NTFS: Fix invalid pointer dereference in ntfs_mft_record_alloc().

In ntfs_mft_record_alloc() when mapping the new extent mft record with
map_extent_mft_record() we overwrite @m with the return value and on
error, we then try to use the old @m but that is no longer there as @m
now contains an error code instead so we crash when dereferencing the
error code as if it were a pointer.

The simple fix is to use a temporary variable to store the return value
thus preserving the original @m for later use.  This is a backport from
the commercial Tuxera-NTFS driver and is well tested...

Thanks go to Julia Lawall for pointing this out (whilst I had fixed it
in the commercial driver I had failed to fix it in the Linux kernel).

Signed-off-by: Anton Altaparmakov <anton@tuxera.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 9 insertions, 4 deletions

diff --git a/Documentation/filesystems/ntfs.txt b/Documentation/filesystems/ntfs.txt
index 6ef8cf3bc9a3..933bc66ccff1 100644
--- a/Documentation/filesystems/ntfs.txt
+++ b/Documentation/filesystems/ntfs.txt
@@ -460,6 +460,8 @@ Note, a technical ChangeLog aimed at kernel hackers is in fs/ntfs/ChangeLog.
 2.1.30:
         - Fix writev() (it kept writing the first segment over and over again
           instead of moving onto subsequent segments).
+        - Fix crash in ntfs_mft_record_alloc() when mapping the new extent mft
+          record failed.
 2.1.29:
         - Fix a deadlock when mounting read-write.
 2.1.28:
diff --git a/fs/ntfs/mft.c b/fs/ntfs/mft.c
index b572b6727181..326e7475a22a 100644
--- a/fs/ntfs/mft.c
+++ b/fs/ntfs/mft.c
@@ -1,7 +1,7 @@
 /**
  * mft.c - NTFS kernel mft record operations. Part of the Linux-NTFS project.
  *
- * Copyright (c) 2001-2006 Anton Altaparmakov
+ * Copyright (c) 2001-2011 Anton Altaparmakov and Tuxera Inc.
  * Copyright (c) 2002 Richard Russon
  *
  * This program/include file is free software; you can redistribute it and/or
@@ -2576,6 +2576,8 @@ mft_rec_already_initialized:
         flush_dcache_page(page);
         SetPageUptodate(page);
         if (base_ni) {
+                MFT_RECORD *m_tmp;
+
                 /*
                  * Setup the base mft record in the extent mft record.  This
                  * completes initialization of the allocated extent mft record
@@ -2588,11 +2590,11 @@ mft_rec_already_initialized:
                  * attach it to the base inode @base_ni and map, pin, and lock
                  * its, i.e. the allocated, mft record.
                  */
-                m = map_extent_mft_record(base_ni, bit, &ni);
-                if (IS_ERR(m)) {
+                m_tmp = map_extent_mft_record(base_ni, bit, &ni);
+                if (IS_ERR(m_tmp)) {
                         ntfs_error(vol->sb, "Failed to map allocated extent "
                                         "mft record 0x%llx.", (long long)bit);
-                        err = PTR_ERR(m);
+                        err = PTR_ERR(m_tmp);
                         /* Set the mft record itself not in use. */
                         m->flags &= cpu_to_le16(
                                         ~le16_to_cpu(MFT_RECORD_IN_USE));
@@ -2603,6 +2605,7 @@ mft_rec_already_initialized:
                         ntfs_unmap_page(page);
                         goto undo_mftbmp_alloc;
                 }
+                BUG_ON(m != m_tmp);
                 /*
                  * Make sure the allocated mft record is written out to disk.
                  * No need to set the inode dirty because the caller is going