Re: [PATCH bpf-next v2 15/15] selftests/bpf: states pruning checks for scalar vs STACK_{MISC,ZERO}

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Maxim Mikityanskiy <maxtram95@gmail.com>
Cc: Eduard Zingerman <eddyz87@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	 Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	 Shung-Hsi Yu <shung-hsi.yu@suse.com>,
	John Fastabend <john.fastabend@gmail.com>,
	 Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>,
	 Yonghong Song <yonghong.song@linux.dev>,
	KP Singh <kpsingh@kernel.org>,
	 Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>,  Mykola Lysenko <mykolal@fb.com>,
	Shuah Khan <shuah@kernel.org>,
	 "David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	 Jesper Dangaard Brouer <hawk@kernel.org>,
	bpf@vger.kernel.org, linux-kselftest@vger.kernel.org,
	 netdev@vger.kernel.org
Subject: Re: [PATCH bpf-next v2 15/15] selftests/bpf: states pruning checks for scalar vs STACK_{MISC,ZERO}
Date: Tue, 9 Jan 2024 16:27:38 -0800	[thread overview]
Message-ID: <CAEf4BzaQzWLJqMuOtBX+wmH4EH43nYjTcuVun5nGUwEY_6E7RA@mail.gmail.com> (raw)
In-Reply-To: <20240108205209.838365-16-maxtram95@gmail.com>

On Mon, Jan 8, 2024 at 12:53 PM Maxim Mikityanskiy <maxtram95@gmail.com> wrote:
>
> From: Eduard Zingerman <eddyz87@gmail.com>
>
> Check that stacksafe() considers the following old vs cur stack spill
> state combinations equivalent:
> - spill of unbound scalar vs combination of STACK_{MISC,ZERO,INVALID}
> - STACK_MISC vs spill of unbound scalar
> - spill of scalar 0 vs STACK_ZERO
> - STACK_ZERO vs spill of scalar 0
>
> Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> ---
>  .../selftests/bpf/progs/verifier_spill_fill.c | 192 ++++++++++++++++++
>  1 file changed, 192 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> index 3764111d190d..3cd3fe30357f 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> @@ -1044,4 +1044,196 @@ l0_%=:  r1 >>= 32;                                      \
>         : __clobber_all);
>  }
>
> +/* stacksafe(): check if spill of unbound scalar in old state is
> + * considered equivalent to any state of the spill in the current state.
> + *
> + * On the first verification path an unbound scalar is written for
> + * fp-8 and later marked precise.
> + * On the second verification path a mix of STACK_MISC/ZERO/INVALID is
> + * written to fp-8. These should be considered equivalent.
> + */
> +SEC("socket")
> +__success __log_level(2)
> +__msg("10: (79) r0 = *(u64 *)(r10 -8)")
> +__msg("10: safe")
> +__msg("processed 16 insns")
> +__flag(BPF_F_TEST_STATE_FREQ)
> +__naked void old_unbound_scalar_vs_cur_anything(void)
> +{
> +       asm volatile(
> +       /* get a random value for branching */
> +       "call %[bpf_ktime_get_ns];"
> +       "r7 = r0;"
> +       /* get a random value for storing at fp-8 */
> +       "call %[bpf_ktime_get_ns];"
> +       "if r7 == 0 goto 1f;"
> +       /* unbound scalar written to fp-8 */
> +       "*(u64*)(r10 - 8) = r0;"
> +       "goto 2f;"
> +"1:"
> +       /* mark fp-8 as mix of STACK_MISC/ZERO/INVALID */
> +       "r1 = 0;"
> +       "*(u8*)(r10 - 8) = r0;"

this is actually a spilled register, not STACK_ZERO. Is it important?

