Re: [PATCH bpf-next v3 3/9] bpf/verifier: refine retval R0 state for bpf_get_stack helper

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Apr 20, 2018 at 03:18:36PM -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@fb.com>
> ---
>  kernel/bpf/verifier.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index aba9425..3c8bb92 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -164,6 +164,8 @@ struct bpf_call_arg_meta {
>  	bool pkt_access;
>  	int regno;
>  	int access_size;
> +	s64 msize_smax_value;
> +	u64 msize_umax_value;
>  };
>  
>  static DEFINE_MUTEX(bpf_verifier_lock);
> @@ -2027,6 +2029,14 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
>  		err = check_helper_mem_access(env, regno - 1,
>  					      reg->umax_value,
>  					      zero_size_allowed, meta);
> +
> +		if (!err && !!meta) {

Please drop !! in the above.

Also what happens when
if (!tnum_is_const(reg->var_off))
  meta = NULL;
?
it seems two new fields of meta will stay zero initialized
that later do_refine_retval_range() will set R0->umax_value = 0
which seems incorrect.

> +			/* remember the mem_size which may be used later
> +			 * to refine return values.
> +			 */
> +			meta->msize_smax_value = reg->smax_value;
> +			meta->msize_umax_value = reg->umax_value;
> +		}
>  	}
>  
>  	return err;
> @@ -2333,6 +2343,21 @@ 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_call_arg_meta *meta)
> +{
> +	struct bpf_reg_state *ret_reg = &regs[BPF_REG_0];
> +
> +	if (ret_type != RET_INTEGER ||
> +	    (func_id != BPF_FUNC_get_stack &&
> +	     func_id != BPF_FUNC_probe_read_str))
> +		return;
> +
> +	ret_reg->smax_value = meta->msize_smax_value;
> +	ret_reg->umax_value = meta->msize_umax_value;
> +}
> +
>  static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn_idx)
>  {
>  	const struct bpf_func_proto *fn = NULL;
> @@ -2456,6 +2481,8 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
>  		return -EINVAL;
>  	}
>  
> +	do_refine_retval_range(regs, fn->ret_type, func_id, &meta);
> +
>  	err = check_map_func_compatibility(env, meta.map_ptr, func_id);
>  	if (err)
>  		return err;
> -- 
> 2.9.5
>