Btrfs: fix clone vs chattr NODATASUM race

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>

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 743c4f1b8001cf516dcb99015d633be2f0ada25a..b9b779a4ab6ebed51149d6cd87444d44644c963b 100644 (file)
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3808,11 +3808,6 @@ static noinline int btrfs_clone_files(struct file *file, struct file *file_src,
            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;
 
@@ -3822,6 +3817,13 @@ static noinline int btrfs_clone_files(struct file *file, struct file *file_src,
                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)