CVE-2017-14340: NULL Pointer Dereference (CWE-476)
value: The XFS_IS_REALTIME_INODE macro in fs/xfs/xfs_linux.h in the Linux kernel before 4.13.2 does not verify that a filesystem has a realtime device, which allows local users to cause a denial of service (NULL pointer dereference and OOPS) via vectors related to setting an RHINHERIT flag on a directory.
CWE description: The product dereferences a pointer that it expects to be valid but is NULL.
CVSS v3 base score: 5.5
Published: 2017-09-15T11:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: fs/xfs/xfs_linux.h
Commit date: 2017-09-12 20:02:22 -0700
Commit message: xfs: XFS_IS_REALTIME_INODE() should be false if no rt device present

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.

Fixes: f538d4da8d52 ("[XFS] write barrier support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Richard Wareing <rwareing@fb.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Diff: diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
index 9301c5a6060b68..dcd1292664b340 100644
--- a/fs/xfs/xfs_linux.h
+++ b/fs/xfs/xfs_linux.h
@@ -268,11 +268,18 @@ static inline uint64_t howmany_64(uint64_t x, uint32_t y)
 
 #endif /* XFS_WARN */
 #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
 
 #endif /* __XFS_LINUX__ */

Vulnerable code: #define XFS_IS_REALTIME_INODE(ip) ((ip)->i_d.di_flags & XFS_DIFLAG_REALTIME)
Fixed code: 
/*
 * 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)