netdev - Re: [PATCH bpf-next v7 03/10] bpf/verifier: refine retval R0 state for bpf_get

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Date:   Fri, 27 Apr 2018 16:22:52 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     ast@...com, daniel@...earbox.net, netdev@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH bpf-next v7 03/10] bpf/verifier: refine retval R0 state
 for bpf_get_stack helper

On Wed, Apr 25, 2018 at 12:29:03PM -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,var_off=(0x0; 0x3ff))
>      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>

Acked-by: Alexei Starovoitov <ast@...nel.org>