CVE-2026-80808ext4: stop retrying saturated xattr cache entries

Official description

In the Linux kernel, the following vulnerability has been resolved: ext4: stop retrying saturated xattr cache entries ext4_xattr_block_set() retries when a cache entry selected for reuse has a saturated reference count after taking the buffer lock. The retry returns to the mbcache lookup without making that entry ineligible, so it can select the same unusable entry indefinitely. A task spinning there can hold the parent directory's i_rwsem and leave concurrent rmdir callers blocked. Normally a reusable entry has a reference count below EXT4_XATTR_REFCOUNT_MAX because the count and MBE_REUSABLE_B are updated under the same buffer lock. A corrupted filesystem can violate that invariant. The syzbot reproducer reports allocator and xattr corruption before triggering this retry loop. Check the untrusted on-disk count before incrementing it, avoiding overflow, and clear MBE_REUSABLE_B when it is already saturated. The next lookup then skips the entry that was just proven unusable. This mirrors the normal transition at EXT4_XATTR_REFCOUNT_MAX; the release path marks the entry reusable again on the exact 1024-to-1023 transition. Using the same QEMU harness and guest parameters, current unpatched Linux hung in 6 of 8 420-second trials with the do_rmdir signature; representative NMI backtraces caught the owner spinning in ext4_xattr_block_set(). The patched kernel completed 28 of 28 trials without a hung-task report; the final twelve trials exercised the reviewed overflow-safe form of the change. syzbot's patch testing also completed without reproducing the hang.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/119a2f053242ed75bdd2ebc95baf3ae7db6ccacf
• https://git.kernel.org/stable/c/61631352a5b405c89be579de00903b72e6888aa4
• https://git.kernel.org/stable/c/8865cd664484517703df5c18a965dc3227572b87
• https://git.kernel.org/stable/c/a40c45268f4358207aa9c53764fed2e05f62986a
• https://git.kernel.org/stable/c/889ec86464d261f026f6c334040cfc6c58c99d58
• https://git.kernel.org/stable/c/4902a5cba21aeaf91e6b29e20e0967a5f6abdcd9
• https://git.kernel.org/stable/c/55ee6533c1db7f7656fa8dd19637f3f8b8c08dc5
• https://git.kernel.org/stable/c/dbd4aea175ad3c46436acb251e817b4374628072
• https://git.kernel.org/stable/c/54b6bd40898de7906acb2bccc9a96d1b8e6b4323