netdev - Re: [PATCH bpf v5] bpf: verifier: prevent userspace memory access

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Date: Sun, 24 Mar 2024 19:34:56 +0000
From: Puranjay Mohan <puranjay12@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern
 <dsahern@...nel.org>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
 <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Martin KaFai
 Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu
 <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, 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>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>, "H. Peter Anvin"
 <hpa@...or.com>, Jean-Philippe Brucker <jean-philippe@...aro.org>, Network
 Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, LKML
 <linux-kernel@...r.kernel.org>, Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: Re: [PATCH bpf v5] bpf: verifier: prevent userspace memory access

Alexei Starovoitov <alexei.starovoitov@...il.com> writes:

> On Sun, Mar 24, 2024 at 11:54 AM Puranjay Mohan <puranjay12@...il.com> wrote:
>>
>> +u64 bpf_arch_uaddress_limit(void)
>> +{
>> +       return max(TASK_SIZE_MAX + PAGE_SIZE, VSYSCALL_ADDR);
>
> This is broken. See my other email.
> Sadly you didn't test it.

Yes, sorry for this. I was relying on the CI for the test this time as
thought it would work. I just realised this would reject all addresses!

Given that the current x86-64 JIT just tests for TASK_SIZE_MAX +
PAGE_SIZE can we go ahead with this and later fix it for VSYSCALL_ADDR
as it is specific for x86-64?

Also, I will spend some time figuring out the best way to do this, there
should be some mathematical trick somewhere.