CVE-2017-18224 (published 2018-03-12T03:29Z)

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.

Severity: LOW
CVSS v2 base score: 1.9 (AV:L/AC:M/Au:N/C:N/I:N/A:P)
CVSS v3 base score: 4.7, MEDIUM (CVSS:3.0/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: CWE-362
cwe_name: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
extended_description: 
            This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.
            A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc.
            A race condition violates these properties, which are closely related:
               
                  Exclusivity - the code sequence is given exclusive access to the shared resource, i.e., no other code sequence can modify properties of the shared resource before the original sequence has completed execution.
                  Atomicity - the code sequence is behaviorally atomic, i.e., no other thread or process can concurrently execute the same sequence of instructions (or a subset) against the same resource.
               
            A race condition exists when an "interfering code sequence" can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single "x++" statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x).
            The interfering code sequence could be "trusted" or "untrusted." A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: alex chen
author_date: 2017-11-15 17:31:44-08:00
msg: 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>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: aops.c
change_type: ModificationType.MODIFY
diff: @@ -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,

programming_language: C