CVE-2026-90179 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
apparmor: fix deadlock in complain-mode change_hat
The use of change_hat when in complain mode can cause a deadlock when the hat doesn't exist and a new learning profile is created for the missing profile. This is because change_hat() has taken the lock to search the hat list and creating the new learning profile needs to take the lock to add it to the list.
From the bug report:
Originally found in 7.0.0 in LTS ubuntu 26.04 with pam_apparmor + su in complain mode set to change hats. Then verified in newest available vanilla kernel I've compiled to see if still present:
7.2-rc7 vanilla -> affected
checked also some other kernels: 6.18.44 vanilla -> affected 6.12.95 with debian patches -> unaffected
On systems without bug (for example 6.12.95 debian) it just prints:
aa_change_hat rc=0
On systems with bug, the executable always hangs, prints nothing and becomes unkillable. (And once stuck this way, it will cause any further hat changes to also cause the changing process to get stuck)
Then in syslog you can find hint about cause:
kernel: INFO: task hat:3409 blocked for more than 483 seconds. kernel: Not tainted 7.2.0-rc7 #1 kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. kernel: task:hat state:D stack:0 pid:3409 tgid:3409 ppid:2605 task_flags:0x400000 flags:0x00080800 kernel: Call Trace: kernel: <TASK> kernel: __schedule+0x48f/0xfe0 kernel: schedule+0x27/0xa0 kernel: schedule_preempt_disabled+0x15/0x30 kernel: __mutex_lock.constprop.0+0x569/0xa10 kernel: aa_new_learning_profile+0x15f/0x210 kernel: build_change_hat+0x19f/0x3b0 kernel: change_hat.isra.0+0x5dd/0xd60 kernel: aa_change_hat+0x2f3/0x710 kernel: aa_setprocattr_changehat+0x121/0x1f0 kernel: do_setattr+0x28c/0x340 kernel: apparmor_setselfattr+0x20/0x50 kernel: security_setselfattr+0xf6/0x110 kernel: __x64_sys_lsm_set_self_attr+0x53/0x90 kernel: do_syscall_64+0xdd/0x5e0 kernel: ? __mod_memcg_lruvec_state+0xfd/0x260 kernel: ? lruvec_stat_mod_folio+0x8d/0xd0 kernel: ? __folio_mod_stat+0x2d/0x90 kernel: ? map_anon_folio_pte_nopf+0xd1/0x1f0 kernel: ? do_anonymous_page+0x184/0xa10 kernel: ? __handle_mm_fault+0x805/0x870 kernel: ? count_memcg_events+0xef/0x230 kernel: ? handle_mm_fault+0x1f0/0x2f0 kernel: ? do_user_addr_fault+0x2bb/0x7b0 kernel: ? do_syscall_64+0x94/0x5e0 kernel: ? exc_page_fault+0x75/0x160 kernel: entry_SYSCALL_64_after_hwframe+0x76/0x7e kernel: RIP: 0033:0x7f815e134c8d kernel: RSP: 002b:00007fff6df94ea8 EFLAGS: 00000246 ORIG_RAX: 00000000000001cc kernel: RAX: ffffffffffffffda RBX: 0000556d8c81d040 RCX: 00007f815e134c8d kernel: RDX: 0000000000000046 RSI: 0000556d8c81d040 RDI: 0000000000000064 kernel: RBP: 00007fff6df94ef0 R08: 00007f815e212ac8 R09: 000000000000000c kernel: R10: 0000000000000000 R11: 0000000000000246 R12: 0000556d8c81d010 kernel: R13: 0000000000000026 R14: 0000000000000046 R15: 0000000000000064 kernel: </TASK> kernel: INFO: task hat:3409 is blocked on a mutex likely owned by task hat:3409.
To fix the issue, lift the locking out of the core of aa_new_learning_profile(), introduce a wrapper function that takes the lock where needed, and have build_change_hat() call the core function that no longer takes the lock.
