Re: [PATCH v7 3/6] stack: Optionally randomize kernel stack offset each syscall

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 3/6] stack: Optionally randomize kernel stack offset each syscall
Date: Mon, 29 Mar 2021 11:41:18 -0700	[thread overview]
Message-ID: <202103291139.54AA7CDE@keescook> (raw)
In-Reply-To: <87eefzcpc4.ffs@nanos.tec.linutronix.de>

On Sun, Mar 28, 2021 at 04:42:03PM +0200, Thomas Gleixner wrote:
> On Fri, Mar 19 2021 at 14:28, Kees Cook wrote:
> > +/*
> > + * Do not use this anywhere else in the kernel. This is used here because
> > + * it provides an arch-agnostic way to grow the stack with correct
> > + * alignment. Also, since this use is being explicitly masked to a max of
> > + * 10 bits, stack-clash style attacks are unlikely. For more details see
> > + * "VLAs" in Documentation/process/deprecated.rst
> 
> VLAs are bad, VLAs to the rescue! :)

I'm aware of the irony, but luto's idea really makes things easy. As
documented there, though, this has a hard-coded (low) upper bound, so
it's not like "regular" VLA use.

> 
> > + * The asm statement is designed to convince the compiler to keep the
> > + * allocation around even after "ptr" goes out of scope.
> > + */
> > +void *__builtin_alloca(size_t size);
> > +
> > +#define add_random_kstack_offset() do {					\
> > +	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
> > +				&randomize_kstack_offset)) {		\
> > +		u32 offset = this_cpu_read(kstack_offset);              \
> 
> Not that it matters on x86, but as this has to be called in the
> interrupt disabled region of the syscall entry, shouldn't this be a
> raw_cpu_read(). The asm-generic version has a preempt_disable/enable
> pair around the raw read for native wordsize reads, otherwise a
> irqsave/restore pair.
> 
> __this_cpu_read() is fine as well, but that has an sanity check before
> the raw read when CONFIG_DEBUG_PREEMPT is on, which is harmless but
> also pointless in this case.
> 
> Probably the same for the counterpart this_cpu_write().

Oh! Excellent point. I think this will make a big difference on arm64. I
will adjust and test.

-- 
Kees Cook