Message-ID: <9db5753d-422b-40ad-973e-27c0c5721b28@huaweicloud.com>
Date: Tue, 30 Apr 2024 11:54:35 +0800
From: Xu Kuohai <xukuohai@...weicloud.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>,
 Edward Cree <ecree.xilinx@...il.com>
Cc: Yonghong Song <yonghong.song@...ux.dev>,
 Eduard Zingerman <eddyz87@...il.com>, bpf@...r.kernel.org,
 netdev@...r.kernel.org, linux-security-module@...r.kernel.org,
 linux-kselftest@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Matt Bobrowski <mattbobrowski@...gle.com>,
 Brendan Jackman <jackmanb@...omium.org>, Paul Moore <paul@...l-moore.com>,
 James Morris <jmorris@...ei.org>, "Serge E . Hallyn" <serge@...lyn.com>,
 Khadija Kamran <kamrankhadijadj@...il.com>,
 Casey Schaufler <casey@...aufler-ca.com>,
 Ondrej Mosnacek <omosnace@...hat.com>, Kees Cook <keescook@...omium.org>,
 John Johansen <john.johansen@...onical.com>,
 Lukas Bulwahn <lukas.bulwahn@...il.com>,
 Roberto Sassu <roberto.sassu@...wei.com>,
 Shung-Hsi Yu <shung-hsi.yu@...e.com>
Subject: Re: [PATCH bpf-next v3 07/11] bpf: Fix a false rejection caused by
 AND operation

