From mboxrd@z Thu Jan  1 00:00:00 1970
Subject: Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack
 at the end of syscalls
References: <1526488097-20611-1-git-send-email-alex.popov@linux.com>
 <1526488097-20611-3-git-send-email-alex.popov@linux.com>
 <20180518065349.GA10080@gmail.com>
From: Alexander Popov <alex.popov@linux.com>
Message-ID: <9db6b23e-e2da-8e95-40da-b26951848c8d@linux.com>
Date: Sat, 19 May 2018 00:12:49 +0300
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To: Ingo Molnar <mingo@kernel.org>
Cc: kernel-hardening@lists.openwall.com, Kees Cook <keescook@chromium.org>, PaX Team <pageexec@freemail.hu>, Brad Spengler <spender@grsecurity.net>, Andy Lutomirski <luto@kernel.org>, Tycho Andersen <tycho@tycho.ws>, Laura Abbott <labbott@redhat.com>, Mark Rutland <mark.rutland@arm.com>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Borislav Petkov <bp@alien8.de>, Richard Sandiford <richard.sandiford@arm.com>, Thomas Gleixner <tglx@linutronix.de>, "H . Peter Anvin" <hpa@zytor.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, "Dmitry V . Levin" <ldv@altlinux.org>, Emese Revfy <re.emese@gmail.com>, Jonathan Corbet <corbet@lwn.net>, Andrey Ryabinin <aryabinin@virtuozzo.com>, "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, Thomas Garnier <thgarnie@google.com>, Andrew Morton <akpm@linux-foundation.org>, Alexei Starovoitov <ast@kernel.org>, Josef Bacik <jbacik@fb.com>, Masami Hiramatsu <mhiramat@kernel.org>, Nicholas Piggin <npiggin@gmail.com>, Al Viro <viro@zeniv.linux.org.uk>, "David S . Miller" <davem@davemloft.net>, Ding Tianhong <dingtianhong@huawei.com>, David Woodhouse <dwmw@amazon.co.uk>, Josh Poimboeuf <jpoimboe@redhat.com>, Steven Rostedt <rostedt@goodmis.org>, Dominik Brodowski <linux@dominikbrodowski.net>, Juergen Gross <jgross@suse.com>, Linus Torvalds <torvalds@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Dan Williams <dan.j.williams@intel.com>, Dave Hansen <dave.hansen@linux.intel.com>, Mathias Krause <minipli@googlemail.com>, Vikas Shivappa <vikas.shivappa@linux.intel.com>, Kyle Huey <me@kylehuey.com>, Dmitry Safonov <dsafonov@virtuozzo.com>, Will Deacon <will.deacon@arm.com>, Arnd Bergmann <arnd@arndb.de>, Florian Weimer <fweimer@redhat.com>, Boris Lukashev <blukashev@sempervictus.com>, Andrey Konovalov <andreyknvl@google.com>, x86@kernel.org, linux-kernel@vger.kernel.org
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Hello Ingo,

Thanks a lot for the review!
I agree with your points. I'll fix the series and return with v13.

There are some comments/questions below.

On 18.05.2018 09:53, Ingo Molnar wrote:
> * Alexander Popov <alex.popov@linux.com> wrote:
>> +	/*
>> +	 * It would be nice not to have p and boundary on the stack.
>> +	 * Setting the register specifier for them is the best we can do.
>> +	 */
>> +	register unsigned long p = current->thread.lowest_stack.val;
>> +	register unsigned long boundary = p & ~(THREAD_SIZE - 1);
> 
> Does the 'register' keyword actually have any effect on the generated code?

No, for gcc it doesn't give any effect for this code. I used it to show the
intention.

However, even if the compiler allocates 'p' and 'boundary' on stack, that will
not break the stack erasing.

Should I drop 'register'?

>> +	unsigned long poison = 0;
>> +	const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
>> +							sizeof(unsigned long);
> 
> Please don't break lines in such an ugly fashion!

Ok. I'll make it look like that:

	const unsigned long check_depth =
			STACKLEAK_POISON_CHECK_DEPTH / sizeof(unsigned long);


> Also, 'poison' is a very weird name for something that looks like an index.
> 
> Plus since it's bound by "check_depth" is the 'unsigned long' justified,
> or could it be 32-bit?

Thanks, I can turn both 'check_depth' and 'poison' (which I'll rename) into
'unsigned int'.

>> +	/* Reset the lowest_stack value for the next syscall */
>> +	current->thread.lowest_stack.val = current_top_of_stack() - 256;
> 
> Magic, unexplained '256' literal.

Here I'm choosing the point from which we'll start the poison search on the next
syscall in case 'lowest_stack' is not updated in track_stack().

Would you like if I use "current_top_of_stack() - THREAD_SIZE / 64" ?

Best regards,
Alexander