ext4: fix possible use after free with metadata csum

Commit fa77dcfafeaa introduces block bitmap checksum calculation into
ext4_new_inode() in the case that block group was uninitialized.
However we brelse() the bitmap buffer before we attempt to checksum it
so we have no guarantee that the buffer is still there.

Fix this by releasing the buffer after the possible checksum
computation.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Acked-by: Darrick J. Wong <darrick.wong@oracle.com>
Cc: stable@vger.kernel.org

diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
index 3a100e7a62a8343d31912a423330e11ea40e2e2a..c7efa88d714922a2cdd877ecee4b464f4221dac9 100644 (file)
--- a/fs/ext4/ialloc.c
+++ b/fs/ext4/ialloc.c
@@ -762,7 +762,6 @@ got:
 
                BUFFER_TRACE(block_bitmap_bh, "dirty block bitmap");
                err = ext4_handle_dirty_metadata(handle, NULL, block_bitmap_bh);
-               brelse(block_bitmap_bh);
 
                /* recheck and clear flag under lock if we still need to */
                ext4_lock_group(sb, group);
@@ -775,6 +774,7 @@ got:
                        ext4_group_desc_csum_set(sb, group, gdp);
                }
                ext4_unlock_group(sb, group);
+               brelse(block_bitmap_bh);
 
                if (err)
                        goto fail;