Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 1 Apr 2021 15:42:08 -0700
From:   Kees Cook <keescook@chromium.org>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     'Will Deacon' <will@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "x86@kernel.org" <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>,
        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" 
        <kernel-hardening@lists.openwall.com>,
        "linux-hardening@vger.kernel.org" <linux-hardening@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset
 each syscall
Message-ID: <202104011447.2E7F543@keescook>
References: <20210330205750.428816-1-keescook@chromium.org>
 <20210330205750.428816-4-keescook@chromium.org>
 <20210401083034.GA8554@willie-the-truck>
 <61ae9398a03d4fe7868b68c9026d5998@AcuMS.aculab.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <61ae9398a03d4fe7868b68c9026d5998@AcuMS.aculab.com>
Precedence: bulk

On Thu, Apr 01, 2021 at 11:15:43AM +0000, David Laight wrote:
> From: Will Deacon
> > Sent: 01 April 2021 09:31
> ...
> > > +/*
> > > + * These macros must be used during syscall entry when interrupts and
> > > + * preempt are disabled, and after user registers have been stored to
> > > + * the stack.
> > > + */
> > > +#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);		\
> > > +		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
> > > +		asm volatile("" : "=m"(*ptr) :: "memory");		\
> > 
> > Using the "m" constraint here is dangerous if you don't actually evaluate it
> > inside the asm. For example, if the compiler decides to generate an
> > addressing mode relative to the stack but with writeback (autodecrement), then
> > the stack pointer will be off by 8 bytes. Can you use "o" instead?

I see other examples of empty asm, but it's true, none are using "=m" read
constraints. But, yes, using "o" appears to work happily.

> Is it allowed to use such a mode?
> It would have to know that the "m" was substituted exactly once.
> I think there are quite a few examples with 'strange' uses of memory
> asm arguments.
> 
> However, in this case, isn't it enough to ensure the address is 'saved'?
> So:
> 	asm volatile("" : "=r"(ptr) );
> should be enough.

It isn't, it seems.

Here's a comparison:

https://godbolt.org/z/xYGn9GfGY

So, I'll resend with "o", and with raw_cpu_*().

Thanks!

-- 
Kees Cook