linux-kernel - [PATCH 3.16 136/192] xfs: XFS_IS_REALTIME_INODE() should be false if no rt device present

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Message-ID: <lsq.1507553065.431219666@decadent.org.uk>
Date:   Mon, 09 Oct 2017 13:44:25 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Richard Wareing" <rwareing@...com>,
        "Dave Chinner" <david@...morbit.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: [PATCH 3.16 136/192] xfs: XFS_IS_REALTIME_INODE() should be false
 if no rt device present

3.16.49-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Richard Wareing <rwareing@...com>

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.

Fixes: f538d4da8d52 ("[XFS] write barrier support")
Signed-off-by: Richard Wareing <rwareing@...com>
Signed-off-by: Dave Chinner <david@...morbit.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/xfs/xfs_dinode.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/fs/xfs/xfs_dinode.h
+++ b/fs/xfs/xfs_dinode.h
@@ -228,7 +228,14 @@ static inline void xfs_dinode_put_rdev(s
 #define XFS_DIFLAG_FILESTREAM    (1 << XFS_DIFLAG_FILESTREAM_BIT)
 
 #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