Patch "xfs: XFS_IS_REALTIME_INODE() should be false if no rt device present" has been added to the 4.13-stable tree

This is a note to let you know that I've just added the patch titled

xfs: XFS_IS_REALTIME_INODE() should be false if no rt device present

to the 4.13-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
xfs-xfs_is_realtime_inode-should-be-false-if-no-rt-device-present.patch
and it can be found in the queue-4.13 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Richard Wareing <[email protected]>
Date: Wed, 13 Sep 2017 09:09:35 +1000
Subject: xfs: XFS_IS_REALTIME_INODE() should be false if no rt device present

From: Richard Wareing <[email protected]>

commit b31ff3cdf540110da4572e3e29bd172087af65cc upstream.

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 xfs_blkdev_issue_flush():

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  IP: xfs_blkdev_issue_flush+0xd/0x20
  .....
  Call Trace:
    xfs_file_fsync+0x188/0x1c0
    vfs_fsync_range+0x3b/0xa0
    do_fsync+0x3d/0x70
    SyS_fsync+0x10/0x20
    do_syscall_64+0x4d/0xb0
    entry_SYSCALL64_slow_path+0x25/0x25

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.

---
 fs/xfs/xfs_linux.h |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

@@ -270,7 +270,14 @@ static inline uint64_t howmany_64(uint64
 #endif /* DEBUG */
 
 #ifdef CONFIG_XFS_RT
-#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)
 #else
 #define XFS_IS_REALTIME_INODE(ip) (0)
 #endif

Patches currently in stable-queue which might be from [email protected] are

queue-4.13/xfs-xfs_is_realtime_inode-should-be-false-if-no-rt-device-present.patch