> +       "*(u8*)(r10 - 7) = r1;"
> +       /* fp-2..fp-6 remain STACK_INVALID */
> +       "*(u8*)(r10 - 1) = r0;"
> +"2:"
> +       /* read fp-8 and force it precise, should be considered safe
> +        * on second visit
> +        */
> +       "r0 = *(u64*)(r10 - 8);"
> +       "r0 &= 0xff;"
> +       "r1 = r10;"
> +       "r1 += r0;"
> +       "exit;"
> +       :
> +       : __imm(bpf_ktime_get_ns)
> +       : __clobber_all);
> +}
> +
> +/* stacksafe(): check if STACK_MISC in old state is considered
> + * equivalent to stack spill of unbound scalar in cur state.
> + */
> +SEC("socket")
> +__success __log_level(2)
> +__msg("8: (79) r0 = *(u64 *)(r10 -8)         ; R0_w=scalar(id=1) R10=fp0 fp-8=scalar(id=1)")
> +__msg("8: safe")
> +__msg("processed 11 insns")
> +__flag(BPF_F_TEST_STATE_FREQ)
> +__naked void old_unbound_scalar_vs_cur_stack_misc(void)
> +{
> +       asm volatile(
> +       /* get a random value for branching */
> +       "call %[bpf_ktime_get_ns];"
> +       "if r0 == 0 goto 1f;"
> +       /* conjure unbound scalar at fp-8 */
> +       "call %[bpf_ktime_get_ns];"
> +       "*(u64*)(r10 - 8) = r0;"
> +       "goto 2f;"
> +"1:"
> +       /* conjure STACK_MISC at fp-8 */
> +       "call %[bpf_ktime_get_ns];"
> +       "*(u64*)(r10 - 8) = r0;"
> +       "*(u32*)(r10 - 4) = r0;"
> +"2:"
> +       /* read fp-8, should be considered safe on second visit */
> +       "r0 = *(u64*)(r10 - 8);"
> +       "exit;"
> +       :
> +       : __imm(bpf_ktime_get_ns)
> +       : __clobber_all);
> +}
> +
> +/* stacksafe(): check if stack spill of unbound scalar in old state is
> + * considered equivalent to STACK_MISC in cur state.
> + */
> +SEC("socket")
> +__success  __log_level(2)
> +__msg("8: (79) r0 = *(u64 *)(r10 -8)         ; R0_w=scalar() R10=fp0 fp-8=mmmmmmmm")
> +__msg("8: safe")
> +__msg("processed 11 insns")
> +__flag(BPF_F_TEST_STATE_FREQ)
> +__naked void old_stack_misc_vs_cur_unbound_scalar(void)
> +{
> +       asm volatile(
> +       /* get a random value for branching */
> +       "call %[bpf_ktime_get_ns];"
> +       "if r0 == 0 goto 1f;"
> +       /* conjure STACK_MISC at fp-8 */
> +       "call %[bpf_ktime_get_ns];"
> +       "*(u64*)(r10 - 8) = r0;"
> +       "*(u32*)(r10 - 4) = r0;"
> +       "goto 2f;"
> +"1:"
> +       /* conjure unbound scalar at fp-8 */
> +       "call %[bpf_ktime_get_ns];"
> +       "*(u64*)(r10 - 8) = r0;"
> +"2:"
> +       /* read fp-8, should be considered safe on second visit */
> +       "r0 = *(u64*)(r10 - 8);"
> +       "exit;"
> +       :
> +       : __imm(bpf_ktime_get_ns)
> +       : __clobber_all);
> +}
> +
> +/* stacksafe(): check if spill of register with value 0 in old state
> + * is considered equivalent to STACK_ZERO.
> + */
> +SEC("socket")
> +__success __log_level(2)
> +__msg("9: (79) r0 = *(u64 *)(r10 -8)")
> +__msg("9: safe")
> +__msg("processed 15 insns")
> +__flag(BPF_F_TEST_STATE_FREQ)
> +__naked void old_spill_zero_vs_stack_zero(void)
> +{
> +       asm volatile(
> +       /* get a random value for branching */
> +       "call %[bpf_ktime_get_ns];"
> +       "r7 = r0;"
> +       /* get a random value for storing at fp-8 */
> +       "call %[bpf_ktime_get_ns];"
> +       "if r7 == 0 goto 1f;"
> +       /* conjure spilled register with value 0 at fp-8 */
> +       "*(u64*)(r10 - 8) = r0;"
> +       "if r0 != 0 goto 3f;"
> +       "goto 2f;"
> +"1:"
> +       /* conjure STACK_ZERO at fp-8 */
> +       "r1 = 0;"
> +       "*(u64*)(r10 - 8) = r1;"

this is not STACK_ZERO, it's full register spill

> +"2:"
> +       /* read fp-8 and force it precise, should be considered safe
> +        * on second visit
> +        */
> +       "r0 = *(u64*)(r10 - 8);"
> +       "r1 = r10;"
> +       "r1 += r0;"
> +"3:"
> +       "exit;"
> +       :
> +       : __imm(bpf_ktime_get_ns)
> +       : __clobber_all);
> +}
> +
> +/* stacksafe(): similar to old_spill_zero_vs_stack_zero() but the
> + * other way around: check if STACK_ZERO is considered equivalent to
> + * spill of register with value 0.
> + */
> +SEC("socket")
> +__success __log_level(2)
> +__msg("8: (79) r0 = *(u64 *)(r10 -8)")
> +__msg("8: safe")
> +__msg("processed 14 insns")
> +__flag(BPF_F_TEST_STATE_FREQ)
> +__naked void old_stack_zero_vs_spill_zero(void)
> +{
> +       asm volatile(
> +       /* get a random value for branching */
> +       "call %[bpf_ktime_get_ns];"
> +       "if r0 == 0 goto 1f;"
> +       /* conjure STACK_ZERO at fp-8 */
> +       "r1 = 0;"
> +       "*(u64*)(r10 - 8) = r1;"

same, please double check this STACK_xxx assumptions, as now we spill
registers instead of STACK_ZERO in a lot of cases

> +       "goto 2f;"
> +"1:"
> +       /* conjure spilled register with value 0 at fp-8 */
> +       "call %[bpf_ktime_get_ns];"
> +       "*(u64*)(r10 - 8) = r0;"
> +       "if r0 != 0 goto 3f;"
> +"2:"
> +       /* read fp-8 and force it precise, should be considered safe
> +        * on second visit
> +        */
> +       "r0 = *(u64*)(r10 - 8);"
> +       "r1 = r10;"
> +       "r1 += r0;"
> +"3:"
> +       "exit;"
> +       :
> +       : __imm(bpf_ktime_get_ns)
> +       : __clobber_all);
> +}
> +
>  char _license[] SEC("license") = "GPL";
> --
> 2.43.0
>