xfs: prevent swapext from operating on write-only files

This patch prevents user "foo" from using the SWAPEXT ioctl to swap
a write-only file owned by user "bar" into a file owned by "foo" and
subsequently reading it.  It does so by checking that the file
descriptors passed to the ioctl are also opened for reading.

Signed-off-by: Dan Rosenberg <dan.j.rosenberg@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 4 insertions, 1 deletions

diff --git a/fs/xfs/xfs_dfrag.c b/fs/xfs/xfs_dfrag.c
index 5bba29a07812..7f159d2a429a 100644
--- a/fs/xfs/xfs_dfrag.c
+++ b/fs/xfs/xfs_dfrag.c
@@ -69,7 +69,9 @@ xfs_swapext(
                 goto out;
         }
 
-        if (!(file->f_mode & FMODE_WRITE) || (file->f_flags & O_APPEND)) {
+        if (!(file->f_mode & FMODE_WRITE) ||
+            !(file->f_mode & FMODE_READ) ||
+            (file->f_flags & O_APPEND)) {
                 error = XFS_ERROR(EBADF);
                 goto out_put_file;
         }
@@ -81,6 +83,7 @@ xfs_swapext(
         }
 
         if (!(tmp_file->f_mode & FMODE_WRITE) ||
+            !(tmp_file->f_mode & FMODE_READ) ||
             (tmp_file->f_flags & O_APPEND)) {
                 error = XFS_ERROR(EBADF);
                 goto out_put_tmp_file;