In addition fix 4 other issues introduced by commit 32e92764d6f8d ("apparmor: grab ns lock and refresh when looking up changehat child profiles") - aa_get_profile_rcu() was replaced-by: aa_get_profile without the accompanying rcu_dereference_protected() - an extra aa_get_label(label) was introduced at the start of change_hat() without an accompanying aa_put_label() causing a reference count leak. - a reference count leak was introduced in the label_is_stale(label) case, where the newest profile would be leaked instead of the label passed to the function. - a potential UAF when the lookup walks up the tree with new_ns != ns the new label refere ---truncated---
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel AppArmor security module contains a critical concurrency flaw within its complain-mode hat change mechanism that results in process deadlocks and resource leaks. This vulnerability arises from improper lock management during the dynamic creation of learning profiles when an application attempts to transition into a profile hat that does not yet exist. In complain mode, AppArmor is designed to learn new policies by creating temporary learning profiles for missing permissions or transitions. The specific defect occurs in the change_hat function where the code acquires a mutex lock to search the existing list of hats. If the target hat is not found, the system attempts to create a new learning profile and add it to that same list. However, this creation process also requires acquiring the identical mutex lock. Since the calling thread already holds the lock during the initial search phase, attempting to re-acquire the non-recursive lock within the same execution context causes an immediate deadlock. This results in the affected task entering a uninterruptible sleep state, effectively hanging the process and rendering it unkillable by standard signals until the system is rebooted or the kernel parameter for hung tasks is modified.
The operational impact of this vulnerability extends beyond simple service interruption. When a privileged application such as those managed via PAM with AppArmor integration attempts to change hats in complain mode, the resulting deadlock can stall critical authentication and session management processes. This creates a denial-of-service condition where system resources are consumed by blocked tasks without progress. Furthermore, the underlying code changes that introduced this bug also exposed several other severe security issues stemming from commit 32e92764d6f8. These include improper reference counting leading to memory leaks and potential use-after-free vulnerabilities. Specifically, a replacement of aa_get_profile_rcu with aa_get_profile without corresponding rcu_dereference_protected calls violates Read-Copy-Update synchronization protocols, potentially allowing access to freed memory structures if the original profile is destroyed concurrently. Additionally, an extra reference increment on labels was introduced without a matching decrement in certain code paths, causing persistent kernel memory leaks that degrade system stability over time.
From a classification perspective, this vulnerability aligns with CWE-833 Deadlock and CWE-401 Missing Release of Resource after Effective Lifetime due to improper cleanup or error handling. The concurrency issue represents a classic lock ordering violation where the same mutex is acquired recursively by a single thread without proper locking primitives designed for reentrancy. In terms of attack vectors, this could be leveraged in an ATT&CK context under T1499 Endpoint Denial of Service if an attacker can trigger the specific sequence of hat changes that leads to the deadlock state. The use-after-free aspects relate to CWE-416 Use After Free and CWE-787 Out-of-bounds Write, as improper RCU handling and reference counting errors allow for memory corruption scenarios where stale pointers are dereferenced after their underlying objects have been freed or reallocated by other kernel threads.
Mitigation strategies require immediate patching of the AppArmor subsystem to resolve these concurrency and resource management defects. The primary fix involves restructuring the locking logic in aa_new_learning_profile so that it does not attempt to acquire locks already held by its callers, thereby eliminating the deadlock condition. This is achieved by introducing a wrapper function that manages lock acquisition while keeping the core profile creation logic lock-free for internal use. Additionally, developers must correct the reference counting errors by ensuring every aa_get_label call has a corresponding aa_put_label and fixing RCU dereference protections to maintain memory safety during concurrent lookups. System administrators should ensure their kernels are updated with these patches applied, particularly if they utilize complain mode features or dynamic policy learning in production environments. Regular auditing of AppArmor logs for hung task warnings can help detect residual instances of this vulnerability before it leads to complete system hangs.