linux-kernel - Re: [PATCH bpf v2] bpf: verifier: prevent userspace memory access

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Message-ID: <CAADnVQLHrmkJ5p2gEUJkf_CRxq9gv8rcSuBm5GeZ_nUJxQOE0Q@mail.gmail.com>
Date: Thu, 21 Mar 2024 04:31:29 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Puranjay Mohan <puranjay12@...il.com>, Ilya Leoshkevich <iii@...ux.ibm.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>
Subject: Re: [PATCH bpf v2] bpf: verifier: prevent userspace memory access

On Thu, Mar 21, 2024 at 4:05 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Mar 21, 2024 at 3:11 AM Puranjay Mohan <puranjay12@...il.com> wrote:
> >
> > diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
> > index e613eebfd349..e61a51a5b4be 100644
> > --- a/arch/s390/net/bpf_jit_comp.c
> > +++ b/arch/s390/net/bpf_jit_comp.c
> > @@ -2691,3 +2691,8 @@ bool bpf_jit_supports_subprog_tailcalls(void)
> >  {
> >         return true;
> >  }
> > +
> > +u64 bpf_arch_uaddress_limit(void)
> > +{
> > +       return -ENOTSUPP;
> > +}
>
> Looks good and should work, but s390 CI is still not happy.
> Ideas?
> sock tests were not failing before. So something is going on.

I think I have an explanation.
-ENOTSUPP and u64... and later:
u64 uaddress_limit = bpf_arch_uaddress_limit()
if (uaddress_limit < 0)

I bet the compiler simply removes this check since unsigned cannot
be negative.
Odd that there is no compiler warning.

pw-bot: cr