) at fs/buffer.c:2064
#3  0xffffffff81f5165a in __block_write_begin (page=page@entry=0xffffea00019a9180, pos=pos@entry=61440, len=len@entry=1, get_block=get_block@entry=0xffffffff821a5390 <reiserfs_get_block>) at ./arch/x86/include/asm/jump_label.h:27
#4  0xffffffff821a3e3d in reiserfs_write_begin (file=<optimized out>, mapping=<optimized out>, pos=61440, len=1, pagep=<optimized out>, fsdata=<optimized out>) at fs/reiserfs/inode.c:2779
#5  0xffffffff81aec252 in generic_perform_write (iocb=iocb@entry=0xffffc9002130fb60, i=i@entry=0xffffc9002130fd00) at mm/filemap.c:3923
#6  0xffffffff81b0604e in __generic_file_write_iter (iocb=iocb@entry=0xffffc9002130fb60, from=from@entry=0xffffc9002130fd00) at mm/filemap.c:4051
#7  0xffffffff81b06383 in generic_file_write_iter (iocb=0xffffc9002130fb60, from=0xffffc9002130fd00) at mm/filemap.c:4083
#8  0xffffffff81e3240b in call_write_iter (file=0xffff888012692d00, iter=0xffffc9002130fd00, kio=0xffffc9002130fb60) at ./include/linux/fs.h:1868
#9  do_iter_readv_writev (filp=filp@entry=0xffff888012692d00, iter=iter@entry=0xffffc9002130fd00, ppos=ppos@entry=0xffffc9002130fe90, type=type@entry=1, flags=flags@entry=0) at fs/read_write.c:735
#10 0xffffffff81e33da4 in do_iter_write (flags=0, pos=0xffffc9002130fe90, iter=0xffffc9002130fd00, file=0xffff888012692d00) at fs/read_write.c:860
#11 do_iter_write (file=0xffff888012692d00, iter=0xffffc9002130fd00, pos=0xffffc9002130fe90, flags=0) at fs/read_write.c:841
#12 0xffffffff81e34611 in vfs_writev (file=file@entry=0xffff888012692d00, vec=vec@entry=0x20000480, vlen=vlen@entry=1, pos=pos@entry=0xffffc9002130fe90, flags=flags@entry=0) at fs/read_write.c:933
#13 0xffffffff81e34fd6 in do_pwritev (fd=fd@entry=5, vec=vec@entry=0x20000480, vlen=vlen@entry=1, pos=pos@entry=61440, flags=flags@entry=0) at fs/read_write.c:1030
#14 0xffffffff81e3b61f in __do_sys_pwritev2 (pos_h=<optimized out>, flags=0, pos_l=61440, vlen=1, vec=0x20000480, fd=5) at fs/read_write.c:1089
#15 __se_sys_pwritev2 (pos_h=<optimized out>, flags=0, pos_l=61440, vlen=1, vec=536872064, fd=5) at fs/read_write.c:1080
#16 __x64_sys_pwritev2 (regs=0xffffc9002130ff58) at fs/read_write.c:1080
#17 0xffffffff880dd279 in do_syscall_x64 (nr=<optimized out>, regs=0xffffc9002130ff58) at arch/x86/entry/common.c:50
#18 do_syscall_64 (regs=0xffffc9002130ff58, nr=<optimized out>) at arch/x86/entry/common.c:80
#19 0xffffffff8820008b in entry_SYSCALL_64 () at arch/x86/entry/entry_64.S:120
#20 0x0000000000406e00 in ?? ()
#21 0x00007f99e21b5000 in ?? ()
#22 0x0000000000000000 in ?? ()

After that, there is a very long loop doing:

Thread 5 hit Breakpoint 3, reiserfs_read_bitmap_block (sb=sb@entry=0xffff888066e28000, bitmap=bitmap@entry=1) at fs/reiserfs/bitmap.c:1417
1417	{
(gdb) c
Continuing.

Thread 5 hit Breakpoint 3, reiserfs_read_bitmap_block (sb=sb@entry=0xffff888066e28000, bitmap=bitmap@entry=2) at fs/reiserfs/bitmap.c:1417
1417	{
(gdb) 
Continuing.

and so on...

[  628.589974][ T6003] REISERFS warning (device loop0): sh-2029: %s: bitmap block (#%u) reading failed reiserfs_read_bitmap_block: reiserfs_read_bitmap_block

This message appears because we are here:

struct buffer_head *reiserfs_read_bitmap_block(struct super_block *sb,
                                               unsigned int bitmap)
{

[...]

	bh = sb_bread(sb, block);
	if (bh == NULL)
		reiserfs_warning(sb, "sh-2029: %s: bitmap block (#%u) "
		                 "reading failed", __func__, block);

The hanging task (kthread) is trying to hold the same lock, which
unfortunately is not going to be released soon:

static int reiserfs_sync_fs(struct super_block *s, int wait)
{

[...]

	reiserfs_write_lock(s);

I didn't get yet if the reason of this long loop is because we cannot
flush at this point, or just because of the test. I tried to
synchronously flush, but didn't make any difference.

I did a very simple change, which can be totally wrong:

@@ -94,7 +96,7 @@ static void flush_old_commits(struct work_struct *work)
         * trylock as reiserfs_cancel_old_flush() may be waiting for this work
         * to complete with s_umount held.
         */
-       if (!down_read_trylock(&s->s_umount)) {
+       if (sbi->lock_owner || !down_read_trylock(&s->s_umount)) {
                /* Requeue work if we are not cancelling it */
                spin_lock(&sbi->old_work_lock);
                if (sbi->work_queued == 1)


If the lock is held, instead of waiting, reschedule the flush.

Anyway, at least this report does not seem to be related to fixing
security xattrs.

Roberto