bpf: Fix bpf_xdp_store_bytes proto for read-only arg

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

bpf: Fix bpf_xdp_store_bytes proto for read-only arg

While making some maps in Cilium read-only from the BPF side, we noticed

that the bpf_xdp_store_bytes proto is incorrect. In particular, the

verifier was throwing the following error:

; ret = ctx_store_bytes(ctx, l3_off + offsetof(struct iphdr, saddr),

&nat->address, 4, 0);

635: (79) r1 = *(u64 *)(r10 -144) ; R1=ctx() R10=fp0 fp-144=ctx()

636: (b4) w2 = 26 ; R2=26

637: (b4) w4 = 4 ; R4=4

638: (b4) w5 = 0 ; R5=0

639: (85) call bpf_xdp_store_bytes#190

write into map forbidden, value_size=6 off=0 size=4

nat comes from a BPF_F_RDONLY_PROG map, so R3 is a PTR_TO_MAP_VALUE.

The verifier checks the helper's memory access to R3 in

check_mem_size_reg, as it reaches ARG_CONST_SIZE argument. The third

argument has expected type ARG_PTR_TO_UNINIT_MEM, which includes the

MEM_WRITE flag. The verifier thus checks for a BPF_WRITE access on R3.

Given R3 points to a read-only map, the check fails.

Conversely, ARG_PTR_TO_UNINIT_MEM can also lead to the helper reading

from uninitialized memory.

This patch simply fixes the expected argument type to match that of

bpf_skb_store_bytes.