bpf: Fix issue in verifying allow_ptr_leaks

After we converted the capabilities of our networking-bpf program from
cap_sys_admin to cap_net_admin+cap_bpf, our networking-bpf program
failed to start. Because it failed the bpf verifier, and the error log
is "R3 pointer comparison prohibited".

A simple reproducer as follows,

SEC("cls-ingress")
int ingress(struct __sk_buff *skb)
{
	struct iphdr *iph = (void *)(long)skb->data + sizeof(struct ethhdr);

	if ((long)(iph + 1) > (long)skb->data_end)
		return TC_ACT_STOLEN;
	return TC_ACT_OK;
}

Per discussion with Yonghong and Alexei [1], comparison of two packet
pointers is not a pointer leak. This patch fixes it.

Our local kernel is 6.1.y and we expect this fix to be backported to
6.1.y, so stable is CCed.

[1]. https://lore.kernel.org/bpf/CAADnVQ+Nmspr7Si+pxWn8zkE7hX-7s93ugwC+94aXSy4uQ9vBg@mail.gmail.com/

Suggested-by: Yonghong Song <yonghong.song@linux.dev>
Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230823020703.3790-2-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3d51c737a034fb5fea3b8af33f587754016680c8..0680569f9bd07b46532f358d9dee064a04155001 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14041,6 +14041,12 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
                return -EINVAL;
        }
 
+       /* check src2 operand */
+       err = check_reg_arg(env, insn->dst_reg, SRC_OP);
+       if (err)
+               return err;
+
+       dst_reg = &regs[insn->dst_reg];
        if (BPF_SRC(insn->code) == BPF_X) {
                if (insn->imm != 0) {
                        verbose(env, "BPF_JMP/JMP32 uses reserved fields\n");
@@ -14052,12 +14058,13 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
                if (err)
                        return err;
 
-               if (is_pointer_value(env, insn->src_reg)) {
+               src_reg = &regs[insn->src_reg];
+               if (!(reg_is_pkt_pointer_any(dst_reg) && reg_is_pkt_pointer_any(src_reg)) &&
+                   is_pointer_value(env, insn->src_reg)) {
                        verbose(env, "R%d pointer comparison prohibited\n",
                                insn->src_reg);
                        return -EACCES;
                }
-               src_reg = &regs[insn->src_reg];
        } else {
                if (insn->src_reg != BPF_REG_0) {
                        verbose(env, "BPF_JMP/JMP32 uses reserved fields\n");
@@ -14065,12 +14072,6 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
                }
        }
 
-       /* check src2 operand */
-       err = check_reg_arg(env, insn->dst_reg, SRC_OP);
-       if (err)
-               return err;
-
-       dst_reg = &regs[insn->dst_reg];
        is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
 
        if (BPF_SRC(insn->code) == BPF_K) {