jfs: Fix inconsistency between memory allocation and ea_buf->max_size

CVE-2018-12233

The code is assuming the buffer is max_size length, but we weren't
allocating enough space for it.
Signed-off-by: Shankara Pailoor <shankarapailoor@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
(cherry picked from commit 92d34134)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/jfs/xattr.c

@@ -493,15 +493,17 @@ static int ea_get(struct inode *inode, struct ea_buffer *ea_buf, int min_size)
 	if (size > PSIZE) {
 		/*
 		 * To keep the rest of the code simple.  Allocate a
-		 * contiguous buffer to work with
+		 * contiguous buffer to work with. Make the buffer large
+		 * enough to make use of the whole extent.
 		 */
-		ea_buf->xattr = kmalloc(size, GFP_KERNEL);
+		ea_buf->max_size = (size + sb->s_blocksize - 1) &
+		    ~(sb->s_blocksize - 1);
+
+		ea_buf->xattr = kmalloc(ea_buf->max_size, GFP_KERNEL);
 		if (ea_buf->xattr == NULL)
 			return -ENOMEM;
 
 		ea_buf->flag = EA_MALLOC;
-		ea_buf->max_size = (size + sb->s_blocksize - 1) &
-		    ~(sb->s_blocksize - 1);
 
 		if (ea_size == 0)
 			return 0;