# CVE-2026-31702: f2fs: fix use-after-free of sbi in f2fs\_compress\_write\_end\_io()

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2026-05-01 · Updated: 2026-06-14

## Affected

- Linux: version range not reported
- Linux: 5.6

## Description

In the Linux kernel, the following vulnerability has been resolved: f2fs: fix use-after-free of sbi in f2fs_compress_write_end_io() In f2fs_compress_write_end_io(), dec_page_count(sbi, type) can bring the F2FS_WB_CP_DATA counter to zero, unblocking f2fs_wait_on_all_pages() in f2fs_put_super() on a concurrent unmount CPU. The unmount path then proceeds to call f2fs_destroy_page_array_cache(sbi), which destroys sbi->page_array_slab via kmem_cache_destroy(), and eventually kfree(sbi). Meanwhile, the bio completion callback is still executing: when it reaches page_array_free(sbi, ...), it dereferences sbi->page_array_slab — a destroyed slab cache — to call kmem_cache_free(), causing a use-after-free. This is the same class of bug as CVE-2026-23234 (which fixed the equivalent race in f2fs_write_end_io() in data.c), but in the compressed writeback completion path that was not covered by that fix. Fix this by moving dec_page_count() to after page_array_free(), so that all sbi accesses complete before the counter decrement that can unblock unmount. For non-last folios (where atomic_dec_return on cic->pending_pages is nonzero), dec_page_count is called immediately before returning — page_array_free is not reached on this path, so there is no post-decrement sbi access. For the last folio, page_array_free runs while the F2FS_WB_CP_DATA counter is still nonzero (this folio has not yet decremented it), keeping sbi alive, and dec_page_count runs as the final operation.

## 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/57bc678f36ac03281e877c6b84877b43f964143f
- https://git.kernel.org/stable/c/ef57cd3329b40c739b9a2e1a8a21ecc4171c6280
- https://git.kernel.org/stable/c/f5154cf3ce1c8193f0c1891d3769f62740cfe6fe
- https://git.kernel.org/stable/c/c76cf339b87975ae5b2c06d2d774d5667d25a12a
- https://git.kernel.org/stable/c/2c97dcb6147c8f7f25c629b93be1e69617de5d4a
- https://git.kernel.org/stable/c/39d4ee19c1e7d753dd655aebee632271b171f43a

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Page: https://cve.now/CVE-2026-31702
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