eventpoll: Fix semi-unbounded recursion

Vulnerability 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 ("epoll: 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 ("eventpoll: 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.

References

• https://git.kernel.org/stable/c/71379495ab70eaba19224bd71b5b9b399eb85e04
• https://git.kernel.org/stable/c/1b13b033062824495554e836a1ff5f85ccf6b039
• https://git.kernel.org/stable/c/2a0c0c974bea9619c6f41794775ae4b97530e0e6
• https://git.kernel.org/stable/c/7a2125962c42d5336ca0495a9ce4cb38a63e9161
• https://git.kernel.org/stable/c/ea5f97dbdcb1651581a22bd10afd2f0dd9dc11d6
• https://git.kernel.org/stable/c/3542c90797bc3ab83ebab54b737d751cf3682036
• https://git.kernel.org/stable/c/f2e467a48287c868818085aa35389a224d226732