eventpoll: Fix semi-unbounded recursion

Overview

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.

Affected

• Linux >= 22bacca48a1755f79b7e0f192ddb9fbb7fc6e64e < 71379495ab70eaba19224bd71b5b9b399eb85e04
• Linux >= 22bacca48a1755f79b7e0f192ddb9fbb7fc6e64e < 1b13b033062824495554e836a1ff5f85ccf6b039
• Linux >= 22bacca48a1755f79b7e0f192ddb9fbb7fc6e64e < 2a0c0c974bea9619c6f41794775ae4b97530e0e6
• Linux >= 22bacca48a1755f79b7e0f192ddb9fbb7fc6e64e < 7a2125962c42d5336ca0495a9ce4cb38a63e9161
• Linux >= 22bacca48a1755f79b7e0f192ddb9fbb7fc6e64e < ea5f97dbdcb1651581a22bd10afd2f0dd9dc11d6
• Linux >= 22bacca48a1755f79b7e0f192ddb9fbb7fc6e64e < 3542c90797bc3ab83ebab54b737d751cf3682036
• Linux >= 22bacca48a1755f79b7e0f192ddb9fbb7fc6e64e < f2e467a48287c868818085aa35389a224d226732
• Linux 8216e1a0d47cae06a75c42346f19dffe14e42d57
• Linux 28a92748aa4bc57d35e7b079498b0ac2e7610a37
• Linux 7eebcd4792c5a341559aed327b6afecbb1c46402
• Linux 0eccd188cfeaf857a26f2d72941d27d298cf6a54
• Linux a72affdbb09f3f24f64ffcbbdf62c2e57c58f379
• Linux >= 2.6.32.30 < 2.6.33
• Linux >= 2.6.33.8 < 2.6.34
• Linux >= 2.6.34.10 < 2.6.35
• Linux >= 2.6.35.12 < 2.6.36
• Linux >= 2.6.37.3 < 2.6.38
• Linux 2.6.38

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.