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From: Puranjay Mohan <puranjay12@gmail.com>
To: Ilya Leoshkevich <iii@linux.ibm.com>, Alexei Starovoitov
 <alexei.starovoitov@gmail.com>
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Subject: Re: [PATCH bpf] bpf: verifier: prevent userspace memory access
In-Reply-To: <ed5cozsc7mduzmgbwrlw3lou4tlb6zpivhs2xrqwgpq2rhvkue@l7aifknll4tb>
References: <20240320105436.4781-1-puranjay12@gmail.com>
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Date: Thu, 21 Mar 2024 10:13:06 +0000
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Ilya Leoshkevich <iii@linux.ibm.com> writes:

> On Wed, Mar 20, 2024 at 11:08:00PM -0700, Alexei Starovoitov wrote:
>> On Wed, Mar 20, 2024 at 3:55=E2=80=AFAM Puranjay Mohan <puranjay12@gmail=
com> wrote:
>> >
>> > The JITs need to implement bpf_arch_uaddress_limit() to define where
>> > the userspace addresses end for that architecture or TASK_SIZE is taken
>> > as default.
>> >
>> > The implementation is as follows:
>> >
>> > REG_AX =3D  SRC_REG
>> > if(offset)
>> >         REG_AX +=3D offset;
>> > REG_AX >>=3D 32;
>> > if (REG_AX <=3D (uaddress_limit >> 32))
>> >         DST_REG =3D 0;
>> > else
>> >         DST_REG =3D *(size *)(SRC_REG + offset);
>>=20
>> The patch looks good, but it seems to be causing s390 CI failures.
>>=20
>> Ilya,
>> could you help us understand is this check needed on s390
>> and if so, what should be the uaddress_limit ?
>
> s390x does not define ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.
> Userspace and kernel run in completely different and isolated address
> spaces, so it's not possible to determine from a pointer value whether
> it's a user or a kernel pointer.
> But the good news is that whatever you deference without using
> special instruction sequences will refer to the kernel address space.
> So I wonder if we could somehow disable this check on s390x altogether?
> And if we are not sure whether it's a valid pointer, use BPF_PROBE_MEM
> as always.

Thanks for the details. I understand that s390x doesn't need this extra
check because all normal accesses are in the kernel address space and they
will be marked with BPF_PROBE_MEM by the verifier if the pointer is
untrusted.

I have sent v2 of this patch with this check disabled on s390x
https://lore.kernel.org/bpf/20240321101058.68530-1-puranjay12@gmail.com/

Thanks,
Puranjay