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 Acked-by: Eduard Zingerman eddyz87@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]; } 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) {