CVE-2025-38614

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 EPMAXNESTS+1 links.

Currently, eploopcheck_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 ("epoll: limit paths").

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

A more thorough check is done in reversepathcheck() 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 ("eventpoll: Fix priority inversion problem").)

To fix it:

1. In eploopcheck_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 epgetupwardsdepthproc() for similarly determining the maximum depth of an upwards walk.
3. In eploopcheck(), use these values to limit the total path length between epoll nodes to EPMAXNESTS edges.