linux-kernel - Re: [PATCH bpf-next v4 9/9] bpf: Fall back to nospec for sanitization-failures

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Message-ID: <CAADnVQLC_zViaCs5Huu63Jr2oCx1NGY3f_VCkJhrKvqst7HL=g@mail.gmail.com>
Date: Mon, 9 Jun 2025 22:31:31 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Luis Gerhorst <luis.gerhorst@....de>
Cc: 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@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Puranjay Mohan <puranjay@...nel.org>, 
	Xu Kuohai <xukuohai@...weicloud.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Hari Bathini <hbathini@...ux.ibm.com>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Naveen N Rao <naveen@...nel.org>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Nicholas Piggin <npiggin@...il.com>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	Henriette Herzog <henriette.herzog@....de>, Saket Kumar Bhaskar <skb99@...ux.ibm.com>, 
	Cupertino Miranda <cupertino.miranda@...cle.com>, Jiayuan Chen <mrpre@....com>, 
	Matan Shachnai <m.shachnai@...il.com>, Dimitar Kanaliev <dimitar.kanaliev@...eground.com>, 
	Shung-Hsi Yu <shung-hsi.yu@...e.com>, Daniel Xu <dxu@...uu.xyz>, bpf <bpf@...r.kernel.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>, 
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, Maximilian Ott <ott@...fau.de>, Milan Stephan <milan.stephan@....de>
Subject: Re: [PATCH bpf-next v4 9/9] bpf: Fall back to nospec for sanitization-failures

On Tue, Jun 3, 2025 at 2:32 PM Luis Gerhorst <luis.gerhorst@....de> wrote:
>
> ALU sanitization was introduced to ensure that a subsequent ptr access
> can never go OOB, even under speculation. This is required because we
> currently allow speculative scalar confusion. Spec. scalar confusion is
> possible because Spectre v4 sanitization only adds a nospec after
> critical stores (e.g., scalar overwritten with a pointer).
>
> If we add a nospec before the ALU op, none of the operands can be
> subject to scalar confusion. As an ADD/SUB can not introduce scalar
> confusion itself, the result will also not be subject to scalar
> confusion. Therefore, the subsequent ptr access is always safe.
>
> We directly fall back to nospec for the sanitization errors
> REASON_BOUNDS, _TYPE, _PATHS, and _LIMIT, even if we are not on a
> speculative path.
>
> For REASON_STACK, we return the error -ENOMEM directly now. Previously,
> sanitize_err() returned -EACCES for this case but we change it to
> -ENOMEM because doing so prevents do_check() from falling back to a
> nospec if we are on a speculative path. This would not be a serious
> issue (the verifier would probably run into the -ENOMEM again shortly on
> the next non-speculative path and still abort verification), but -ENOMEM
> is more fitting here anyway. An alternative would be -EFAULT, which is
> also returned for some of the other cases where push_stack() fails, but
> this is more frequently used for verifier-internal bugs.
>
> Signed-off-by: Luis Gerhorst <luis.gerhorst@....de>
> Acked-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> Acked-by: Henriette Herzog <henriette.herzog@....de>
> Cc: Maximilian Ott <ott@...fau.de>
> Cc: Milan Stephan <milan.stephan@....de>
> ---
>  kernel/bpf/verifier.c                         | 89 ++++++-----------
>  .../selftests/bpf/progs/verifier_bounds.c     |  5 +-
>  .../bpf/progs/verifier_bounds_deduction.c     | 45 ++++++---
>  .../selftests/bpf/progs/verifier_map_ptr.c    | 20 +++-
>  .../bpf/progs/verifier_value_ptr_arith.c      | 97 ++++++++++++++++---
>  5 files changed, 160 insertions(+), 96 deletions(-)

I applied the first 8, but this patch had odd merge conflicts
and I didn't want to risk it. Please rebase.