[PATCH] blkdev: Fix blkdev_open to release the bdev on error

[Suzuki K. Poulose]

blkdev_open() doesn't release the bdev, it attached to a given
inode, if blkdev_get() fails (e.g, due to absence of a device).
This can cause kernel crashes when the original filesystem
tries to flush the data during evict_inode.

This can be triggered easily with virtio-9p fs using the following
simple steps.

root@localhost:~# mknod disk b 9 1
root@localhost:~# cat disk
Unable to handle kernel NULL pointer dereference at virtual address 00000214
pgd = bea40000
[00000214] *pgd=be9eb831, *pte=00000000, *ppte=00000000
Internal error: Oops: 17 [#1] SMP ARM
Modules linked in:
CPU: 0 PID: 1094 Comm: cat Not tainted 4.3.0 #3
Hardware name: Generic DT based system
task: bf186600 ti: be822000 task.ti: be822000
PC is at blk_get_backing_dev_info+0x4/0x10
LR is at __filemap_fdatawrite_range+0x88/0x94
pc : [<80317e00>]    lr : [<801995d4>]    psr: 60010013
sp : be823db0  ip : 00000000  fp : 00000024
r10: fffffffa  r9 : be86a240  r8 : bec87e58
r7 : 00000001  r6 : 80615640  r5 : 7fffffff  r4 : bec03354
r3 : 00000000  r2 : bf006c00  r1 : 7fffffff  r0 : bec03200
Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
Control: 10c5383d  Table: bea4006a  DAC: 00000051
Process cat (pid: 1094, stack limit = 0xbe822210)

[... stack contents trimmed ...]

[<80317e00>] (blk_get_backing_dev_info) from [<801995d4>] (__filemap_fdatawrite_range+0x88/0x94)
[<801995d4>] (__filemap_fdatawrite_range) from [<80199608>] (filemap_fdatawrite+0x28/0x30)
[<80199608>] (filemap_fdatawrite) from [<802fa830>] (v9fs_evict_inode+0x20/0x3c)
[<802fa830>] (v9fs_evict_inode) from [<801ef4fc>] (evict+0xb0/0x188)
[<801ef4fc>] (evict) from [<801eb998>] (__dentry_kill+0x1ec/0x250)
[<801eb998>] (__dentry_kill) from [<801ec2d8>] (dput+0x188/0x28c)
[<801ec2d8>] (dput) from [<801e0858>] (path_put+0x10/0x1c)
[<801e0858>] (path_put) from [<801e08a0>] (terminate_walk+0x3c/0x98)
[<801e08a0>] (terminate_walk) from [<801e3d54>] (path_openat+0x1ec/0xeac)
[<801e3d54>] (path_openat) from [<801e56c8>] (do_filp_open+0x60/0xb4)
[<801e56c8>] (do_filp_open) from [<801d7850>] (do_sys_open+0x124/0x1d0)
[<801d7850>] (do_sys_open) from [<80107340>] (ret_fast_syscall+0x0/0x3c)
Code: 806d3ca0 80941b7c 807175d8 e590305c (e5930214)
---[ end trace b61b160a3217ae29 ]---

Fixes: e525fd89d380c4a94c0d63913a1dd1a593ed25e7
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Suzuki K. Poulose <suzuki.poulose@xxxxxxx>
---
 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 c25639e..7d7f322 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1484,6 +1484,7 @@ EXPORT_SYMBOL(blkdev_get_by_dev);
 
 static int blkdev_open(struct inode * inode, struct file * filp)
 {
+	int rc;
 	struct block_device *bdev;
 
 	/*
@@ -1507,7 +1508,11 @@ static int blkdev_open(struct inode * inode, struct file * filp)
 
 	filp->f_mapping = bdev->bd_inode->i_mapping;
 
-	return blkdev_get(bdev, filp->f_mode, filp);
+	rc = blkdev_get(bdev, filp->f_mode, filp);
+	if (rc)
+		bd_forget(inode);
+
+	return rc;
 }
 
 static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
-- 
1.7.9.5

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