AS-2026-016: Linux Kernel (Bad Epoll, pedit COW)

2026-08-03

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Statement

A Linux kernel local privilege escalation vulnerability, named "Bad Epoll" (with CVE-2026-46242), has been identified in the Linux kernel's eventpoll / epoll subsystem. If the vulnerability is exploited, it allows a local unprivileged attacker to corrupt kernel memory and escalate privileges to root on affected Linux systems.

CVE-2026-46331, also known as "pedit COW", is a Local Privilege Escalation (LPE) vulnerability in the Linux kernel's traffic control (tc) subsystem. If the vulnerability is exploited, it allows any unprivileged local user to inject payloads into memory-cached binaries to gain root access.

• CVE-2026-46242 and CVE-2026-46331 affects certain ASUSTOR products have Linux kernel versions higher than 5.18 and ADM 5.0.
• Linux Kernel Patch has been updated on ADM 5.1.4.RJV2 to resolve the issues.

Affected Products

Product	Severity	Fixed Release Availability
ADM 5.0 and 5.1	Moderate	Upgrade to ADM 5.1.4.RJV2 or above.

NOT affected NAS models:

• Drivestor (AS1102T, AS1104T)
• Drivestor Lite (AS1102TL)
• Drivestor Gen2 (AS1202T, AS1204T)
• Drivestor Pro (AS3302T, AS3304T)
• Drivestor Pro Gen2 (AS3302T v2, AS3304T v2)

Detail

• CVE-2026-46242
  • Severity: High
  • CVSS3.1 Base Score: 7.8
  • CVSS3.1 Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  • In the Linux kernel, the following vulnerability has been resolved: eventpoll: fix ep_remove struct eventpoll / struct file UAF ep_remove() (via ep_remove_file()) cleared file->f_ep under file->f_lock but then kept using @file inside the critical section (is_file_epoll(), hlist_del_rcu() through the head, spin_unlock). A concurrent __fput() taking the eventpoll_release() fastpath in that window observed the transient NULL, skipped eventpoll_release_file() and ran to f_op->release / file_free(). For the epoll-watches-epoll case, f_op->release is ep_eventpoll_release() -> ep_clear_and_put() -> ep_free(), which kfree()s the watched struct eventpoll. Its embedded ->refs hlist_head is exactly where epi->fllink.pprev points, so the subsequent hlist_del_rcu()'s "*pprev = next" scribbles into freed kmalloc-192 memory. In addition, struct file is SLAB_TYPESAFE_BY_RCU, so the slot backing @file could be recycled by alloc_empty_file() -- reinitializing f_lock and f_ep -- while ep_remove() is still nominally inside that lock. The upshot is an attacker-controllable kmem_cache_free() against the wrong slab cache. Pin @file via epi_fget() at the top of ep_remove() and gate the critical section on the pin succeeding. With the pin held @file cannot reach refcount zero, which holds __fput() off and transitively keeps the watched struct eventpoll alive across the hlist_del_rcu() and the f_lock use, closing both UAFs. If the pin fails @file has already reached refcount zero and its __fput() is in flight. Because we bailed before clearing f_ep, that path takes the eventpoll_release() slow path into eventpoll_release_file() and blocks on ep->mtx until the waiter side's ep_clear_and_put() drops it. The bailed epi's share of ep->refcount stays intact, so the trailing ep_refcount_dec_and_test() in ep_clear_and_put() cannot free the eventpoll out from under eventpoll_release_file(); the orphaned epi is then cleaned up there. A successful pin also proves we are not racing eventpoll_release_file() on this epi, so drop the now-redundant re-check of epi->dying under f_lock. The cheap lockless READ_ONCE(epi->dying) fast-path bailout stays.
• CVE-2026-46331
  • Severity: High
  • CVSS3.1 Base Score: 7.8
  • CVSS3.1 Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  • In the Linux kernel, the following vulnerability has been resolved: net/sched: fix pedit partial COW leading to page cache corruption tcf_pedit_act() computes the COW range for skb_ensure_writable() once before the key loop using tcfp_off_max_hint, but the hint does not account for the runtime header offset added by typed keys. This can leave part of the write region un-COW'd. Fix by moving skb_ensure_writable() inside the per-key loop where the actual write offset is known, and add overflow checking on the offset arithmetic. For negative offsets (e.g. Ethernet header edits at ingress), use skb_cow() to COW the headroom instead. Guard offset_valid() against INT_MIN, where negation is undefined.

Reference

• CVE-2026-46242
• CVE-2026-46331

Revision

Revision	Date	Description
1	2026-07-20	Initial public release.
2	2026-08-03	ADM 5.1.4.RJV2 have been released for fixing the issues.