ext4: stop retrying saturated xattr cache entries

Overview

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

• Linux >= 1a56cd972ce121b6cf2517a47a578782bbd2ec95 < 119a2f053242ed75bdd2ebc95baf3ae7db6ccacf
• Linux >= 1be97463696c7291a3e1547614e96432b0bd3add < 61631352a5b405c89be579de00903b72e6888aa4
• Linux >= 65f8b80053a1b2fd602daa6814e62d6fa90e5e9b < 8865cd664484517703df5c18a965dc3227572b87
• Linux >= 65f8b80053a1b2fd602daa6814e62d6fa90e5e9b < a40c45268f4358207aa9c53764fed2e05f62986a
• Linux >= 65f8b80053a1b2fd602daa6814e62d6fa90e5e9b < 889ec86464d261f026f6c334040cfc6c58c99d58
• Linux >= 65f8b80053a1b2fd602daa6814e62d6fa90e5e9b < 4902a5cba21aeaf91e6b29e20e0967a5f6abdcd9
• Linux >= 65f8b80053a1b2fd602daa6814e62d6fa90e5e9b < 55ee6533c1db7f7656fa8dd19637f3f8b8c08dc5
• Linux >= 65f8b80053a1b2fd602daa6814e62d6fa90e5e9b < dbd4aea175ad3c46436acb251e817b4374628072
• Linux >= 65f8b80053a1b2fd602daa6814e62d6fa90e5e9b < 54b6bd40898de7906acb2bccc9a96d1b8e6b4323
• Linux 98953044b3cdb2cb7d82e7365b659e2ed4f4ca4d
• Linux af8ecc8d20e72130771cc076bce7fcf17ccda6c4
• Linux c6fac5cf5a5098732623bcd00a8a3eb9f5465144
• Linux 96fa141fa295ae9428da73c56c9852053b575c04
• Linux >= 5.10.163 < 5.10.267
• Linux >= 5.15.61 < 5.15.218
• Linux >= 4.19.270 < 4.20
• Linux >= 5.4.229 < 5.5
• Linux >= 5.18.18 < 5.19
• Linux >= 5.19.2 < 5.20
• Linux 6.0

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.