# CVE-2025-38415: Squashfs: check return result of sb\_min\_blocksize

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2025-07-25 · Updated: 2026-06-18

## Affected

- Linux: version range not reported
- Linux: 2.6.29

## 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().

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
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## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/db7096ea160e40d78c67fce52e7cc51bde049497
- https://git.kernel.org/stable/c/549f9e3d7b60d53808c98b9fde49b4f46d0524a5
- https://git.kernel.org/stable/c/5c51aa862cbeed2f3887f0382a2708956710bd68
- https://git.kernel.org/stable/c/6abf6b78c6fb112eee495f5636ffcc350dd2ce25
- https://git.kernel.org/stable/c/4f99357dadbf9c979ad737156ad4c37fadf7c56b
- https://git.kernel.org/stable/c/0aff95d9bc7fb5400ca8af507429c4b067bdb425
- https://git.kernel.org/stable/c/295ab18c2dbce8d0ac6ecf7c5187e16e1ac8b282
- https://git.kernel.org/stable/c/734aa85390ea693bb7eaf2240623d41b03705c84

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