On 4/30/2024 4:58 AM, Andrii Nakryiko wrote:
> On Sun, Apr 28, 2024 at 8:15 AM Xu Kuohai <xukuohai@...weicloud.com> wrote:
>>
>> On 4/27/2024 4:36 AM, Andrii Nakryiko wrote:
>>> On Tue, Apr 23, 2024 at 7:26 PM Xu Kuohai <xukuohai@...weicloud.com> wrote:
>>>>
>>>> On 4/24/2024 5:55 AM, Yonghong Song wrote:
>>>>>
>>>>> On 4/20/24 1:33 AM, Xu Kuohai wrote:
>>>>>> On 4/20/2024 7:00 AM, Eduard Zingerman wrote:
>>>>>>> On Thu, 2024-04-11 at 20:27 +0800, Xu Kuohai wrote:
>>>>>>>> From: Xu Kuohai <xukuohai@...wei.com>
>>>>>>>>
>>>>>>>> With lsm return value check, the no-alu32 version test_libbpf_get_fd_by_id_opts
>>>>>>>> is rejected by the verifier, and the log says:
>>>>>>>>
>>>>>>>>      0: R1=ctx() R10=fp0
>>>>>>>>      ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) @ test_libbpf_get_fd_by_id_opts.c:27
>>>>>>>>      0: (b7) r0 = 0                        ; R0_w=0
>>>>>>>>      1: (79) r2 = *(u64 *)(r1 +0)
>>>>>>>>      func 'bpf_lsm_bpf_map' arg0 has btf_id 916 type STRUCT 'bpf_map'
>>>>>>>>      2: R1=ctx() R2_w=trusted_ptr_bpf_map()
>>>>>>>>      ; if (map != (struct bpf_map *)&data_input) @ test_libbpf_get_fd_by_id_opts.c:29
>>>>>>>>      2: (18) r3 = 0xffff9742c0951a00       ; R3_w=map_ptr(map=data_input,ks=4,vs=4)
>>>>>>>>      4: (5d) if r2 != r3 goto pc+4         ; R2_w=trusted_ptr_bpf_map() R3_w=map_ptr(map=data_input,ks=4,vs=4)
>>>>>>>>      ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) @ test_libbpf_get_fd_by_id_opts.c:27
>>>>>>>>      5: (79) r0 = *(u64 *)(r1 +8)          ; R0_w=scalar() R1=ctx()
>>>>>>>>      ; if (fmode & FMODE_WRITE) @ test_libbpf_get_fd_by_id_opts.c:32
>>>>>>>>      6: (67) r0 <<= 62                     ; R0_w=scalar(smax=0x4000000000000000,umax=0xc000000000000000,smin32=0,smax32=umax32=0,var_off=(0x0; 0xc000000000000000))
>>>>>>>>      7: (c7) r0 s>>= 63                    ; R0_w=scalar(smin=smin32=-1,smax=smax32=0)
>>>>>>>>      ;  @ test_libbpf_get_fd_by_id_opts.c:0
>>>>>>>>      8: (57) r0 &= -13                     ; R0_w=scalar(smax=0x7ffffffffffffff3,umax=0xfffffffffffffff3,smax32=0x7ffffff3,umax32=0xfffffff3,var_off=(0x0; 0xfffffffffffffff3))
>>>>>>>>      ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) @ test_libbpf_get_fd_by_id_opts.c:27
>>>>>>>>      9: (95) exit
>>>
>>> [...]
>>>
>>>>
>>>>        As suggested by Eduard, this patch makes a special case for source
>>>>        or destination register of '&=' operation being in range [-1, 0].
>>>>
>>>>        Meaning that one of the '&=' operands is either:
>>>>        - all ones, in which case the counterpart is the result of the operation;
>>>>        - all zeros, in which case zero is the result of the operation.
>>>>
>>>>        And MIN and MAX values could be derived based on above two observations.
>>>>
>>>>        [0] https://lore.kernel.org/bpf/e62e2971301ca7f2e9eb74fc500c520285cad8f5.camel@gmail.com/
>>>>        [1] https://github.com/llvm/llvm-project/blob/4523a267829c807f3fc8fab8e5e9613985a51565/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
>>>>
>>>>        Suggested-by: Eduard Zingerman <eddyz87@...il.com>
>>>>        Signed-off-by: Xu Kuohai <xukuohai@...wei.com>
>>>>
>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>>> index 640747b53745..30c551d39329 100644
>>>> --- a/kernel/bpf/verifier.c
>>>> +++ b/kernel/bpf/verifier.c
>>>> @@ -13374,6 +13374,24 @@ static void scalar32_min_max_and(struct bpf_reg_state *dst_reg,
>>>>            dst_reg->u32_min_value = var32_off.value;
>>>>            dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);
>>>>
>>>> +       /* Special case: src_reg is known and dst_reg is in range [-1, 0] */
>>>> +       if (src_known &&
>>>> +               dst_reg->s32_min_value == -1 && dst_reg->s32_max_value == 0 &&
>>>> +               dst_reg->smin_value == -1 && dst_reg->smax_value == 0) {
>>>
>>> please keep if () condition aligned across multiple lines, it's super
>>> confusing this way
>>>
>>
>> OK, will update the align style
>>
>>>> +               dst_reg->s32_min_value = min_t(s32, src_reg->s32_min_value, 0);
>>>> +               dst_reg->s32_max_value = max_t(s32, src_reg->s32_min_value, 0);
>>>
>>> do we need to update tnum parts as well (or reset and re-derive, probably)?
>>>
>>> btw, can't we support src being a range here? the idea is that dst_reg
>>> either all ones or all zeros. For and it means that it either stays
>>> all zero, or will be *exactly equal* to src, right? So I think the
>>> logic would be:
>>>
>>> a) if [s32_min, s32_max] is on the same side of zero, then resulting
>>> range would be [min(s32_min, 0), max(s32_max, 0)], just like you have
>>> here
>>>
>>> b) if [s32_min, s32_max] contains zero, then resulting range will be
>>> exactly [s32_min, s32_max]
>>>
>>> Or did I make a mistake above?
>>>
>>
>> Totally agree, the AND of any set with the range [-1,0] is equivalent
>> to adding number 0 to the set!
>>
>> Based on this observation, I've rewritten the patch as follows.
>>
>> diff --git a/include/linux/tnum.h b/include/linux/tnum.h
>> index 3c13240077b8..5e795d728b9f 100644
>> --- a/include/linux/tnum.h
>> +++ b/include/linux/tnum.h
>> @@ -52,6 +52,9 @@ struct tnum tnum_mul(struct tnum a, struct tnum b);
>>    /* Return a tnum representing numbers satisfying both @a and @b */
>>    struct tnum tnum_intersect(struct tnum a, struct tnum b);
>>
>> +/* Return a tnum representing numbers satisfying either @a or @b */
>> +struct tnum tnum_union(struct tnum a, struct tnum b);
>> +
>>    /* Return @a with all but the lowest @size bytes cleared */
>>    struct tnum tnum_cast(struct tnum a, u8 size);
>>
>> diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
>> index 9dbc31b25e3d..9d4480a683ca 100644
>> --- a/kernel/bpf/tnum.c
>> +++ b/kernel/bpf/tnum.c
>> @@ -150,6 +150,29 @@ struct tnum tnum_intersect(struct tnum a, struct tnum b)
>>           return TNUM(v & ~mu, mu);
>>    }
>>
>> +/*
>> + * Each bit has 3 states: unkown, known 0, known 1. If using x to represent
>> + * unknown state, the result of the union of two bits is as follows:
>> + *
>> + *         | x    0    1
>> + *    -----+------------
>> + *     x   | x    x    x
>> + *     0   | x    0    x
>> + *     1   | x    x    1
>> + *
>> + * For tnum a and b, only the bits that are both known 0 or known 1 in a
>> + * and b are known in the result of union a and b.
>> + */
>> +struct tnum tnum_union(struct tnum a, struct tnum b)
>> +{
>> +       u64 v0, v1, mu;
>> +
>> +       mu = a.mask | b.mask; // unkown bits either in a or b
>> +       v1 = (a.value & b.value) & ~mu; // "known 1" bits in both a and b
>> +       v0 = (~a.value & ~b.value) & ~mu; // "known 0" bits in both a and b
> 
> no C++-style comments, please
>

OK, will fix in the formal patch.

