bpf: Fix bpf_xdp_store_bytes proto for read-only arg

Overview

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.

Affected

• Linux >= 3f364222d032eea6b245780e845ad213dab28cdd < ffb5d1c5e3933b947fc7303ad68bf0c536d0c85e
• Linux >= 3f364222d032eea6b245780e845ad213dab28cdd < ddc34a1b85505c919026ddc82fafdada9a160b15
• Linux >= 3f364222d032eea6b245780e845ad213dab28cdd < 0db169a91381a473b7974021d1c02f8da72c5775
• Linux >= 3f364222d032eea6b245780e845ad213dab28cdd < d7b87adeb0eb539b9b824b101bb14fb01e41240b
• Linux >= 3f364222d032eea6b245780e845ad213dab28cdd < 57f7f6a0ad04a65c8a7a067b2f56cbbf2aec9e52
• Linux >= 3f364222d032eea6b245780e845ad213dab28cdd < 6557f1565d779851c4db9c488c49c05a47a6e72f
• Linux 5.18

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.