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
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:
- In ep_loop_check_proc(), note the subtree depth of each visited node,
- Add ep_get_upwards_depth_proc() for similarly determining the maximum
- In ep_loop_check(), use these values to limit the total path length