Re: [PATCH bpf-next 1/2] bpf: Fix issue in verifying allow_ptr_leaks

On Fri, 2023-08-18 at 08:39 +0000, Yafang Shao wrote:

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+94aXSy4uQ9...

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 Cc: stable@vger.kernel.org

---

kernel/bpf/verifier.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 4ccca1f..b6b60cd 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -14047,6 +14047,12 @@ 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");

@@ -14058,12 +14064,13 @@ 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];
  

I tested this change and it seem to work as intended. Was worried a bit that there are three places in this function where such checks are applied: 1. upon entry for BPF_X case (this one): checks if dst_reg/src_reg are pointers to packet or packet end or packet meta; 2. when attempting to predict branch: prediction would be triggered only when dst/src is packet/packet_end (or vice-versa); 3. when prediction failed and both branches have to be visited (`try_match_pkt_pointers`): dst/src have to be packet/packet_end or meta/packet-start (or vice versa).

Check (1) is more permissive than (2) or (3) but either (2) or (3) would be applied before exit, so there is no contradiction.

Acked-by: Eduard Zingerman eddyz87@gmail.com

} else { if (insn->src_reg != BPF_REG_0) { verbose(env, "BPF_JMP/JMP32 uses reserved fields\n"); @@ -14071,12 +14078,6 @@ 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) {