CVE-2026-63827 in Linux
Summary
by MITRE • 07/19/2026
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.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability resides in the Linux kernel's AppArmor security module where a use-after-free condition occurs during policy blob deduplication operations. This flaw manifests specifically within the aa_replace_profiles() function which processes the ns->rawdata_list to identify and eliminate duplicate policy data against already loaded profiles. The underlying technical issue stems from improper reference counting handling when traversing list entries that may have been marked for deletion but not yet fully removed from the system. According to kernel documentation, struct aa_loaddata list membership does not maintain an active reference, with actual reference tracking handled through pcount counters on profiles. When the final pcount decrements to zero, the do_ploaddata_rmfs() cleanup function is scheduled via workqueue to acquire ns->lock and remove the entry from the list. However, a temporal window exists between the decrement of pcount and the workqueue execution where entries remain listed with pcount equal to zero, creating an exploitable race condition.
The operational impact of this vulnerability is significant as it triggers kernel refcount_t warnings that indicate potential memory corruption and use-after-free scenarios. The aa_get_profile_loaddata() function performs an unconditional kref_get() operation on pcount without proper validation against zero values, causing the deduplication loop to process entries with invalid reference counts. This leads to immediate refcount_t addition operations on zero values which are flagged by kernel refcount hardening mechanisms as use-after-free conditions. The subsequent "saturated" and "underflow" warnings occur when the same poisoned loaddata structures are referenced again, potentially leading to system instability or memory corruption. This vulnerability directly relates to CWE-416 Use After Free and CWE-1321 Improper Reference Counting as it demonstrates a classic race condition in reference management where objects are accessed after their reference count has reached zero.
The mitigation implemented involves introducing a new function aa_get_profile_loaddata_not0() that follows the existing _not0 convention established by aa_get_profile_not0(). This approach ensures that during rawdata_list deduplication lookups, entries with zero pcount values are explicitly skipped rather than processed, preventing access to potentially freed memory structures. The fix addresses the root cause identified in the split-refcount refactor where the previous get_unless_zero-style helper with meaningful "if (tmp)" guards was replaced with plain kref_get() semantics that no longer provided adequate protection. Testing confirmed that without this patch, stress-ng workloads with AppArmor enabled trigger three refcount_t warnings within seconds, while the patched version maintains clean operation during extended testing periods. The solution aligns with ATT&CK technique T1490 Inhibit System Recovery by preventing potential system crashes or instability resulting from memory corruption, and follows best practices for reference counting in concurrent systems as recommended by kernel security guidelines and the Linux kernel documentation on proper object lifecycle management.