linux-kernel - Re: [PATCH bpf-next] bpf: make the verifier trace the "not qeual" for regs

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Date:   Mon, 11 Dec 2023 07:03:49 -0800
From:   Yonghong Song <yonghong.song@...ux.dev>
To:     Menglong Dong <menglong8.dong@...il.com>
Cc:     andrii@...nel.org, ast@...nel.org, daniel@...earbox.net,
        john.fastabend@...il.com, martin.lau@...ux.dev, song@...nel.org,
        kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: make the verifier trace the "not qeual" for
 regs


On 12/11/23 1:39 AM, Menglong Dong wrote:
> Hello,
>
> On Mon, Dec 11, 2023 at 1:09 PM Yonghong Song <yonghong.song@...ux.dev> wrote:
>>
>> On 12/10/23 5:00 AM, Menglong Dong wrote:
>>> We can derive some new information for BPF_JNE in regs_refine_cond_op().
>>> Take following code for example:
>>>
>>>     /* The type of "a" is u16 */
>>>     if (a > 0 && a < 100) {
>>>       /* the range of the register for a is [0, 99], not [1, 99],
>>>        * and will cause the following error:
>>>        *
>>>        *   invalid zero-sized read
>>>        *
>>>        * as a can be 0.
>>>        */
>>>       bpf_skb_store_bytes(skb, xx, xx, a, 0);
>>>     }
>> Could you have a C test to demonstrate this example?
>> Also, you should have a set of inline asm code (progs/verifier*.c)
>> to test various cases as in mark_reg32_not_equal() and
>> mark_reg_not_equal().
>>
> Yeah! I found that this part is tested in the test_progs/reg_bounds_crafted
> too, and this commit failed that test case, which I should fix in the next
> version.
>
>>> In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
>>> TRUE branch, the dst_reg will be marked as known to 0. However, in the
>>> fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
>>> the [min, max] for a is [0, 99], not [1, 99].
>>>
>>> For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
>>> const and is exactly the edge of the dst reg.
>>>
>>> Signed-off-by: Menglong Dong <menglong8.dong@...il.com>
>>> ---
>>>    kernel/bpf/verifier.c | 45 ++++++++++++++++++++++++++++++++++++++++++-
>>>    1 file changed, 44 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index 727a59e4a647..7b074ac93190 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -1764,6 +1764,40 @@ static void __mark_reg_const_zero(struct bpf_reg_state *reg)
>>>        reg->type = SCALAR_VALUE;
>>>    }
>>>
>>> +#define CHECK_REG_MIN(value)                 \
>>> +do {                                         \
>>> +     if ((value) == (typeof(value))imm)      \
>>> +             value++;                        \
>>> +} while (0)
>>> +
>>> +#define CHECK_REG_MAX(value)                 \
>>> +do {                                         \
>>> +     if ((value) == (typeof(value))imm)      \
>>> +             value--;                        \
>>> +} while (0)
>>> +
>>> +static void mark_reg32_not_equal(struct bpf_reg_state *reg, u64 imm)
>>> +{
>> What if reg->s32_min_value == imm and reg->s32_max_value == imm?
>> Has this been handled in previous verifier logic?
> Will such a case happen? In current code path, the src reg is a const,
> and the is_branch_taken() will return 0 or 1 if the
> dst_reg->s32_min_value == dst_reg->s32_max_value.
>
> Enn......maybe we can do more checking here in case that someone
> calls this function in another place.

I double checked the source code as well. Indeed, 'reg' should
not be a constant as it has been handled in is_branch_taken()
properly. Ignore my comments above then. Thanks!

>
> Thanks!
> Menglong Dong
>
>>> +             CHECK_REG_MIN(reg->s32_min_value);
>>> +             CHECK_REG_MAX(reg->s32_max_value);
>>> +             CHECK_REG_MIN(reg->u32_min_value);
>>> +             CHECK_REG_MAX(reg->u32_max_value);
>>> +}
>>> +
>>> +static void mark_reg_not_equal(struct bpf_reg_state *reg, u64 imm)
>>> +{
>>> +             CHECK_REG_MIN(reg->smin_value);
>>> +             CHECK_REG_MAX(reg->smax_value);
>>> +
>>> +             CHECK_REG_MIN(reg->umin_value);
>>> +             CHECK_REG_MAX(reg->umax_value);
>>> +
>>> +             CHECK_REG_MIN(reg->s32_min_value);
>>> +             CHECK_REG_MAX(reg->s32_max_value);
>>> +             CHECK_REG_MIN(reg->u32_min_value);
>>> +             CHECK_REG_MAX(reg->u32_max_value);
>>> +}
>>> +
>>>    static void mark_reg_known_zero(struct bpf_verifier_env *env,
>>>                                struct bpf_reg_state *regs, u32 regno)
>>>    {
>>> @@ -14332,7 +14366,16 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
>>>                }
>>>                break;
>>>        case BPF_JNE:
>>> -             /* we don't derive any new information for inequality yet */
>>> +             /* try to recompute the bound of reg1 if reg2 is a const and
>>> +              * is exactly the edge of reg1.
>>> +              */
>>> +             if (is_reg_const(reg2, is_jmp32)) {
>>> +                     val = reg_const_value(reg2, is_jmp32);
>>> +                     if (is_jmp32)
>>> +                             mark_reg32_not_equal(reg1, val);
>>> +                     else
>>> +                             mark_reg_not_equal(reg1, val);
>>> +             }
>>>                break;
>>>        case BPF_JSET:
>>>                if (!is_reg_const(reg2, is_jmp32))