Re: [PATCH bpf-next v2] bpf: Detect jumping to reserved code during check_cfg()

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2023-10-10 at 14:03 +0200, 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:
> 
> func#0 @0
> jump to reserved code from insn 8 to 7
> 
> Also adjust existing tests in ld_imm64.c, testing forward/back jump to
> reserved code.
> 
> Signed-off-by: Hao Sun <sunhao.th@gmail.com>

Please see a nitpick below.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

> ---
> Changes in v2:
> - Adjust existing test cases
> - Link to v1: https://lore.kernel.org/bpf/20231009-jmp-into-reserved-fields-v1-1-d8006e2ac1f6@gmail.com/
> ---
>  kernel/bpf/verifier.c                           | 7 +++++++
>  tools/testing/selftests/bpf/verifier/ld_imm64.c | 8 +++-----
>  2 files changed, 10 insertions(+), 5 deletions(-)
> 
> 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);
> diff --git a/tools/testing/selftests/bpf/verifier/ld_imm64.c b/tools/testing/selftests/bpf/verifier/ld_imm64.c
> index f9297900cea6..c34aa78f1877 100644
> --- a/tools/testing/selftests/bpf/verifier/ld_imm64.c
> +++ b/tools/testing/selftests/bpf/verifier/ld_imm64.c
> @@ -9,22 +9,20 @@
>  	BPF_MOV64_IMM(BPF_REG_0, 2),
>  	BPF_EXIT_INSN(),
>  	},
> -	.errstr = "invalid BPF_LD_IMM insn",
> -	.errstr_unpriv = "R1 pointer comparison",
> +	.errstr = "jump to reserved code",
>  	.result = REJECT,
>  },
>  {
>  	"test2 ld_imm64",
>  	.insns = {
> -	BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 0, 1),
>  	BPF_LD_IMM64(BPF_REG_0, 0),
> +	BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 0, -2),

This change is not really necessary, the test reports same error
either way.

>  	BPF_LD_IMM64(BPF_REG_0, 0),
>  	BPF_LD_IMM64(BPF_REG_0, 1),
>  	BPF_LD_IMM64(BPF_REG_0, 1),
>  	BPF_EXIT_INSN(),
>  	},
> -	.errstr = "invalid BPF_LD_IMM insn",
> -	.errstr_unpriv = "R1 pointer comparison",
> +	.errstr = "jump to reserved code",
>  	.result = REJECT,
>  },
>  {
> 
> ---
> base-commit: 3157b7ce14bbf468b0ca8613322a05c37b5ae25d
> change-id: 20231009-jmp-into-reserved-fields-fc1a98a8e7dc
> 
> Best regards,