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

program's 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.