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Date: Thu, 19 Apr 2018 16:37:52 -0700
From: Yonghong Song <yhs@...com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
CC: <ast@...com>, <daniel@...earbox.net>, <netdev@...r.kernel.org>,
<kernel-team@...com>
Subject: Re: [PATCH bpf-next v2 3/9] bpf/verifier: refine retval R0 state for
bpf_get_stack helper
On 4/18/18 9:33 PM, Alexei Starovoitov wrote:
> On Wed, Apr 18, 2018 at 09:54:38AM -0700, Yonghong Song wrote:
>> The special property of return values for helpers bpf_get_stack
>> and bpf_probe_read_str are captured in verifier.
>> Both helpers return a negative error code or
>> a length, which is equal to or smaller than the buffer
>> size argument. This additional information in the
>> verifier can avoid the condition such as "retval > bufsize"
>> in the bpf program. For example, for the code blow,
>> usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
>> if (usize < 0 || usize > max_len)
>> return 0;
>> The verifier may have the following errors:
>> 52: (85) call bpf_get_stack#65
>> R0=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R1_w=ctx(id=0,off=0,imm=0)
>> R2_w=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R3_w=inv800 R4_w=inv256
>> R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0)
>> R9_w=inv800 R10=fp0,call_-1
>> 53: (bf) r8 = r0
>> 54: (bf) r1 = r8
>> 55: (67) r1 <<= 32
>> 56: (bf) r2 = r1
>> 57: (77) r2 >>= 32
>> 58: (25) if r2 > 0x31f goto pc+33
>> R0=inv(id=0) R1=inv(id=0,smax_value=9223372032559808512,
>> umax_value=18446744069414584320,
>> var_off=(0x0; 0xffffffff00000000))
>> R2=inv(id=0,umax_value=799,var_off=(0x0; 0x3ff))
>> R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0)
>> R8=inv(id=0) R9=inv800 R10=fp0,call_-1
>> 59: (1f) r9 -= r8
>> 60: (c7) r1 s>>= 32
>> 61: (bf) r2 = r7
>> 62: (0f) r2 += r1
>> math between map_value pointer and register with unbounded
>> min value is not allowed
>> The failure is due to llvm compiler optimization where register "r2",
>> which is a copy of "r1", is tested for condition while later on "r1"
>> is used for map_ptr operation. The verifier is not able to track such
>> inst sequence effectively.
>>
>> Without the "usize > max_len" condition, there is no llvm optimization
>> and the below generated code passed verifier:
>> 52: (85) call bpf_get_stack#65
>> R0=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R1_w=ctx(id=0,off=0,imm=0)
>> R2_w=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R3_w=inv800 R4_w=inv256
>> R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0)
>> R9_w=inv800 R10=fp0,call_-1
>> 53: (b7) r1 = 0
>> 54: (bf) r8 = r0
>> 55: (67) r8 <<= 32
>> 56: (c7) r8 s>>= 32
>> 57: (6d) if r1 s> r8 goto pc+24
>> R0=inv(id=0,umax_value=800) R1=inv0 R6=ctx(id=0,off=0,imm=0)
>> R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0)
>> R8=inv(id=0,umax_value=800,var_off=(0x0; 0x3ff)) R9=inv800
>> R10=fp0,call_-1
>> 58: (bf) r2 = r7
>> 59: (0f) r2 += r8
>> 60: (1f) r9 -= r8
>> 61: (bf) r1 = r6
>>
>> Signed-off-by: Yonghong Song <yhs@...com>
>> ---
>> kernel/bpf/verifier.c | 31 ++++++++++++++++++++++++++++++-
>> 1 file changed, 30 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index aba9425..a8302c3 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -2333,10 +2333,32 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
>> return 0;
>> }
>>
>> +static void do_refine_retval_range(struct bpf_reg_state *regs, int ret_type,
>> + int func_id,
>> + struct bpf_reg_state *retval_state,
>> + bool is_check)
>> +{
>> + struct bpf_reg_state *src_reg, *dst_reg;
>> +
>> + if (ret_type != RET_INTEGER ||
>> + (func_id != BPF_FUNC_get_stack &&
>> + func_id != BPF_FUNC_probe_read_str))
>> + return;
>> +
>> + dst_reg = is_check ? retval_state : ®s[BPF_REG_0];
>> + if (func_id == BPF_FUNC_get_stack)
>> + src_reg = is_check ? ®s[BPF_REG_3] : retval_state;
>> + else
>> + src_reg = is_check ? ®s[BPF_REG_2] : retval_state;
>> +
>> + dst_reg->smax_value = src_reg->smax_value;
>> + dst_reg->umax_value = src_reg->umax_value;
>> +}
>
> I think this part can be made more generic, by using 'meta' logic.
> check_func_arg(.. &meta);
> can remember smax/umax into meta for arg_type_is_mem_size()
> and later refine_retval_range() can be applied to r0.
> This will help avoid mistakes with specifying reg by position (r2 or r3)
> like above snippet is doing.
Good suggestion. Let me try this.
>
>> +
>> static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn_idx)
>> {
>> const struct bpf_func_proto *fn = NULL;
>> - struct bpf_reg_state *regs;
>> + struct bpf_reg_state *regs, retval_state;
>> struct bpf_call_arg_meta meta;
>> bool changes_data;
>> int i, err;
>> @@ -2415,6 +2437,10 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
>> }
>>
>> regs = cur_regs(env);
>> +
>> + /* before reset caller saved regs, check special ret value */
>> + do_refine_retval_range(regs, fn->ret_type, func_id, &retval_state, 1);
>> +
>> /* reset caller saved regs */
>> for (i = 0; i < CALLER_SAVED_REGS; i++) {
>> mark_reg_not_init(env, regs, caller_saved[i]);
>> @@ -2456,6 +2482,9 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
>> return -EINVAL;
>> }
>>
>> + /* apply additional constraints to ret value */
>> + do_refine_retval_range(regs, fn->ret_type, func_id, &retval_state, 0);
>> +
>> err = check_map_func_compatibility(env, meta.map_ptr, func_id);
>> if (err)
>> return err;
>> --
>> 2.9.5
>>
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