On Thu, Dec 1, 2016 at 12:18 AM, Jan Kara <[email protected]> wrote:
> From: Rabin Vincent <[email protected]>
>
> If a block device is closed while iterate_bdevs() is handling it, the
> following NULL pointer dereference occurs because bdev->b_disk is NULL
> in bdev_get_queue(), which is called from blk_get_backing_dev_info() (in
> turn called by the mapping_cap_writeback_dirty() call in
> __filemap_fdatawrite_range()):
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000508
> IP: [<ffffffff81314790>] blk_get_backing_dev_info+0x10/0x20
> PGD 9e62067 PUD 9ee8067 PMD 0
> Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> Modules linked in:
> CPU: 1 PID: 2422 Comm: sync Not tainted 4.5.0-rc7+ #400
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
> task: ffff880009f4d700 ti: ffff880009f5c000 task.ti: ffff880009f5c000
> RIP: 0010:[<ffffffff81314790>] [<ffffffff81314790>]
> blk_get_backing_dev_info+0x10/0x20
> RSP: 0018:ffff880009f5fe68 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff88000ec17a38 RCX: ffffffff81a4e940
> RDX: 7fffffffffffffff RSI: 0000000000000000 RDI: ffff88000ec176c0
> RBP: ffff880009f5fe68 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000001 R11: 0000000000000000 R12: ffff88000ec17860
> R13: ffffffff811b25c0 R14: ffff88000ec178e0 R15: ffff88000ec17a38
> FS: 00007faee505d700(0000) GS:ffff88000fb00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: 0000000000000508 CR3: 0000000009e8a000 CR4: 00000000000006e0
> Stack:
> ffff880009f5feb8 ffffffff8112e7f5 0000000000000000 7fffffffffffffff
> 0000000000000000 0000000000000000 7fffffffffffffff 0000000000000001
> ffff88000ec178e0 ffff88000ec17860 ffff880009f5fec8 ffffffff8112e81f
> Call Trace:
> [<ffffffff8112e7f5>] __filemap_fdatawrite_range+0x85/0x90
> [<ffffffff8112e81f>] filemap_fdatawrite+0x1f/0x30
> [<ffffffff811b25d6>] fdatawrite_one_bdev+0x16/0x20
> [<ffffffff811bc402>] iterate_bdevs+0xf2/0x130
> [<ffffffff811b2763>] sys_sync+0x63/0x90
> [<ffffffff815d4272>] entry_SYSCALL_64_fastpath+0x12/0x76
> Code: 0f 1f 44 00 00 48 8b 87 f0 00 00 00 55 48 89 e5 <48> 8b 80 08 05 00 00
> 5d
> RIP [<ffffffff81314790>] blk_get_backing_dev_info+0x10/0x20
> RSP <ffff880009f5fe68>
> CR2: 0000000000000508
> ---[ end trace 2487336ceb3de62d ]---
>
> The crash is easily reproducible by running the following command, if an
> msleep(100) is inserted before the call to func() in iterate_devs():
>
> while :; do head -c1 /dev/nullb0; done > /dev/null & while :; do sync; done
>
> Fix it by holding the bd_mutex across the func() call and only calling
> func() if the bdev is opened.
>
> Cc: [email protected]
> Fixes: 5c0d6b60a0ba46d45020547eacf7199171920935
> Reported-and-tested-by: Wei Fang <[email protected]>
> Signed-off-by: Rabin Vincent <[email protected]>
> Signed-off-by: Jan Kara <[email protected]>
> ---
> fs/block_dev.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 05b553368bb4..899fa8ccc347 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1950,6 +1950,7 @@ void iterate_bdevs(void (*func)(struct block_device *,
> void *), void *arg)
> spin_lock(&blockdev_superblock->s_inode_list_lock);
> list_for_each_entry(inode, &blockdev_superblock->s_inodes, i_sb_list)
> {
> struct address_space *mapping = inode->i_mapping;
> + struct block_device *bdev;
>
> spin_lock(&inode->i_lock);
> if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW) ||
> @@ -1970,8 +1971,12 @@ void iterate_bdevs(void (*func)(struct block_device *,
> void *), void *arg)
> */
> iput(old_inode);
> old_inode = inode;
> + bdev = I_BDEV(inode);
>
> - func(I_BDEV(inode), arg);
> + mutex_lock(&bdev->bd_mutex);
> + if (bdev->bd_openers)
> + func(bdev, arg);
> + mutex_unlock(&bdev->bd_mutex);
>
> spin_lock(&blockdev_superblock->s_inode_list_lock);
> }
> --
Hi,
I hit a bug with a similar signature back in October:
http://marc.info/?l=linux-block&m=147769594003740&w=2
...and still see it in 4.10-rc2.
