CVE-2026-93100
UnknownSummary
A use-after-free vulnerability exists in the Linux kernel's rdtgroup_kn_put() function within the fs/resctrl subsystem. The rdtgroup structure can be freed by two racing mechanisms, leading to use-after-free and potentially double-free. The fix replaces atomic_dec_and_test() with atomic_dec_and_mutex_lock() to ensure mutual exclusion with bulk free paths.
Risk Assessment
The vulnerability can cause kernel memory corruption, system crash (panic), or potentially arbitrary code execution with kernel privileges. It affects systems with the resctrl subsystem active (e.g., servers using cache resource control features).
Recommendation
Update the Linux kernel to a version containing the fix for CVE-2026-93100. If updating is not possible, consider restricting access to resctrl interfaces or disabling the resctrl subsystem if not in use.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: fs/resctrl: Prevent use-after-free in rdtgroup_kn_put() A struct rdtgroup is reference counted via rdtgroup::waitcount. Callers that need the structure to remain valid across a sleep (while waiting on acquiring rdtgroup_mutex) take a reference with rdtgroup_kn_get() and release it with rdtgroup_kn_put(). The release path is intended to serve as the fallback freer: if the count drops to zero and the group has already been marked RDT_DELETED, rdtgroup_kn_put() frees the structure. The bulk teardown paths free_all_child_rdtgrp() and rmdir_all_sub() resulting from a resctrl directory remove or resctrl fs unmount act as the primary freer: they hold rdtgroup_mutex and free each rdtgroup whose waitcount is zero, otherwise they set RDT_DELETED and leave the freeing to the last waiter. These two freers race. rdtgroup_kn_put() commits waitcount == 0 with atomic_dec_and_test() outside rdtgroup_mutex, then reads rdtgroup::flags. Between those two operations a concurrent caller of free_all_child_rdtgrp() or rmdir_all_sub() (which holds the mutex) can observe waitcount == 0 via atomic_read(), call rdtgroup_remove(), and kfree() the structure. The subsequent read of rdtgroup::flags in rdtgroup_kn_put() is then a use-after-free, and the structure may even be freed twice if the freed memory happens to satisfy the RDT_DELETED flag check. Replace the bare atomic_dec_and_test() with atomic_dec_and_mutex_lock() so that the decrement-to-zero takes rdtgroup_mutex before the count becomes globally visible. The inspection of rdtgroup::flags then runs under the same mutex held by the bulk freers, making the two paths mutually exclusive. The common case where the count does not reach zero remains lock-free. Defer kernfs_unbreak_active_protection() until after the mutex is dropped since kernfs active protections functionally wrap rdtgroup_mutex. Remove resource group, which in turn drops its kernfs reference, after kernfs protection is restored. [ bp: Split the commit messsages into smaller, easier-parseable paragraphs. ]
Vulnerability data from NVD (NIST) · CISA KEV · EPSS