Vulnerability: In the Linux kernel before 4.15, fs/ocfs2/aops.c omits use of a semaphore and consequently has a race condition for access to the extent tree during read operations in DIRECT mode, which allows local users to cause a denial of service (BUG) by modifying a certain e_cpos field.
Vulnerability ID: CVE-2017-18224
CWE: CWE-362
Severity score: 2.9
Project: https://github.com/torvalds/linux
Published: 2018-03-12
File: fs/ocfs2/aops.c
Commit message: ocfs2: ip_alloc_sem should be taken in ocfs2_get_block()

ip_alloc_sem should be taken in ocfs2_get_block() when reading file in
DIRECT mode to prevent concurrent access to extent tree with
ocfs2_dio_end_io_write(), which may cause BUGON in the following
situation:

read file 'A'                                  end_io of writing file 'A'
vfs_read
 __vfs_read
  ocfs2_file_read_iter
   generic_file_read_iter
    ocfs2_direct_IO
     __blockdev_direct_IO
      do_blockdev_direct_IO
       do_direct_IO
        get_more_blocks
         ocfs2_get_block
          ocfs2_extent_map_get_blocks
           ocfs2_get_clusters
            ocfs2_get_clusters_nocache()
             ocfs2_search_extent_list
              return the index of record which
              contains the v_cluster, that is
              v_cluster > rec[i]->e_cpos.
                                                ocfs2_dio_end_io
                                                 ocfs2_dio_end_io_write
                                                  down_write(&oi->ip_alloc_sem);
                                                  ocfs2_mark_extent_written
                                                   ocfs2_change_extent_flag
                                                    ocfs2_split_extent
                                                     ...
                                                 --> modify the rec[i]->e_cpos, resulting
                                                     in v_cluster < rec[i]->e_cpos.
             BUG_ON(v_cluster < le32_to_cpu(rec->e_cpos))

[alex.chen@huawei.com: v3]
  Link: http://lkml.kernel.org/r/59EF3614.6050008@huawei.com
Link: http://lkml.kernel.org/r/59EF3614.6050008@huawei.com
Fixes: c15471f79506 ("ocfs2: fix sparse file & data ordering issue in direct io")
Signed-off-by: Alex Chen <alex.chen@huawei.com>
Reviewed-by: Jun Piao <piaojun@huawei.com>
Reviewed-by: Joseph Qi <jiangqi903@gmail.com>
Reviewed-by: Gang He <ghe@suse.com>
Acked-by: Changwei Ge <ge.changwei@h3c.com>
Cc: Mark Fasheh <mfasheh@versity.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Commit author: alex chen

Vulnerable version (before fix):
 static int ocfs2_symlink_get_block(struct inode *inode, sector_t iblock, 	return err; }  int ocfs2_get_block(struct inode *inode, sector_t iblock, 		    struct buffer_head *bh_result, int create) { static void ocfs2_dio_free_write_ctx(struct inode *inode,  * called like this: dio->get_blocks(dio->inode, fs_startblk,  * 					fs_count, map_bh, dio->rw == WRITE);  */ static int ocfs2_dio_get_block(struct inode *inode, sector_t iblock, 			       struct buffer_head *bh_result, int create) { 	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb); static int ocfs2_dio_get_block(struct inode *inode, sector_t iblock, 	 * while file size will be changed. 	 */ 	if (pos + total_len <= i_size_read(inode)) { 		down_read(&oi->ip_alloc_sem); 		/* This is the fast path for re-write. */ 		ret = ocfs2_get_block(inode, iblock, bh_result, create);  		up_read(&oi->ip_alloc_sem);  		if (buffer_mapped(bh_result) && 		    !buffer_new(bh_result) && 		    ret == 0) static ssize_t ocfs2_direct_IO(struct kiocb *iocb, struct iov_iter *iter) 		return 0;  	if (iov_iter_rw(iter) == READ) 		get_block = ocfs2_get_block; 	else 		get_block = ocfs2_dio_get_block;  	return __blockdev_direct_IO(iocb, inode, inode->i_sb->s_bdev, 				    iter, get_block,

