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Date: Wed, 8 Mar 2023 18:05:39 +0800
From: Xu Kuohai <xukuohai@...wei.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
Xu Kuohai <xukuohai@...weicloud.com>
CC: bpf <bpf@...r.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
John Fastabend <john.fastabend@...il.com>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf: update 32-bit bounds when the lower
32-bit value is not wrapping
On 3/8/2023 1:22 AM, Alexei Starovoitov wrote:
> On Tue, Mar 7, 2023 at 1:05 AM Xu Kuohai <xukuohai@...weicloud.com> wrote:
>>
>> The following XDP prog is accepted by verifier.
>>
>> 0: (61) r2 = *(u32 *)(r1 +0) ; R2_w=pkt(off=0,r=0,imm=0)
>> 1: (61) r3 = *(u32 *)(r1 +4) ; R3_w=pkt_end(off=0,imm=0)
>> 2: (bf) r1 = r2
>> 3: (07) r1 += 1
>> 4: (2d) if r1 > r3 goto pc+6
>> 5: (71) r1 = *(u8 *)(r2 +0) ; R1_w=scalar(umax=255,var_off=(0x0; 0xff))
>> 6: (b4) w0 = 0x7fffff10
>> 7: (0c) w1 += w0 ; R1_w=scalar(umin=0x7fffff10,umax=0x8000000f,var_off=(0x0; 0xffffffff))
>> 8: (b4) w0 = 0x80000000
>> 9: (04) w0 += 1
>> 10: (ae) if w0 < w1 goto pc-2
>> 11: (b7) r0 = 0
>> 12: (95) exit
>>
>> while the following 64-bit version is rejected.
>>
>> 0: (61) r2 = *(u32 *)(r1 +0) ; R2_w=pkt(off=0,r=0,imm=0)
>> 1: (61) r3 = *(u32 *)(r1 +4) ; R3_w=pkt_end(off=0,imm=0)
>> 2: (bf) r1 = r2
>> 3: (07) r1 += 1
>> 4: (2d) if r1 > r3 goto pc+8
>> 5: (71) r1 = *(u8 *)(r2 +0) ; R1_w=scalar(umax=255,var_off=(0x0; 0xff))
>> 6: (18) r0 = 0x7fffffffffffff10
>> 8: (0f) r1 += r0 ; R1_w=scalar(umin=0x7fffffffffffff10,umax=0x800000000000000f)
>> 9: (18) r0 = 0x8000000000000000
>> 11: (07) r0 += 1
>> 12: (ad) if r0 < r1 goto pc-2
>> 13: (b7) r0 = 0
>> 14: (95) exit
>
> These two programs are not equivalent.
> Not clear how apples to oranges comparison explains anything.
>
Yes, they are not equivalent. I assumed the 32-bit prog being accepted
implies it is unreasonable for the 64-bit prog to be rejected. Regardless
of this assumption and the 32-bit prog, the above 64-bit prog is expected
to be accepted, right?
>> The verifier log says:
>>
>> [...]
>>
>> from 12 to 11: R0_w=-9223372036854775794 R1=scalar(umin=9223372036854775823,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff))
>> 11: (07) r0 += 1 ; R0_w=-9223372036854775793
>> 12: (ad) if r0 < r1 goto pc-2 ; R0_w=-9223372036854775793 R1=scalar(umin=9223372036854775823,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff))
>> 13: safe
>>
>> from 12 to 11: R0_w=-9223372036854775793 R1=scalar(umin=9223372036854775824,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff))
>
> First thing to debug is why umin is higher than umax.
>
Well, it's because the loop does not stop, when r0 increases to -9223372036854775793,
the following code in reg_set_min_max() sets umin_value to 9223372036854775824:
case BPF_JGT:
{
if (is_jmp32) {
[...]
} else {
u64 false_umax = opcode == BPF_JGT ? val : val - 1;
u64 true_umin = opcode == BPF_JGT ? val + 1 : val;
false_reg->umax_value = min(false_reg->umax_value, false_umax);
true_reg->umin_value = max(true_reg->umin_value, true_umin);
}
break;
}
To avoid umin > umax, it could be changed it to:
case BPF_JGT:
{
if (is_jmp32) {
[...]
} else {
u64 false_umax = opcode == BPF_JGT ? val : val - 1;
u64 true_umin = opcode == BPF_JGT ? val + 1 : val;
false_reg->umax_value = min(false_reg->umax_value, false_umax);
false_reg->umax_value = max(false_reg->umax_value, false_reg->umin_value);
true_reg->umin_value = max(true_reg->umin_value, true_umin);
true_reg->umin_value = min(true_reg->umax_value, true_reg->umin_value);
}
break;
}
The problem is that the loop still does not stop because tnum_is_const(src_reg->var_off)
always returns false and is_branch_taken() is skipped:
if (BPF_SRC(insn->code) == BPF_K) {
[...]
