CVE-2021-47114

/// Technical description

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

ocfs2: fix data corruption by fallocate

When fallocate punches holes out of inode size, if original isize is in

the middle of last cluster, then the part from isize to the end of the

cluster will be zeroed with buffer write, at that time isize is not yet

updated to match the new size, if writeback is kicked in, it will invoke

ocfs2_writepage()->block_write_full_page() where the pages out of inode

size will be dropped. That will cause file corruption. Fix this by

zero out eof blocks when extending the inode size.

Running the following command with qemu-image 4.2.1 can get a corrupted

coverted image file easily.

qemu-img convert -p -t none -T none -f qcow2 $qcow_image \

-O qcow2 -o compat=1.1 $qcow_image.conv

The usage of fallocate in qemu is like this, it first punches holes out

of inode size, then extend the inode size.

fallocate(11, FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE, 2276196352, 655

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References