CVE-2025-38614 Information

Aug 20, 2025 cve

Description

In the Linux kernel the following vulnerability has been resolved:

eventpoll: Fix semi-unbounded recursion

Ensure that epoll instances can never form a graph deeper than EP_MAX_NESTS+1 links.

Currently ep_loop_check_proc() ensures that the graph is loop-free and does some recursion depth checks but those recursion depth checks don’t limit the depth of the resulting tree for two reasons:

  • They don’t look upwards in the tree.
  • If there are multiple downwards paths of different lengths only one of the paths is actually considered for the depth check since commit 28d82dc1c4ed (poll: limit paths).

Essentially the current recursion depth check in ep_loop_check_proc() just serves to prevent it from recursing too deeply while checking for loops.

A more thorough check is done in reverse_path_check() after the new graph edge has already been created; this checks among other things that no paths going upwards from any non-epoll file with a length of more than 5 edges exist. However this check does not apply to non-epoll files.

As a result it is possible to recurse to a depth of at least roughly 500 tested on v6.15. (I am unsure if deeper recursion is possible; and this may have changed with commit 8c44dac8add7 (ventpoll: Fix priority inversion problem).)

To fix it:

  1. In ep_loop_check_proc() note the subtree depth of each visited node and use subtree depths for the total depth calculation even when a subtree has already been visited.
  2. Add ep_get_upwards_depth_proc() for similarly determining the maximum depth of an upwards walk.
  3. In ep_loop_check() use these values to limit the total path length between epoll nodes to EP_MAX_NESTS edges.

Reference

https://git.kernel.org/stable/c/3542c90797bc3ab83ebab54b737d751cf3682036 https://git.kernel.org/stable/c/f2e467a48287c868818085aa35389a224d226732

CNNVD-202508-2178 (Published: 2025-08-19)

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