Subject: Re: [PATCH net 1/2] selftests/bpf: subtraction bounds test

From: Daniel Borkmann via iovisor-dev <iovisor-dev-9jONkmmOlFHEE9lA1F8Ukti2O/[email-masked]>
Date: Fri, 21 Jul 2017 16:29:22 +0200

On 07/21/2017 03:36 PM, Edward Cree wrote:
> There is a bug in the verifier's handling of BPF_SUB: [a,b] - [c,d] yields
>   was [a-c, b-d] rather than the correct [a-d, b-c].  So here is a test
>   which, with the bogus handling, will produce ranges of [0,0] and thus
>   allowed accesses; whereas the correct handling will give a range of
>   [-255, 255] (and hence the right-shift will give a range of [0, 255]) and
>   the accesses will be rejected.
>
> Signed-off-by: Edward Cree <ecree-s/n/[email-masked]>

Acked-by: Daniel Borkmann <[email-masked]>

>   tools/testing/selftests/bpf/test_verifier.c | 28 ++++++++++++++++++++++++++++
>   1 file changed, 28 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> index af7d173..addea82 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -5980,6 +5980,34 @@ static struct bpf_test tests[] = {
>   		.result = REJECT,
>   		.result_unpriv = REJECT,
>   	},
> +	{
> +		"subtraction bounds (map value)",
> +		.insns = {
> +			BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
> +			BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
> +			BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
> +			BPF_LD_MAP_FD(BPF_REG_1, 0),
> +			BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
> +				     BPF_FUNC_map_lookup_elem),
> +			BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 9),
> +			BPF_LDX_MEM(BPF_B, BPF_REG_1, BPF_REG_0, 0),
> +			BPF_JMP_IMM(BPF_JGT, BPF_REG_1, 0xff, 7),
> +			BPF_LDX_MEM(BPF_B, BPF_REG_3, BPF_REG_0, 1),
> +			BPF_JMP_IMM(BPF_JGT, BPF_REG_3, 0xff, 5),
> +			BPF_ALU64_REG(BPF_SUB, BPF_REG_1, BPF_REG_3),
> +			BPF_ALU64_IMM(BPF_RSH, BPF_REG_1, 56),
> +			BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),
> +			BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_0, 0),
> +			BPF_EXIT_INSN(),
> +			BPF_MOV64_IMM(BPF_REG_0, 0),
> +			BPF_EXIT_INSN(),
> +		},
> +		.fixup_map1 = { 3 },
> +		.errstr_unpriv = "R0 pointer arithmetic prohibited",
> +		.errstr = "R0 min value is negative, either use unsigned index or do a if (index >=0) check.",
> +		.result = REJECT,
> +		.result_unpriv = REJECT,
> +	},
>   };
>
>   static int probe_filter_length(const struct bpf_insn *fp)
>