CVE-2024-56786

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

bpf: put bpf_links program when link is safe to be deallocated

In general, BPF links underlying BPF program should be considered to be reachable through attach hook -> link -> prog chain, and, pessimistically, we have to assume that as long as links memory is not safe to free, attach hooks code might hold a pointer to BPF program and use it.

As such, its not (generally) correct to put links program early before waiting for RCU GPs to go through. More eager bpf_prog_put() that we currently do is mostly correct due to BPF programs release code doing similar RCU GP waiting, but as will be shown in the following patches, BPF program can be non-sleepable (and, thus, reliant on only classic RCU GP), while BPF links attach hook can have sleepable semantics and needs to be protected by RCU Tasks Trace, and for such cases BPF link has to go through RCU Tasks Trace + classic RCU GPs before being deallocated. And so, if we put BPF program early, we might free BPF program before we free BPF link, leading to use-after-free situation.

So, this patch defers bpf_prog_put() until we are ready to perform bpf_links deallocation. At worst, this delays BPF program freeing by one extra RCU GP, but that seems completely acceptable. Alternatively, wed need more elaborate ways to determine BPF hook, BPF link, and BPF program lifetimes, and how they relate to each other, which seems like an unnecessary complication.

Note, for most BPF links we still will perform eager bpf_prog_put() and link dealloc, so for those BPF links there are no observable changes whatsoever. Only BPF links that use deferred dealloc might notice slightly delayed freeing of BPF programs.

Also, to reduce code and logic duplication, extract program put + link dealloc logic into bpf_link_dealloc() helper.

Affected Software

References

• https://git.kernel.org/stable/c/2fcb921c2799c49ac5e365cf4110f94a64ae4885
• https://git.kernel.org/stable/c/5fe23c57abadfd46a7a66e81f3536e4757252a0b
• https://git.kernel.org/stable/c/f44ec8733a8469143fde1984b5e6931b2e2f6f3f