} else if (src_reg->type == SCALAR_VALUE &&
is_jmp32 && tnum_is_const(tnum_subreg(src_reg->var_off))) {
[...]
} else if (src_reg->type == SCALAR_VALUE &&
!is_jmp32 && tnum_is_const(src_reg->var_off)) {
pred = is_branch_taken(dst_reg, // could not reach here
src_reg->var_off.value,
opcode,
is_jmp32);
} else if (reg_is_pkt_pointer_any(dst_reg) &&
reg_is_pkt_pointer_any(src_reg) &&
!is_jmp32) {
[...]
}
Why tnum_is_const(src_reg->var_off) returns false is because the lower 32-bit
is not constant since the lower 32-bit range is [U32_MIN, U32_MAX].
>> 11: (07) r0 += 1 ; R0_w=-9223372036854775792
>> 12: (ad) if r0 < r1 goto pc-2 ; R0_w=-9223372036854775792 R1=scalar(umin=9223372036854775824,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff))
>> 13: safe
>>
>> [...]
>>
>> The loop crosses termination condition r0 == r1.umax, and does not stop.
>>
>> The reason is that when the verifier enumerates to r1.umin == r1.umax, the value
>> 0x800000000000000f of r1.umin is greater than U32_MAX, so __reg_combine_64_into_32
>> sets the u32 range of r1 to [0, U32_MAX] instead of marking r1 as a constant,
>> making is_branch_taken() in check_cond_jmp_op() be skipped.
>
> And it's fine. The verifier is conservative.
>
>>
>> To fix it, update 32-bit bounds when the lower 32-bit value is not wrapping,
>> even if the 64-bit value is beyond the range of [0, U32_MAX] or [S32_MIN, S32_MAX].
>
> That's not safe in general.
>
>>
>> Signed-off-by: Xu Kuohai <xukuohai@...weicloud.com>
>> ---
>> kernel/bpf/verifier.c | 27 +++++++++++----------------
>> 1 file changed, 11 insertions(+), 16 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index b2116ca78d9a..64c9ee3857ec 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -2013,26 +2013,21 @@ static void __reg_combine_32_into_64(struct bpf_reg_state *reg)
>> reg_bounds_sync(reg);
>> }
>>
>> -static bool __reg64_bound_s32(s64 a)
>> -{
>> - return a >= S32_MIN && a <= S32_MAX;
>> -}
>> -
>> -static bool __reg64_bound_u32(u64 a)
>> -{
>> - return a >= U32_MIN && a <= U32_MAX;
>> -}
>> -
>> static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
>> {
>> + s64 smin = reg->smin_value;
>> + s64 smax = reg->smax_value;
>> + u64 umin = reg->umin_value;
>> + u64 umax = reg->umax_value;
>> +
>> __mark_reg32_unbounded(reg);
>> - if (__reg64_bound_s32(reg->smin_value) && __reg64_bound_s32(reg->smax_value)) {
>> - reg->s32_min_value = (s32)reg->smin_value;
>> - reg->s32_max_value = (s32)reg->smax_value;
>> + if ((u64)(smax - smin) <= (u64)U32_MAX && (s32)smin <= (s32)smax) {
>> + reg->s32_min_value = (s32)smin;
>> + reg->s32_max_value = (s32)smax;
>> }
>> - if (__reg64_bound_u32(reg->umin_value) && __reg64_bound_u32(reg->umax_value)) {
>> - reg->u32_min_value = (u32)reg->umin_value;
>> - reg->u32_max_value = (u32)reg->umax_value;
>> + if (umax - umin <= U32_MAX && (u32)umin <= (u32)umax) {
>> + reg->u32_min_value = (u32)umin;
>> + reg->u32_max_value = (u32)umax;
>
> This looks like a workaround for umin > umax issue.
> Please debug that instead.
>
"__reg64_bound_u32(umin) && __reg64_bound_u32(max)" is a special case of
"umax - umin <= U32_MAX && (u32)umin <= (u32)umax " when umax <= U32_MAX.
If it's only safe to set lower 32-bit range to [U32_MIN, U32_MAX] when
umax > U32_MAX, could we infer the 64-bit value is a constant from umin == umax?
>> }
>> reg_bounds_sync(reg);
>> }
>> --
>> 2.30.2
>>
> .
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