From: Will Deacon <will@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
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>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Potapenko <glider@google.com>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Jann Horn <jannh@google.com>,
"Perla, Enrico" <enrico.perla@intel.com>,
kernel-hardening@lists.openwall.com,
linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support
Date: Tue, 21 Apr 2020 08:02:06 +0100 [thread overview]
Message-ID: <20200421070206.GB14448@willie-the-truck> (raw)
In-Reply-To: <202004201529.BB787BB@keescook>
On Mon, Apr 20, 2020 at 03:34:57PM -0700, Kees Cook wrote:
> On Mon, Apr 20, 2020 at 09:54:58PM +0100, Will Deacon wrote:
> > On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote:
> > > + /*
> > > + * Since the compiler chooses a 4 bit alignment for the stack,
> > > + * let's save one additional bit (9 total), which gets us up
> > > + * near 5 bits of entropy.
> > > + */
> > > + choose_random_kstack_offset(get_random_int() & 0x1FF);
> >
> > Hmm, this comment doesn't make any sense to me. I mean, I get that 0x1ff
> > is 9 bits, and that is 4+5 but so what?
>
> Er, well, yes. I guess I was just trying to explain why there were 9
> bits saved here and to document what I was seeing the compiler actually
> doing with the values. (And it serves as a comparison to the x86 comment
> which is explaining similar calculations in the face of x86_64 vs ia32.)
>
> Would something like this be better?
>
> /*
> * Since the compiler uses 4 bit alignment for the stack (1 more than
> * x86_64), let's try to match the 5ish-bit entropy seen in x86_64,
> * instead of having needlessly lower entropy. As a result, keep the
> * low 9 bits.
> */
Yes, thank you! I was missing the comparison to x86_64 and so the one
"additional" bit didn't make sense to me.
With the new comment:
Acked-by: Will Deacon <will@kernel.org>
I'm assuming you're merging this via some other tree, but let me know
if you need anything else from me.
Cheers,
Will
next prev parent reply other threads:[~2020-04-21 7:02 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-24 20:32 [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-24 20:32 ` [PATCH v2 1/5] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2020-03-24 22:06 ` Peter Zijlstra
2020-03-24 20:32 ` [PATCH v2 2/5] init_on_alloc: Unpessimize default-on builds Kees Cook
2020-03-26 15:48 ` Alexander Potapenko
2020-03-24 20:32 ` [PATCH v2 3/5] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-30 11:25 ` Mark Rutland
2020-03-30 18:18 ` Kees Cook
2020-03-30 18:27 ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 4/5] x86/entry: Enable random_kstack_offset support Kees Cook
2020-03-28 22:26 ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 5/5] arm64: entry: " Kees Cook
2020-03-25 13:21 ` Mark Rutland
2020-03-25 20:22 ` Kees Cook
2020-03-26 11:15 ` Mark Rutland
2020-03-26 16:31 ` Kees Cook
2020-03-30 11:26 ` Mark Rutland
2020-04-20 20:54 ` Will Deacon
2020-04-20 22:34 ` Kees Cook
2020-04-21 7:02 ` Will Deacon [this message]
2020-03-24 21:28 ` [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Jann Horn
2020-03-24 23:07 ` Kees Cook
2020-03-25 12:15 ` Reshetova, Elena
2020-03-25 20:27 ` Kees Cook
2020-03-25 23:20 ` Jann Horn
2020-03-26 17:18 ` Kees Cook
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