Fixed version (after fix):
 static int ocfs2_symlink_get_block(struct inode *inode, sector_t iblock, 	return err; }  static int ocfs2_lock_get_block(struct inode *inode, sector_t iblock, 		    struct buffer_head *bh_result, int create) { 	int ret = 0; 	struct ocfs2_inode_info *oi = OCFS2_I(inode);  	down_read(&oi->ip_alloc_sem); 	ret = ocfs2_get_block(inode, iblock, bh_result, create); 	up_read(&oi->ip_alloc_sem);  	return ret; }  int ocfs2_get_block(struct inode *inode, sector_t iblock, 		    struct buffer_head *bh_result, int create) { static void ocfs2_dio_free_write_ctx(struct inode *inode,  * called like this: dio->get_blocks(dio->inode, fs_startblk,  * 					fs_count, map_bh, dio->rw == WRITE);  */ static int ocfs2_dio_wr_get_block(struct inode *inode, sector_t iblock, 			       struct buffer_head *bh_result, int create) { 	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb); static int ocfs2_dio_get_block(struct inode *inode, sector_t iblock, 	 * while file size will be changed. 	 */ 	if (pos + total_len <= i_size_read(inode)) {  		/* This is the fast path for re-write. */ 		ret = ocfs2_lock_get_block(inode, iblock, bh_result, create); 		if (buffer_mapped(bh_result) && 		    !buffer_new(bh_result) && 		    ret == 0) static ssize_t ocfs2_direct_IO(struct kiocb *iocb, struct iov_iter *iter) 		return 0;  	if (iov_iter_rw(iter) == READ) 		get_block = ocfs2_lock_get_block; 	else 		get_block = ocfs2_dio_wr_get_block;  	return __blockdev_direct_IO(iocb, inode, inode->i_sb->s_bdev, 				    iter, get_block,

Patch:
patch: @@ -134,6 +134,19 @@ static int ocfs2_symlink_get_block(struct inode *inode, sector_t iblock,
 	return err;
 }
 
+static int ocfs2_lock_get_block(struct inode *inode, sector_t iblock,
+		    struct buffer_head *bh_result, int create)
+{
+	int ret = 0;
+	struct ocfs2_inode_info *oi = OCFS2_I(inode);
+
+	down_read(&oi->ip_alloc_sem);
+	ret = ocfs2_get_block(inode, iblock, bh_result, create);
+	up_read(&oi->ip_alloc_sem);
+
+	return ret;
+}
+
 int ocfs2_get_block(struct inode *inode, sector_t iblock,
 		    struct buffer_head *bh_result, int create)
 {
@@ -2128,7 +2141,7 @@ static void ocfs2_dio_free_write_ctx(struct inode *inode,
  * called like this: dio->get_blocks(dio->inode, fs_startblk,
  * 					fs_count, map_bh, dio->rw == WRITE);
  */
-static int ocfs2_dio_get_block(struct inode *inode, sector_t iblock,
+static int ocfs2_dio_wr_get_block(struct inode *inode, sector_t iblock,
 			       struct buffer_head *bh_result, int create)
 {
 	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb);
@@ -2154,12 +2167,9 @@ static int ocfs2_dio_get_block(struct inode *inode, sector_t iblock,
 	 * while file size will be changed.
 	 */
 	if (pos + total_len <= i_size_read(inode)) {
-		down_read(&oi->ip_alloc_sem);
-		/* This is the fast path for re-write. */
-		ret = ocfs2_get_block(inode, iblock, bh_result, create);
-
-		up_read(&oi->ip_alloc_sem);
 
+		/* This is the fast path for re-write. */
+		ret = ocfs2_lock_get_block(inode, iblock, bh_result, create);
 		if (buffer_mapped(bh_result) &&
 		    !buffer_new(bh_result) &&
 		    ret == 0)
@@ -2424,9 +2434,9 @@ static ssize_t ocfs2_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 		return 0;
 
 	if (iov_iter_rw(iter) == READ)
-		get_block = ocfs2_get_block;
+		get_block = ocfs2_lock_get_block;
 	else
-		get_block = ocfs2_dio_get_block;
+		get_block = ocfs2_dio_wr_get_block;
 
 	return __blockdev_direct_IO(iocb, inode, inode->i_sb->s_bdev,
 				    iter, get_block,
path: fs/ocfs2/aops.c