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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
Date: Thu, 1 Apr 2021 15:42:08 -0700	[thread overview]
Message-ID: <202104011447.2E7F543@keescook> (raw)
In-Reply-To: <61ae9398a03d4fe7868b68c9026d5998@AcuMS.aculab.com>

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

  reply	other threads:[~2021-04-01 22:42 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-30 20:57 [PATCH v8 0/6] " Kees Cook
2021-03-30 20:57 ` [PATCH v8 1/6] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2021-03-30 20:57 ` [PATCH v8 2/6] init_on_alloc: Optimize static branches Kees Cook
2021-03-30 20:57 ` [PATCH v8 3/6] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2021-03-31  7:53   ` Thomas Gleixner
2021-03-31 21:54     ` Kees Cook
2021-03-31 22:38       ` Thomas Gleixner
2021-04-01  6:31         ` Kees Cook
2021-04-01  8:30   ` Will Deacon
2021-04-01 11:15     ` David Laight
2021-04-01 22:42       ` Kees Cook [this message]
2021-03-30 20:57 ` [PATCH v8 4/6] x86/entry: Enable random_kstack_offset support Kees Cook
2021-03-31  7:50   ` Thomas Gleixner
2021-03-30 20:57 ` [PATCH v8 5/6] arm64: entry: " Kees Cook
2021-03-30 20:57 ` [PATCH v8 6/6] lkdtm: Add REPORT_STACK for checking stack offsets Kees Cook
2021-04-01 19:17 ` [PATCH] Where we are for this patch? Roy Yang
2021-04-01 19:48   ` Al Viro
2021-04-01 20:13     ` Theodore Ts'o
2021-04-01 21:46   ` [PATCH v8 0/6] Optionally randomize kernel stack offset each syscall Kees Cook

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