From: Eduard Zingerman <eddyz87@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Martin KaFai Lau <martin.lau@linux.dev>,
Kernel Team <kernel-team@fb.com>,
Yonghong Song <yonghong.song@linux.dev>,
"Jose E. Marchesi" <jose.marchesi@oracle.com>
Subject: Re: [bpf-next v3 11/12] bpf: do check_nocsr_stack_contract() for ARG_ANYTHING helper params
Date: Tue, 16 Jul 2024 11:15:18 -0700 [thread overview]
Message-ID: <f27a6146f8ef01fe01efc8b69cba1263b3f45ce9.camel@gmail.com> (raw)
In-Reply-To: <86c8004aab94e0e833b438ef2fba25f0835a9aa8.camel@gmail.com>
On Tue, 2024-07-16 at 03:03 -0700, Eduard Zingerman wrote:
> On Mon, 2024-07-15 at 19:00 -0700, Alexei Starovoitov wrote:
> > On Mon, Jul 15, 2024 at 4:02 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> [...]
>
> > > This might lead to a surprising behavior in combination with nocsr
> > > rewrites, e.g. consider the program below:
> > >
> > > 1: r1 = 1;
> > > /* nocsr pattern with stack offset -16 */
> > > 2: *(u64 *)(r10 - 16) = r1;
> > > 3: call %[bpf_get_smp_processor_id];
> > > 4: r1 = *(u64 *)(r10 - 16);
> > > 5: r1 = r10;
> > > 6: r1 += -8;
> > > 7: r2 = 1;
> > > 8: r3 = r10;
> > > 9: r3 += -16;
> > > /* bpf_probe_read_kernel(dst: &fp[-8], size: 1, src: &fp[-16]) */
> > > 10: call %[bpf_probe_read_kernel];
> > > 11: exit;
> > >
> > > Here nocsr rewrite logic would remove instructions (2) and (4).
> > > However, (2) writes a value that is later read by a call at (10).
> >
> > This makes no sense to me.
> > This bpf prog is broken.
> > If probe_read is used to read stack it will read garbage.
> > JITs and the verifier are allowed to do any transformation
> > that keeps the program semantics and safety.
Ok, my bad, the following program works at the moment:
SEC("socket") // <---- used wrong program type
__retval(42)
__success
int bpf_probe_read_kernel_stack_ptr(void *ctx)
{
unsigned long a = 17;
unsigned long b = 42;
int err;
err = bpf_probe_read_kernel(&a, 8, &b);
if (err)
return 1;
return a;
}
And it is compiled to BPF as one would expect:
... fp[-8,-16] setup ...
4: r1 = r10
5: r1 += -0x8
6: r3 = r10
7: r3 += -0x10
8: w2 = 0x8
9: call 0x71
... return check ...
So, the point stands: from C compiler pov pointer &b escapes,
and compiler is not really allowed to replace object at that offset
with garbage. Why do you think the program is broken?
I don't mind dropping the patch in question, but I agree with Andrii's
viewpoint that there is nothing wrong with this use case.
next prev parent reply other threads:[~2024-07-16 18:15 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-07-15 23:01 [bpf-next v3 00/12] no_caller_saved_registers attribute for helper calls Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 01/12] bpf: add a get_helper_proto() utility function Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 02/12] bpf: no_caller_saved_registers attribute for helper calls Eduard Zingerman
2024-07-16 1:51 ` Alexei Starovoitov
2024-07-16 5:34 ` Eduard Zingerman
2024-07-20 2:00 ` Alexei Starovoitov
2024-07-22 18:42 ` Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 03/12] bpf, x86, riscv, arm: no_caller_saved_registers for bpf_get_smp_processor_id() Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 04/12] selftests/bpf: extract utility function for BPF disassembly Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 05/12] selftests/bpf: print correct offset for pseudo calls in disasm_insn() Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 06/12] selftests/bpf: no need to track next_match_pos in struct test_loader Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 07/12] selftests/bpf: extract test_loader->expect_msgs as a data structure Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 08/12] selftests/bpf: allow checking xlated programs in verifier_* tests Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 09/12] selftests/bpf: __arch_* macro to limit test cases to specific archs Eduard Zingerman
2024-07-15 23:01 ` [bpf-next v3 10/12] selftests/bpf: test no_caller_saved_registers spill/fill removal Eduard Zingerman
2024-07-15 23:02 ` [bpf-next v3 11/12] bpf: do check_nocsr_stack_contract() for ARG_ANYTHING helper params Eduard Zingerman
2024-07-16 2:00 ` Alexei Starovoitov
2024-07-16 10:03 ` Eduard Zingerman
2024-07-16 18:15 ` Eduard Zingerman [this message]
2024-07-20 1:54 ` Alexei Starovoitov
2024-07-20 1:58 ` Eduard Zingerman
2024-07-15 23:02 ` [bpf-next v3 12/12] selftests/bpf: check nocsr contract for bpf_probe_read_kernel() Eduard Zingerman
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