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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: f538d4d ("[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>
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RichWpg authored and torvalds committed Sep 12, 2017
1 parent 5226971 commit b31ff3cdf540110da4572e3e29bd172087af65cc
Showing with 8 additions and 1 deletion.
  1. +8 −1 fs/xfs/xfs_linux.h
@@ -270,7 +270,14 @@ static inline uint64_t howmany_64(uint64_t x, uint32_t y)
#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

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