>> +       return TNUM(v1, mu | ~(v0 | v1));
>> +}
>> +
> 
> I've CC'ed Edward, hopefully he can take a look as well. Please CC him
> on future patches touching tnum as well.
> 

Sure

>>    struct tnum tnum_cast(struct tnum a, u8 size)
>>    {
>>           a.value &= (1ULL << (size * 8)) - 1;
>>    {
>>           a.value &= (1ULL << (size * 8)) - 1;
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 8f0f2e21699e..b69c89bc5cfc 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -13478,6 +13478,28 @@ static void scalar32_min_max_and(struct bpf_reg_state *dst_reg,
>>                   return;
>>           }
>>
>> +       /* Special case: dst_reg is in range [-1, 0] */
>> +       if (dst_reg->s32_min_value == -1 && dst_reg->s32_max_value == 0) {
>> +               var32_off = tnum_union(src_reg->var_off, tnum_const(0));
>> +               dst_reg->var_off = tnum_with_subreg(dst_reg->var_off, var32_off);
>> +               dst_reg->u32_min_value = var32_off.value;
>> +               dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);
> 
> can you explain the logic behing u32 min/max updates, especially that
> we use completely different values for min/max and it's not clear why
> u32_min <= u32_max invariant will always hold. Same below
>

We're adding 0 to the existing range, and 0 is the smallest unsigned
number, so the resulted unsigned min can only get smaller, and the
unsigned max will not be affected. In fact, since 0 is added to the
range, var32_off.value should be 0. And since -1 is in included in
dst_reg, dst_reg->u32_max_value should be -1U, the maximum unsigned
integer. So we can just set u32_min to 0, and set u32_max to umax_val.

>> +               dst_reg->s32_min_value = min_t(s32, src_reg->s32_min_value, 0);
>> +               dst_reg->s32_max_value = max_t(s32, src_reg->s32_max_value, 0);
>> +               return;
>> +       }
>> +
>> +       /* Special case: src_reg is in range [-1, 0] */
>> +       if (src_reg->s32_min_value == -1 && src_reg->s32_max_value == 0) {
>> +               var32_off = tnum_union(dst_reg->var_off, tnum_const(0));
>> +               dst_reg->var_off = tnum_with_subreg(dst_reg->var_off, var32_off);
>> +               dst_reg->u32_min_value = var32_off.value;
>> +               dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);
>> +               dst_reg->s32_min_value = min_t(s32, dst_reg->s32_min_value, 0);
>> +               dst_reg->s32_max_value = max_t(s32, dst_reg->s32_max_value, 0);
>> +               return;
>> +       }
>> +
>>           /* We get our minimum from the var_off, since that's inherently
>>            * bitwise.  Our maximum is the minimum of the operands' maxima.
>>            */
>> @@ -13508,6 +13530,26 @@ static void scalar_min_max_and(struct bpf_reg_state *dst_reg,
>>                   return;
>>           }
>>
>> +       /* Special case: dst_reg is in range [-1, 0] */
>> +       if (dst_reg->smin_value == -1 && dst_reg->smax_value == 0) {
>> +               dst_reg->var_off = tnum_union(src_reg->var_off, tnum_const(0));
>> +               dst_reg->umin_value = dst_reg->var_off.value;
>> +               dst_reg->umax_value = min(dst_reg->umax_value, umax_val);
>> +               dst_reg->smin_value = min_t(s64, src_reg->smin_value, 0);
>> +               dst_reg->smax_value = max_t(s64, src_reg->smax_value, 0);
>> +               return;
>> +       }
>> +
>> +       /* Special case: src_reg is in range [-1, 0] */
>> +       if (src_reg->smin_value == -1 && src_reg->smax_value == 0) {
>> +               dst_reg->var_off = tnum_union(dst_reg->var_off, tnum_const(0));
>> +               dst_reg->umin_value = dst_reg->var_off.value;
>> +               dst_reg->umax_value = min(dst_reg->umax_value, umax_val);
>> +               dst_reg->smin_value = min_t(s64, dst_reg->smin_value, 0);
>> +               dst_reg->smax_value = max_t(s64, dst_reg->smax_value, 0);
>> +               return;
>> +       }
>> +
>>
>>>> +               return;
>>>> +       }
>>>> +
>>>> +       /* Special case: dst_reg is known and src_reg is in range [-1, 0] */
>>>> +       if (dst_known &&
>>>> +               src_reg->s32_min_value == -1 && src_reg->s32_max_value == 0 &&
>>>> +               src_reg->smin_value == -1 && src_reg->smax_value == 0) {
>>>> +               dst_reg->s32_min_value = min_t(s32, dst_reg->s32_min_value, 0);
>>>> +               dst_reg->s32_max_value = max_t(s32, dst_reg->s32_min_value, 0);
>>>> +               return;
>>>> +       }
>>>> +
>>>>            /* Safe to set s32 bounds by casting u32 result into s32 when u32
>>>>             * doesn't cross sign boundary. Otherwise set s32 bounds to unbounded.
>>>>             */
>>>
>>> [...]
>>>
>>

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