AZL-69932

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

bpf: consider that tail calls invalidate packet pointers

Tail-called programs could execute any of the helpers that invalidate packet pointers. Hence, conservatively assume that each tail call invalidates packet pointers.

Making the change in bpf_helper_changes_pkt_data() automatically makes use of check_cfg() logic that computes 'changes_pkt_data' effect for global sub-programs, such that the following program could be rejected:

int tail_call(struct __sk_buff *sk)
{
	bpf_tail_call_static(sk, &jmp_table, 0);
	return 0;
}

SEC("tc")
int not_safe(struct __sk_buff *sk)
{
	int *p = (void *)(long)sk->data;
	... make p valid ...
	tail_call(sk);
	*p = 42; /* this is unsafe */
	...
}

The tc_bpf2bpf.c:subprog_tc() needs change: mark it as a function that can invalidate packet pointers. Otherwise, it can't be freplaced with tailcall_freplace.c:entry_freplace() that does a tail call.