From: Kees Cook <keescook@chromium.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Elena Reshetova <elena.reshetova@intel.com>,
x86@kernel.org, Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Potapenko <glider@google.com>,
Alexander Popov <alex.popov@linux.com>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Jann Horn <jannh@google.com>, Vlastimil Babka <vbabka@suse.cz>,
David Hildenbrand <david@redhat.com>,
Mike Rapoport <rppt@linux.ibm.com>,
Andrew Morton <akpm@linux-foundation.org>,
Jonathan Corbet <corbet@lwn.net>,
Randy Dunlap <rdunlap@infradead.org>,
kernel-hardening@lists.openwall.com,
linux-hardening@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 4/6] x86/entry: Enable random_kstack_offset support
Date: Mon, 29 Mar 2021 11:43:23 -0700 [thread overview]
Message-ID: <202103291141.EC2A77731@keescook> (raw)
In-Reply-To: <87h7kvcqen.ffs@nanos.tec.linutronix.de>
On Sun, Mar 28, 2021 at 04:18:56PM +0200, Thomas Gleixner wrote:
> On Fri, Mar 19 2021 at 14:28, Kees Cook wrote:
> > +
> > + /*
> > + * x86_64 stack alignment means 3 bits are ignored, so keep
> > + * the top 5 bits. x86_32 needs only 2 bits of alignment, so
> > + * the top 6 bits will be used.
> > + */
> > + choose_random_kstack_offset(rdtsc() & 0xFF);
>
> Comment mumbles about 5/6 bits and the TSC value is masked with 0xFF and
> then the applied offset is itself limited with 0x3FF.
>
> Too many moving parts for someone who does not have the details of all
> this memorized.
Each piece is intentional -- I will improve the comments to explain
each level of masking happening (implicit compiler stack alignment mask,
explicit per-arch mask, and the VLA upper-bound protection mask).
--
Kees Cook
next prev parent reply other threads:[~2021-03-29 18:43 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-19 21:28 [PATCH v7 0/6] Optionally randomize kernel stack offset each syscall Kees Cook
2021-03-19 21:28 ` [PATCH v7 1/6] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2021-03-19 21:28 ` [PATCH v7 2/6] init_on_alloc: Optimize static branches Kees Cook
2021-03-19 21:28 ` [PATCH v7 3/6] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2021-03-28 14:42 ` Thomas Gleixner
2021-03-29 18:41 ` Kees Cook
2021-03-19 21:28 ` [PATCH v7 4/6] x86/entry: Enable random_kstack_offset support Kees Cook
2021-03-20 11:58 ` Ingo Molnar
2021-03-21 17:03 ` Kees Cook
2021-03-28 14:18 ` Thomas Gleixner
2021-03-29 18:43 ` Kees Cook [this message]
2021-03-19 21:28 ` [PATCH v7 5/6] arm64: entry: " Kees Cook
2021-04-01 8:34 ` Will Deacon
2021-03-19 21:28 ` [PATCH v7 6/6] lkdtm: Add REPORT_STACK for checking stack offsets Kees Cook
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