linux-kernel - Re: INFO: task hung in __get

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Message-ID: <20190515102133.GA16193@quack2.suse.cz>
Date:   Wed, 15 May 2019 12:21:33 +0200
From:   Jan Kara <jack@...e.cz>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        syzbot <syzbot+10007d66ca02b08f0e60@...kaller.appspotmail.com>,
        dvyukov@...gle.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        linux-block@...r.kernel.org
Subject: Re: INFO: task hung in __get_super

On Wed 15-05-19 10:02:37, Tetsuo Handa wrote:
> Since lo_ioctl() is calling sb_set_blocksize() immediately after udf_load_vrs()
> called sb_set_blocksize(), udf_tread() can't use expected i_blkbits settings...

Thanks for debugging this but this doesn't quiet make sense to me. See
below:

> [   48.685672][ T7322] fs/block_dev.c:135 bdev=0000000014fa0ec2 12 -> 9
> [   48.694675][ T7322] CPU: 4 PID: 7322 Comm: a.out Not tainted 5.1.0+ #196
> [   48.701321][ T7322] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/13/2018
> [   48.710265][ T7322] Call Trace:
> [   48.710272][ T7322]  dump_stack+0xaa/0xd8
> [   48.715633][ T7322]  set_blocksize+0xff/0x140
> [   48.822094][ T7322]  sb_set_blocksize+0x27/0x70
> [   48.824843][ T7322]  udf_load_vrs+0x4b/0x500
> [   48.827322][ T7322]  udf_fill_super+0x32e/0x890
> [   48.830125][ T7322]  ? snprintf+0x66/0x90
> [   48.832572][ T7322]  mount_bdev+0x1c7/0x210
> [   48.835293][ T7322]  ? udf_load_vrs+0x500/0x500
> [   48.838009][ T7322]  udf_mount+0x34/0x40
> [   48.840504][ T7322]  legacy_get_tree+0x2d/0x80
> [   48.843192][ T7322]  vfs_get_tree+0x30/0x140
> [   48.845787][ T7322]  do_mount+0x830/0xc30
> [   48.848325][ T7322]  ? copy_mount_options+0x152/0x1c0
> [   48.851066][ T7322]  ksys_mount+0xab/0x120
> [   48.853627][ T7322]  __x64_sys_mount+0x26/0x30
> [   48.856168][ T7322]  do_syscall_64+0x7c/0x1a0
> [   48.858943][ T7322]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

So this is normal - UDF sets block size it wants on the device during
mount. Now we have the block device exclusively open so nobody should be
changing it.

> [   48.978376][ T7332] fs/block_dev.c:135 bdev=0000000014fa0ec2 9 -> 12
> [   49.079394][ T7332] CPU: 6 PID: 7332 Comm: a.out Not tainted 5.1.0+ #196
> [   49.082769][ T7332] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/13/2018
> [   49.089007][ T7332] Call Trace:
> [   49.091410][ T7332]  dump_stack+0xaa/0xd8
> [   49.094053][ T7332]  set_blocksize+0xff/0x140
> [   49.096734][ T7332]  lo_ioctl+0x570/0xc60
> [   49.099366][ T7332]  ? loop_queue_work+0xdb0/0xdb0
> [   49.102079][ T7332]  blkdev_ioctl+0xb69/0xc10
> [   49.104667][ T7332]  block_ioctl+0x56/0x70
> [   49.107267][ T7332]  ? blkdev_fallocate+0x230/0x230
> [   49.110035][ T7332]  do_vfs_ioctl+0xc1/0x7e0
> [   49.112728][ T7332]  ? tomoyo_file_ioctl+0x23/0x30
> [   49.115452][ T7332]  ksys_ioctl+0x94/0xb0
> [   49.118008][ T7332]  __x64_sys_ioctl+0x1e/0x30
> [   49.120686][ T7332]  do_syscall_64+0x7c/0x1a0
> [   49.123470][ T7332]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

And this is strange. set_blocksize() is only called from loop_set_fd() but
that means that the loop device must already be in lo->lo_state ==
Lo_unbound. But loop device that is being used should never be in
Lo_unbound state... Except if... Oh, I see what the problem is:

UDF opens unbound loop device (through mount_bdev() ->
blkdev_get_by_path()). That succeeds as loop allows opens on unbound
devices so that ioctl can be run to set it up. UDF sets block size for the
block device. Someone else comes and calls LOOP_SET_FD for the device and
plop, block device block size changes under UDF's hands.

The question is how to fix this problem. The simplest fix I can see is that
we'd just refuse to do LOOP_SET_FD if someone has the block device
exclusively open as there are high chances such user will be unpleasantly
surprised by the device changing under him. OTOH this has some potential
for userspace visible regressions. But I guess it's worth a try. Something
like attached patch?

Let syzbot test the patch as well:

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git v5.1

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-loop-Don-t-change-loop-device-under-exclusive-opener.patch" of type "text/x-patch" (1705 bytes)