In the Linux kernel, the following vulnerability has been resolved:

ocfs2: fix races between hole punching and AIO+DIO

After commit ocfs2: return real error code in ocfs2_dio_wr_get_block, fstests/generic/300 become from always failed to sometimes failed:

======================================================================== [ 473.293420 ] run fstests generic/300

[ 475.296983 ] JBD2: Ignoring recovery information on journal [ 475.302473 ] ocfs2: Mounting device (253,1) on (node local, slot 0) with ordered data mode. [ 494.290998 ] OCFS2: ERROR (device dm-1): ocfs2_change_extent_flag: Owner 5668 has an extent at cpos 78723 which can no longer be found [ 494.291609 ] On-disk corruption discovered. Please run fsck.ocfs2 once the filesystem is unmounted. [ 494.292018 ] OCFS2: File system is now read-only. [ 494.292224 ] (kworker/19:11,2628,19):ocfs2_mark_extent_written:5272 ERROR: status = -30 [ 494.292602 ] (kworker/19:11,2628,19):ocfs2_dio_end_io_write:2374 ERROR: status = -3 fio: io_u error on file /mnt/scratch/racer: Read-only file system: write offset=460849152, buflen=131072

In __blockdev_direct_IO, ocfs2_dio_wr_get_block is called to add unwritten extents to a list. extents are also inserted into extent tree in ocfs2_write_begin_nolock. Then another thread call fallocate to puch a hole at one of the unwritten extent. The extent at cpos was removed by ocfs2_remove_extent(). At end io worker thread, ocfs2_search_extent_list found there is no such extent at the cpos.

T1                        T2                T3
                          inode lock
                            ...
                            insert extents
                            ...
                          inode unlock

ocfs2_fallocate __ocfs2_change_file_space inode lock lock ip_alloc_sem ocfs2_remove_inode_range inode ocfs2_remove_btree_range ocfs2_remove_extent ^—remove the extent at cpos 78723 … unlock ip_alloc_sem inode unlock ocfs2_dio_end_io ocfs2_dio_end_io_write lock ip_alloc_sem ocfs2_mark_extent_written ocfs2_change_extent_flag ocfs2_search_extent_list ^—failed to find extent … unlock ip_alloc_sem

In most filesystems, fallocate is not compatible with racing with AIO+DIO, so fix it by adding to wait for all dio before fallocate/punch_hole like ext4.

Weakness

The product contains a concurrent code sequence that 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 operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it 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:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. 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.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/050ce8af6838c71e872e982b50d3f1bec21da40e
• https://git.kernel.org/stable/c/117b9c009b72a6c2ebfd23484354dfee2d9570d2
• https://git.kernel.org/stable/c/38825ff9da91d2854dcf6d9ac320a7e641e10f25
• https://git.kernel.org/stable/c/3c26b5d21b1239e9c7fd31ba7d9b2d7bdbaa68d9
• https://git.kernel.org/stable/c/3c361f313d696df72f9bccf058510e9ec737b9b1
• https://git.kernel.org/stable/c/952b023f06a24b2ad6ba67304c4c84d45bea2f18
• https://git.kernel.org/stable/c/e8e2db1adac47970a6a9225f3858e9aa0e86287f
• https://git.kernel.org/stable/c/ea042dc2bea19d72e37c298bf65a9c341ef3fff3
• https://git.kernel.org/stable/c/050ce8af6838c71e872e982b50d3f1bec21da40e
• https://git.kernel.org/stable/c/117b9c009b72a6c2ebfd23484354dfee2d9570d2
• https://git.kernel.org/stable/c/38825ff9da91d2854dcf6d9ac320a7e641e10f25
• https://git.kernel.org/stable/c/3c26b5d21b1239e9c7fd31ba7d9b2d7bdbaa68d9
• https://git.kernel.org/stable/c/3c361f313d696df72f9bccf058510e9ec737b9b1
• https://git.kernel.org/stable/c/952b023f06a24b2ad6ba67304c4c84d45bea2f18
• https://git.kernel.org/stable/c/e8e2db1adac47970a6a9225f3858e9aa0e86287f
• https://git.kernel.org/stable/c/ea042dc2bea19d72e37c298bf65a9c341ef3fff3