[PATCH SRU X/Z] xfs: XFS_IS_REALTIME_INODE() should be false if no rt device present

Thadeu Lima de Souza Cascardo cascardo at canonical.com
Wed Sep 27 13:27:30 UTC 2017

From: Richard Wareing <rwareing at fb.com>

If using a kernel with CONFIG_XFS_RT=y and we set the RHINHERIT flag on
a directory in a filesystem that does not have a realtime device and
create a new file in that directory, it gets marked as a real time file.
When data is written and a fsync is issued, the filesystem attempts to
flush a non-existent rt device during the fsync process.

This results in a crash dereferencing a null buftarg pointer in

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  IP: xfs_blkdev_issue_flush+0xd/0x20
  Call Trace:

Setting RT inode flags does not require special privileges so any
unprivileged user can cause this oops to occur.  To reproduce, confirm
kernel is compiled with CONFIG_XFS_RT=y and run:

  # mkfs.xfs -f /dev/pmem0
  # mount /dev/pmem0 /mnt/test
  # mkdir /mnt/test/foo
  # xfs_io -c 'chattr +t' /mnt/test/foo
  # xfs_io -f -c 'pwrite 0 5m' -c fsync /mnt/test/foo/bar

Or just run xfstests with MKFS_OPTIONS="-d rtinherit=1" and wait.

Kernels built with CONFIG_XFS_RT=n are not exposed to this bug.

Fixes: f538d4da8d52 ("[XFS] write barrier support")
Cc: <stable at vger.kernel.org>
Signed-off-by: Richard Wareing <rwareing at fb.com>
Signed-off-by: Dave Chinner <david at fromorbit.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit b31ff3cdf540110da4572e3e29bd172087af65cc)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
 fs/xfs/xfs_linux.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
index ec0e239a0fa9..201aae0b2662 100644
--- a/fs/xfs/xfs_linux.h
+++ b/fs/xfs/xfs_linux.h
@@ -369,7 +369,14 @@ static inline __uint64_t howmany_64(__uint64_t x, __uint32_t y)
 #endif /* DEBUG */
-#define XFS_IS_REALTIME_INODE(ip) ((ip)->i_d.di_flags & XFS_DIFLAG_REALTIME)
+ * make sure we ignore the inode flag if the filesystem doesn't have a
+ * configured realtime device.
+ */
+#define XFS_IS_REALTIME_INODE(ip)			\
+	(((ip)->i_d.di_flags & XFS_DIFLAG_REALTIME) &&	\
+	 (ip)->i_mount->m_rtdev_targp)
 #define XFS_IS_REALTIME_INODE(ip) (0)

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