CVE-2024-35860

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

bpf: support deferring bpf_link dealloc to after RCU grace period

BPF link for some program types is passed as a context which can be used by those BPF programs to look up additional information. E.g., for multi-kprobes and multi-uprobes, link is used to fetch BPF cookie values.

Because of this runtime dependency, when bpf_link refcnt drops to zero there could still be active BPF programs running accessing link data.

This patch adds generic support to defer bpf_link dealloc callback to after RCU GP, if requested. This is done by exposing two different deallocation callbacks, one synchronous and one deferred. If deferred one is provided, bpf_link_free() will schedule dealloc_deferred() callback to happen after RCU GP.

BPF is using two flavors of RCU: classic non-sleepable one and RCU tasks trace one. The latter is used when sleepable BPF programs are used. bpf_link_free() accommodates that by checking underlying BPF programs sleepable flag, and goes either through normal RCU GP only for non-sleepable, or through RCU tasks trace GP and then normal RCU GP (taking into account rcu_trace_implies_rcu_gp() optimization), if BPF program is sleepable.

We use this for multi-kprobe and multi-uprobe links, which dereference link during program run. We also preventively switch raw_tp link to use deferred dealloc callback, as upcoming changes in bpf-next tree expose raw_tp link data (specifically, cookie value) to BPF program at runtime as well.

Affected Software

References

• https://git.kernel.org/stable/c/1a80dbcb2dbaf6e4c216e62e30fa7d3daa8001ce
• https://git.kernel.org/stable/c/5d8d447777564b35f67000e7838e7ccb64d525c8
• https://git.kernel.org/stable/c/876941f533e7b47fc69977fc4551c02f2d18af97