Copyright © 2012 Red Hat, Inc. All rights reserved. Moderate 2026-09-11T00:00:00 kernel: apparmor: fix cred UAF caused by begin_current_label_crit_section() 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H CWE-825

In the Linux kernel, the following vulnerability has been resolved: apparmor: fix cred UAF caused by begin_current_label_crit_section() AppArmor's begin_current_label_crit_section() is a scary function called from lots of LSM hooks (in particular VFS/socket-related ones) that checks if the label referenced by the current creds is marked FLAG_STALE, and if so, attempts to use aa_replace_current_label() to replace the creds with an updated version that uses a new label. The first problem with this is that it would directly lead to UAF of `struct cred` if anything in the kernel takes a pointer to the current creds and accesses these past a security hook invocation that replaces creds, like so: ``` const struct cred *cred = current_cred(); alloc_file_pseudo(...); uid_t uid = cred->euid; ``` I don't know if anything in the kernel actually does this, but I think it is very surprising that this pattern could lead to UAF. The second problem is that things go wrong when aa_replace_current_label() runs with overridden credentials. aa_replace_current_label() bails out if `current_cred() != current_real_cred()` (mirroring the check in proc_pid_attr_write()), but this check can't actually reliably detect overridden credentials because the overridden creds can be the same as the objective creds. So in approximately the following scenario, things go wrong: 1. task begins with <creds A> (as both objective and subjective creds), with refcount=2 2. task grabs an extra reference on <creds A> for overriding 3. task calls override_creds(<creds A>), which returns a pointer to the old subjective creds (<creds A>) 4. task enters AppArmor LSM hook 5. AppArmor checks that objective/subjective creds are equal 6. AppArmor replaces both cred pointers with <creds B> and drops 2 refs on <creds A> 7. task leaves AppArmor LSM hook 8. task calls revert_creds(<creds A>) 9. now task->cred is <creds A> while task->real_cred is <creds B>, but the task_struct logically holds two references to <creds B> 10. another task drops the extra reference on <creds A> that was used for overriding, refcount drops to 0 11. now task->real_cred points to freed creds At this point, any access to current_cred() will be UAF. I have a test case where I run aa-disable on a profile while a process using that profile is blocked on splice() from a FUSE passthrough file into a full pipe; after the profile update, the pipe becomes empty, splice() resumes, the credentials go out of sync, and a subsequent getuid() syscall results in a KASAN UAF splat. To fix this, instead of directly replacing creds, do it via task_work that will run at the end of the current syscall. (The point in time at which the cred replacement happens should have no correctness impact; it is just a performance optimization to avoid unnecessarily touching the refcount of the new label.) Note that AppArmor still performs direct cred replacements in the sb_pivotroot LSM hook after this change, and that direct cred replacements can still happen in VFS ->write() callbacks via proc_pid_attr_write(). There are two options for what to do with aa_dup_task_ctx(): Either explicitly reset new->label_replacement_pending after the entire aa_task_ctx has been copied, or switch to manually copying members over. I am switching to manually copying members over because that should make bugs more obvious.

A flaw was found in the Linux kernel's AppArmor security module. The begin_current_label_crit_section() function, used in various Linux Security Module (LSM) hooks, attempts to replace user credentials when a security label becomes stale. This process can lead to a Use-After-Free (UAF) vulnerability if the old credentials are still referenced or if the replacement occurs with overridden credentials. A local attacker could potentially exploit this flaw to achieve privilege escalation or cause a denial of service.

Red Hat Enterprise Linux 10 Not affected kernel Red Hat Enterprise Linux 10 Not affected kernel-rt Red Hat Enterprise Linux 6 Not affected kernel Red Hat Enterprise Linux 7 Not affected kernel Red Hat Enterprise Linux 7 Not affected kernel-rt Red Hat Enterprise Linux 8 Not affected kernel Red Hat Enterprise Linux 8 Not affected kernel-rt Red Hat Enterprise Linux 9 Not affected kernel Red Hat Enterprise Linux 9 Not affected kernel-rt Red Hat OpenShift Container Platform 4 Not affected openshift/ose-rhel-coreos-8 Red Hat OpenShift Container Platform 4 Not affected openshift/ose-rhel-coreos-9 https://www.cve.org/CVERecord?id=CVE-2026-89762 https://nvd.nist.gov/vuln/detail/CVE-2026-89762 https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89762.mbox