[PATCH 4.16 20/64] Btrfs: fix clone vs chattr NODATASUM race

Greg Kroah-Hartman
2018-06-24T15:33:03Z

From: Omar Sandoval <osandov@fb.com>

commit b5c40d598f5408bd0ca22dfffa82f03cd9433f23 upstream.

In btrfs_clone_files(), we must check the NODATASUM flag while the
inodes are locked. Otherwise, it's possible that btrfs_ioctl_setflags()
will change the flags after we check and we can end up with a party
checksummed file.

The race window is only a few instructions in size, between the if and
the locks which is:

3834 if (S_ISDIR(src->i_mode) || S_ISDIR(inode->i_mode))
3835 return -EISDIR;

where the setflags must be run and toggle the NODATASUM flag (provided
the file size is 0). The clone will block on the inode lock, segflags
takes the inode lock, changes flags, releases log and clone continues.

Not impossible but still needs a lot of bad luck to hit unintentionally.

Fixes: 0e7b824c4ef9 ("Btrfs: don't make a file partly checksummed through file clone")
CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/ioctl.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)

--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3837,11 +3837,6 @@ static noinline int btrfs_clone_files(st
src->i_sb != inode->i_sb)
return -EXDEV;

- /* don't make the dst file partly checksummed */
- if ((BTRFS_I(src)->flags & BTRFS_INODE_NODATASUM) !=
- (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM))
- return -EINVAL;
-
if (S_ISDIR(src->i_mode) || S_ISDIR(inode->i_mode))
return -EISDIR;

@@ -3851,6 +3846,13 @@ static noinline int btrfs_clone_files(st
inode_lock(src);
}

+ /* don't make the dst file partly checksummed */
+ if ((BTRFS_I(src)->flags & BTRFS_INODE_NODATASUM) !=
+ (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)) {
+ ret = -EINVAL;
+ goto out_unlock;
+ }
+
/* determine range to clone */
ret = -EINVAL;
if (off + len > src->i_size || off + len < off)