linux-kernel - Re: [PATCH bpf-next 1/2] bpf: Skip bounds adjustment for conditional jumps on same register

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Message-ID: <CAADnVQ+Tt4GkhV74P_bekvCWbZOBgR5PEYt7sQHEzMTDeE4j9w@mail.gmail.com>
Date: Fri, 24 Oct 2025 09:40:03 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: KaFai Wan <kafai.wan@...ux.dev>
Cc: Eduard Zingerman <eddyz87@...il.com>, Yonghong Song <yonghong.song@...ux.dev>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	John Fastabend <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Shuah Khan <shuah@...nel.org>, Paul Chaignon <paul.chaignon@...il.com>, 
	Matan Shachnai <m.shachnai@...il.com>, Luis Gerhorst <luis.gerhorst@....de>, colin.i.king@...il.com, 
	Harishankar Vishwanathan <harishankar.vishwanathan@...il.com>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, Kaiyan Mei <M202472210@...t.edu.cn>, 
	Yinhao Hu <dddddd@...t.edu.cn>
Subject: Re: [PATCH bpf-next 1/2] bpf: Skip bounds adjustment for conditional
 jumps on same register

On Fri, Oct 24, 2025 at 9:38 AM KaFai Wan <kafai.wan@...ux.dev> wrote:
>
> On Fri, 2025-10-24 at 09:21 -0700, Eduard Zingerman wrote:
> > On Sat, 2025-10-25 at 00:13 +0800, KaFai Wan wrote:
> >
> > [...]
> >
> > > For non-scalar cases we only allow pointer comparison on pkt_ptr, this check is before
> > > is_branch_taken()
> > >
> > >     src_reg = &regs[insn->src_reg];
> > >     if (!(reg_is_pkt_pointer_any(dst_reg) && reg_is_pkt_pointer_any(src_reg)) &&
> > >         is_pointer_value(env, insn->src_reg)) {
> > >             verbose(env, "R%d pointer comparison prohibited\n",
> > >                     insn->src_reg);
> > >             return -EACCES;
> > >     }
> > >
> > > and in the end of check_cond_jmp_op() (after is_branch_taken()), we checked again
> > >
> > >     } else if (!try_match_pkt_pointers(insn, dst_reg, &regs[insn->src_reg],
> > >                                        this_branch, other_branch) &&
> > >                is_pointer_value(env, insn->dst_reg)) {
> > >             verbose(env, "R%d pointer comparison prohibited\n",
> > >                     insn->dst_reg);
> > >             return -EACCES;
> > >     }
> > >
> > > this time we check if it is valid comparison on pkt_ptr in try_match_pkt_pointers().
> > >
> > > Currently we just allow 4 opcode (BPF_JGT, BPF_JLT, BPF_JGE, BPF_JLE) on pkt_ptr, and with
> > > conditions. But we bypass these prohibits in privileged mode (is_pointer_value() always
> > > return false in privileged mode).
> > >
> > > So the logic skip these prohibits for pkt_ptr in unprivileged mode.
> >
> > Well, yes, but do you really need to do forbid `if r0 > r0 goto ...` in unpriv?
>
> Currently `if r0 > r0 goto ...` is forbid in unpriv, but we can allow it.

Let's not relax unpriv. We don't need new threads with researchers
whether such things can be exploited.