CVE-2025-38415: Squashfs: check return result of sb_min_blocksize

Vulnerability summary

CVE-2025-38415 is a high (CVSS 7.8) Out-of-bounds Write vulnerability in debian debian_linux. EPSS estimates a 0.22% chance of exploitation in the next 30 days, higher than 12.3% of scored vulnerabilities.

Timeline

1. 25-07-2025
   PublishedVulnerability first disclosed.
2. 18-06-2026
   Last modifiedVulnerability information updated.

Description

In the Linux kernel, the following vulnerability has been resolved: Squashfs: check return result of sb_min_blocksize Syzkaller reports an "UBSAN: shift-out-of-bounds in squashfs_bio_read" bug. Syzkaller forks multiple processes which after mounting the Squashfs filesystem, issues an ioctl("/dev/loop0", LOOP_SET_BLOCK_SIZE, 0x8000). Now if this ioctl occurs at the same time another process is in the process of mounting a Squashfs filesystem on /dev/loop0, the failure occurs. When this happens the following code in squashfs_fill_super() fails. ---- msblk->devblksize = sb_min_blocksize(sb, SQUASHFS_DEVBLK_SIZE); msblk->devblksize_log2 = ffz(~msblk->devblksize); ---- sb_min_blocksize() returns 0, which means msblk->devblksize is set to 0. As a result, ffz(~msblk->devblksize) returns 64, and msblk->devblksize_log2 is set to 64. This subsequently causes the UBSAN: shift-out-of-bounds in fs/squashfs/block.c:195:36 shift exponent 64 is too large for 64-bit type 'u64' (aka 'unsigned long long') This commit adds a check for a 0 return by sb_min_blocksize().

• CWE-787Out-of-bounds WriteThe product writes data past the end, or before the beginning, of the intended buffer.