CVE-2026-63827

In the Linux kernel, the following vulnerability has been resolved:

apparmor: fix use-after-free in rawdata dedup loop

aa_replace_profiles() walks ns->rawdata_list to dedup the incoming policy blob against entries already attached to existing profiles. Per the kernel-doc on struct aa_loaddata, list membership does not hold a reference: profiles hold pcount, and when the last pcount drops, do_ploaddata_rmfs() is queued on a workqueue that takes ns->lock and removes the entry. Between dropping the last pcount and the workqueue running, an entry remains on the list with pcount == 0.

aa_get_profile_loaddata() is an unconditional kref_get() on pcount, so when the dedup loop hits such an entry, refcount hardening reports

refcount_t: addition on 0; use-after-free.

inside aa_replace_profiles(), and the poisoned counter then trips saturated and underflow warnings on the subsequent uses of the same loaddata.

Before commit a0b7091c4de4 (apparmor: fix race on rawdata dereference) the dedup path used a get_unless_zero-style helper on a single counter, so the existing if (tmp) guard was meaningful. The split-refcount refactor introduced aa_get_profile_loaddata(), which has plain kref_get() semantics, and the guard quietly became a no-op.

Introduce aa_get_profile_loaddata_not0(), matching the existing _not0 convention used by aa_get_profile_not0(), and use it for the rawdata_list dedup lookup so dying entries are skipped.

Reproduced on x86_64 with v7.1-rc5 in QEMU+KVM running Ubuntu 24.04 + stress-ng 0.17.06:

stress-ng –apparmor 1 –klog-check –timeout 60s

Without this patch the three refcount_t warnings fire within a few seconds. With it the same 60 s run is clean. Coverage is a smoke-test only; a longer soak with CONFIG_KASAN, CONFIG_KCSAN and CONFIG_PROVE_LOCKING would be welcome from anyone with the cycles.

Affected Software

References

• https://git.kernel.org/stable/c/15fd83a1e42ede15070968806bb6c8b1a5170688
• https://git.kernel.org/stable/c/5e34fa9f6f7cd688ae153fff13139a5cf2d42339
• https://git.kernel.org/stable/c/6f060496d03e4dc560a40f73770bd08335cb7a27
• https://git.kernel.org/stable/c/a7a2890028f16e5b0af0bb005d80fcb32559cca3
• https://git.kernel.org/stable/c/c3ca2631073b2cef06824fd2bfc452ff7a1023de
• https://git.kernel.org/stable/c/ce261a20b41db522e320a41bbf1292bf85af66df