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

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Date:   Mon, 29 Mar 2021 11:41:18 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Elena Reshetova <elena.reshetova@...el.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Alexander Popov <alex.popov@...ux.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <jannh@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/6] stack: Optionally randomize kernel stack offset
 each syscall

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