bpf: Fix a UAF issue in bpf_trampoline_link_cgroup_shim

Overview

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix a UAF issue in bpf_trampoline_link_cgroup_shim

The root cause of this bug is that when 'bpf_link_put' reduces the refcount of 'shim_link->link.link' to zero, the resource is considered released but may still be referenced via 'tr->progs_hlist' in 'cgroup_shim_find'. The actual cleanup of 'tr->progs_hlist' in 'bpf_shim_tramp_link_release' is deferred. During this window, another process can cause a use-after-free via 'bpf_trampoline_link_cgroup_shim'.

Based on Martin KaFai Lau's suggestions, I have created a simple patch.

To fix this: Add an atomic non-zero check in 'bpf_trampoline_link_cgroup_shim'. Only increment the refcount if it is not already zero.

Testing: I verified the fix by adding a delay in 'bpf_shim_tramp_link_release' to make the bug easier to trigger:

static void bpf_shim_tramp_link_release(struct bpf_link link) { / ... */ if (!shim_link->trampoline) return;

• msleep(100); WARN_ON_ONCE(bpf_trampoline_unlink_prog(&shim_link->link, shim_link->trampoline, NULL)); bpf_trampoline_put(shim_link->trampoline); }

Before the patch, running a PoC easily reproduced the crash(almost 100%) with a call trace similar to KaiyanM's report. After the patch, the bug no longer occurs even after millions of iterations.

Affected

• Linux >= 69fd337a975c7e690dfe49d9cb4fe5ba1e6db44e < 529e685e522b9d7fb379dbe6929dcdf520e34c8c
• Linux >= 69fd337a975c7e690dfe49d9cb4fe5ba1e6db44e < 9b02c5c4147f8af8ed783c8deb5df927a55c3951
• Linux >= 69fd337a975c7e690dfe49d9cb4fe5ba1e6db44e < cfcfa0ca0212162aa472551266038e8fd6768cff
• Linux >= 69fd337a975c7e690dfe49d9cb4fe5ba1e6db44e < 3eeddb80191f7626ec1ef742bfff51ec3b0fa5c2
• Linux >= 69fd337a975c7e690dfe49d9cb4fe5ba1e6db44e < 4e8a0005d633a4adc98e3b65d5080f93b90d356b
• Linux >= 69fd337a975c7e690dfe49d9cb4fe5ba1e6db44e < 56145d237385ca0e7ca9ff7b226aaf2eb8ef368b
• Linux 6.0

Remediation

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