2020-04-28 20:28:33

by David Howells

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Subject: [PATCH] Fix use after free in get_tree_bdev()

Commit 6fcf0c72e4b9, a fix to get_tree_bdev() put a missing blkdev_put() in
the wrong place, before a warnf() that displays the bdev under
consideration rather after it.

This results in a silent lockup in printk("%pg") called via warnf() from
get_tree_bdev() under some circumstances when there's a race with the
blockdev being frozen. This can be caused by xfstests/tests/generic/085 in
combination with Lukas Czerner's ext4 mount API conversion patchset. It
looks like it ought to occur with other users of get_tree_bdev() such as
XFS, but apparently doesn't.

Fix this by switching the order of the lines.

Fixes: 6fcf0c72e4b9 ("vfs: add missing blkdev_put() in get_tree_bdev()")
Reported-by: Lukas Czerner <[email protected]>
Signed-off-by: David Howells <[email protected]>
cc: Ian Kent <[email protected]>
cc: Al Viro <[email protected]>

fs/super.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/super.c b/fs/super.c
index cd352530eca9..a288cd60d2ae 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1302,8 +1302,8 @@ int get_tree_bdev(struct fs_context *fc,
if (bdev->bd_fsfreeze_count > 0) {
- blkdev_put(bdev, mode);
warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
+ blkdev_put(bdev, mode);
return -EBUSY;

2020-04-29 16:50:40

by Linus Torvalds

[permalink] [raw]
Subject: Re: [PATCH] Fix use after free in get_tree_bdev()

On Tue, Apr 28, 2020 at 11:06 PM Lukas Czerner <[email protected]> wrote:
> This fixes the problem I was seeing. Thanks David.
> Reviewed-by: Lukas Czerner <[email protected]>

Well, it got applied as obvious before this, so the commit log won't
show your testing.

Commit dd7bc8158b41 ("Fix use after free in get_tree_bdev()") in case
anybody cares. Didn't make -rc3, but will be in -rc4.