CVE-2026-31702

Summary

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.

Affected Software

Weaknesses

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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