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Date:   Mon, 22 Jun 2020 16:07:11 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
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>,
        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>,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
 each syscall
Message-ID: <202006221604.871B13DE3@keescook>
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 <20200622225615.GA3511702@rani.riverdale.lan>
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On Mon, Jun 22, 2020 at 06:56:15PM -0400, Arvind Sankar wrote:
> On Mon, Jun 22, 2020 at 12:31:44PM -0700, Kees Cook wrote:
> > +
> > +#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(offset & 0x3FF);		\
> > +		asm volatile("" : "=m"(*ptr));				\
> > +	}								\
> > +} while (0)
> 
> This feels a little fragile. ptr doesn't escape the block, so the
> compiler is free to restore the stack immediately after this block. In
> fact, given that all you've said is that the asm modifies *ptr, but
> nothing uses that output, the compiler could eliminate the whole thing,
> no?
> 
> https://godbolt.org/z/HT43F5
> 
> gcc restores the stack immediately, if no function calls come after it.
> 
> clang completely eliminates the code if no function calls come after.

nothing uses the stack in your example. And adding a barrier (which is
what the "=m" is, doesn't change it.

> I'm not sure why function calls should affect it, though, given that
> those functions can't possibly access ptr or the memory it points to.

It seems to work correctly:
https://godbolt.org/z/c3W5BW

-- 
Kees Cook