bpf: Fix issue in verifying allow_ptr_leaks

laoar · gregkh · commit c96c67991aac · 2023-09-13T09:43:02.000+02:00
commit d75e30d upstream. 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> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
@@ -10401,6 +10401,12 @@ 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");
@@ -10412,25 +10418,20 @@ 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");
 			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];
 	is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
 
 	if (BPF_SRC(insn->code) == BPF_K) {
