{"id":66510,"date":"2025-07-18T12:32:02","date_gmt":"2025-07-18T09:02:02","guid":{"rendered":"https:\/\/afaghhosting.net\/blog\/cve-2025-38349-linux-kernel-epoll-use-after-free-vulnerability\/"},"modified":"2025-07-18T12:32:02","modified_gmt":"2025-07-18T09:02:02","slug":"cve-2025-38349-linux-kernel-epoll-use-after-free-vulnerability","status":"publish","type":"post","link":"https:\/\/afaghhosting.net\/blog\/cve-2025-38349-linux-kernel-epoll-use-after-free-vulnerability\/","title":{"rendered":"CVE-2025-38349 – Linux Kernel Epoll Use-After-Free Vulnerability"},"content":{"rendered":"

CVE ID : <\/strong>CVE-2025-38349
\n
\nPublished : <\/strong> 18. Juli 2025 08:15 | 28\u00a0Minuten ago
\n
\nDescription : <\/strong>In the Linux kernel, the following vulnerability has been resolved:<\/p>\n

eventpoll: don’t decrement ep refcount while still holding the ep mutex<\/p>\n

Jann Horn points out that epoll is decrementing the ep refcount and then
\ndoing a<\/p>\n

mutex_unlock(&ep->mtx);<\/p>\n

afterwards. That’s very wrong, because it can lead to a use-after-free.<\/p>\n

That pattern is actually fine for the very last reference, because the
\ncode in question will delay the actual call to “ep_free(ep)” until after
\nit has unlocked the mutex.<\/p>\n

But it’s wrong for the much subtler “next to last” case when somebody
\n*else* may also be dropping their reference and free the ep while we’re
\nstill using the mutex.<\/p>\n

Note that this is true even if that other user is also using the same ep
\nmutex: mutexes, unlike spinlocks, can not be used for object ownership,
\neven if they guarantee mutual exclusion.<\/p>\n

A mutex “unlock” operation is not atomic, and as one user is still
\naccessing the mutex as part of unlocking it, another user can come in
\nand get the now released mutex and free the data structure while the
\nfirst user is still cleaning up.<\/p>\n

See our mutex documentation in Documentation\/locking\/mutex-design.rst,
\nin particular the section [1] about semantics:<\/p>\n

\t“mutex_unlock() may access the mutex structure even after it has
\n\t internally released the lock already – so it’s not safe for
\n\t another context to acquire the mutex and assume that the
\n\t mutex_unlock() context is not using the structure anymore”<\/p>\n

So if we drop our ep ref before the mutex unlock, but we weren’t the
\nlast one, we may then unlock the mutex, another user comes in, drops
\n_their_ reference and releases the ‘ep’ as it now has no users – all
\nwhile the mutex_unlock() is still accessing it.<\/p>\n

Fix this by simply moving the ep refcount dropping to outside the mutex:
\nthe refcount itself is atomic, and doesn’t need mutex protection (that’s
\nthe whole _point_ of refcounts: unlike mutexes, they are inherently
\nabout object lifetimes).
\n
\nSeverity:<\/strong> 0.0 | NA
\n
\nVisit the link for more details, such as CVSS details, affected products, timeline, and more…<\/p>\n","protected":false},"excerpt":{"rendered":"

CVE ID : CVE-2025-38349 Published : 18. Juli 2025 08:15 | 28\u00a0Minuten ago Description : In the Linux kernel, the following vulnerability has been resolved: eventpoll: don’t decrement ep refcount while still holding the ep mutex Jann Horn points out that epoll is decrementing the ep refcount and then doing a mutex_unlock(&ep->mtx); afterwards. That’s very …<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[26],"tags":[],"class_list":["post-66510","post","type-post","status-publish","format-standard","hentry","category-vulnerability"],"_links":{"self":[{"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/posts\/66510","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/comments?post=66510"}],"version-history":[{"count":0,"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/posts\/66510\/revisions"}],"wp:attachment":[{"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/media?parent=66510"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/categories?post=66510"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/afaghhosting.net\/blog\/wp-json\/wp\/v2\/tags?post=66510"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}