My reproducer is not very reliable. I'm thinking this fix doesn't work
because it assumes the only race is close vs sync, but my case is
device-shutdown vs sync. In fact iterate_bdevs() is not in my
backtrace:
[ 5750.941063] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000568
[..]
[ 5750.959449] CPU: 1 PID: 8910 Comm: lt-libndctl Tainted: G
O 4.10.0-rc2+ #672
[ 5750.961283] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org 04/01/2014
[ 5750.963538] task: ffff88033173c600 task.stack: ffffc90006a80000
[ 5750.964694] RIP: 0010:blk_get_backing_dev_info+0x10/0x20
[ 5750.965774] RSP: 0018:ffffc90006a83b00 EFLAGS: 00010246
[ 5750.966842] RAX: 0000000000000000 RBX: ffffc90006a83b60 RCX: 0000000000000000
[ 5750.968136] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff88031b3a8040
[ 5750.969429] RBP: ffffc90006a83b00 R08: ffffffff82b013d0 R09: ffffffff81ea8107
[ 5750.970732] R10: 0000000000000001 R11: ffffffff82acd5c0 R12: ffff88031b3a83d0
[ 5750.972046] R13: ffff88031b3a81d0 R14: ffff8801efa934a0 R15: ffff88031b3a81d0
[ 5750.973344] FS: 00007f2cc4ed5d80(0000) GS:ffff88033ed00000(0000)
knlGS:0000000000000000
[ 5750.975171] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 5750.976295] CR2: 0000000000000568 CR3: 0000000333fcf000 CR4: 00000000000406e0
[ 5750.977608] Call Trace:
[ 5750.978345] __inode_attach_wb+0x3a7/0x5d0
[ 5750.979281] __filemap_fdatawrite_range+0xf8/0x100
[ 5750.980303] filemap_write_and_wait+0x40/0x90
[ 5750.981270] fsync_bdev+0x54/0x60
[ 5750.982110] ? bdget_disk+0x30/0x40
[ 5750.982967] invalidate_partition+0x24/0x50
[ 5750.983910] del_gendisk+0xfa/0x230
[ 5750.984768] pmem_release_disk+0x12/0x20 [nd_pmem]
[ 5750.985787] devm_action_release+0xf/0x20
[ 5750.986697] release_nodes+0x16d/0x2b0
[ 5750.987595] devres_release_all+0x3c/0x60
[ 5750.988510] device_release_driver_internal+0x16d/0x210
[ 5750.989586] device_release_driver+0x12/0x20
[ 5750.990537] unbind_store+0x10f/0x160
[ 5750.991425] drv_attr_store+0x25/0x30
[ 5750.992301] sysfs_kf_write+0x45/0x60
[ 5750.993176] kernfs_fop_write+0x13c/0x1c0
[ 5750.994099] __vfs_write+0x37/0x160
[ 5750.994967] ? rcu_read_lock_sched_held+0x5d/0x70
[ 5750.995975] ? rcu_sync_lockdep_assert+0x2f/0x60
[ 5750.996975] ? __sb_start_write+0xce/0x1d0
[ 5750.997913] ? vfs_write+0x17d/0x1a0
[ 5750.998790] vfs_write+0xb5/0x1a0
[ 5750.999626] SyS_write+0x58/0xc0
[ 5751.000458] entry_SYSCALL_64_fastpath+0x1f/0xc2
...so I'm going to take a look at having blk_get_backing_dev_info()
take its own blkdev_get() reference.
--
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