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

[Daniel Borkmann <daniel@iogearbox.net>]

On 10/10/23 4:46 PM, Eduard Zingerman wrote:
> 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.

If we don't have a backward jump covered, we could probably also make this
a new test case rather than modifying an existing one. Aside from that it
would probably also make sense to make this a separate commit, so it eases
backporting a bit.

>>   	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,
>