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Date: Thu, 21 Mar 2024 09:45:05 +0100
From: Ilya Leoshkevich <iii@linux.ibm.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Puranjay Mohan <puranjay12@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>, David Ahern <dsahern@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf] bpf: verifier: prevent userspace memory access
Message-ID: <ed5cozsc7mduzmgbwrlw3lou4tlb6zpivhs2xrqwgpq2rhvkue@l7aifknll4tb>
References: <20240320105436.4781-1-puranjay12@gmail.com>
 <CAADnVQJ3o6DsURi=N_KXx+mbW9r7__3LrwYLyYwuoMOsqFHPkw@mail.gmail.com>
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On Wed, Mar 20, 2024 at 11:08:00PM -0700, Alexei Starovoitov wrote:
> On Wed, Mar 20, 2024 at 3:55 AM 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 =  SRC_REG
> > if(offset)
> >         REG_AX += offset;
> > REG_AX >>= 32;
> > if (REG_AX <= (uaddress_limit >> 32))
> >         DST_REG = 0;
> > else
> >         DST_REG = *(size *)(SRC_REG + offset);
> 
> The patch looks good, but it seems to be causing s390 CI failures.
> 
> 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.