[PATCH 3.16 066/370] block: protect iterate_bdevs() against concurrent close

Ben Hutchings
2017-03-10T14:23:51Z

From: Rabin Vincent <rabinv@axis.com>

commit af309226db916e2c6e08d3eba3fa5c34225200c4 upstream.

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.

Fixes: 5c0d6b60a0ba ("vfs: Create function for iterating over block devices")
Reported-and-tested-by: Wei Fang <fangwei1@huawei.com>
Signed-off-by: Rabin Vincent <rabinv@axis.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/block_dev.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1731,6 +1731,7 @@ void iterate_bdevs(void (*func)(struct b
spin_lock(&inode_sb_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) ||
@@ -1751,8 +1752,12 @@ void iterate_bdevs(void (*func)(struct b
*/
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(&inode_sb_list_lock);
}