RE: [PATCH bpf-next] Detect jumping to reserved code during check_cfg()

[John Fastabend <john.fastabend@gmail.com>]

Hao Sun wrote:
> Currently, we don't check if the branch-taken of a jump is reserved code of
> ld_imm64. Instead, such a issue is captured in check_ld_imm(). The verifier
> gives the following log in such case:
> 
> func#0 @0
> 0: R1=ctx(off=0,imm=0) R10=fp0
> 0: (18) r4 = 0xffff888103436000       ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
> 2: (18) r1 = 0x1d                     ; R1_w=29
> 4: (55) if r4 != 0x0 goto pc+4        ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
> 5: (1c) w1 -= w1                      ; R1_w=0
> 6: (18) r5 = 0x32                     ; R5_w=50
> 8: (56) if w5 != 0xfffffff4 goto pc-2
> mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx -1
> mark_precise: frame0: regs=r5 stack= before 6: (18) r5 = 0x32
> 7: R5_w=50
> 7: BUG_ld_00
> invalid BPF_LD_IMM insn
> 
> Here the verifier rejects the program because it thinks insn at 7 is an
> invalid BPF_LD_IMM, but such a error log is not accurate since the issue
> is jumping to reserved code not because the program contains invalid insn.
> Therefore, make the verifier check the jump target during check_cfg(). For
> the same program, the verifier reports the following log:

I think we at least would want a test case for this. Also how did you create
this case? Is it just something you did manually and noticed a strange error?

> 
> func#0 @0
> jump to reserved code from insn 8 to 7
> 
> ---
> 
> 
> Signed-off-by: Hao Sun <sunhao.th@gmail.com>
> ---
>  kernel/bpf/verifier.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index eed7350e15f4..725ac0b464cf 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14980,6 +14980,7 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
>  {
>  	int *insn_stack = env->cfg.insn_stack;
>  	int *insn_state = env->cfg.insn_state;
> +	struct bpf_insn *insns = env->prog->insnsi;
>  
>  	if (e == FALLTHROUGH && insn_state[t] >= (DISCOVERED | FALLTHROUGH))
>  		return DONE_EXPLORING;
> @@ -14993,6 +14994,12 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
>  		return -EINVAL;
>  	}
>  
> +	if (e == BRANCH && insns[w].code == 0) {
> +		verbose_linfo(env, t, "%d", t);
> +		verbose(env, "jump to reserved code from insn %d to %d\n", t, w);
> +		return -EINVAL;
> +	}
> +
>  	if (e == BRANCH) {
>  		/* mark branch target for state pruning */
>  		mark_prune_point(env, w);
> 
> ---
> base-commit: 3157b7ce14bbf468b0ca8613322a05c37b5ae25d
> change-id: 20231009-jmp-into-reserved-fields-fc1a98a8e7dc
> 
> Best regards,
> -- 
> Hao Sun <sunhao.th@gmail.com>
>