[kernel-hardening] Re: [PATCH RFC v5 2/5] gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack

From: Alexander Popov <alex.popov@linux.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: kernel-hardening@lists.openwall.com, keescook@chromium.org,
	pageexec@freemail.hu, spender@grsecurity.net,
	Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@kernel.org>,
	tycho@docker.com, Laura Abbott <labbott@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org
Subject: [kernel-hardening] Re: [PATCH RFC v5 2/5] gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack
Date: Mon, 30 Oct 2017 21:06:30 +0300	[thread overview]
Message-ID: <586ee584-df0d-3c82-a254-16ac1573255a@linux.com> (raw)
In-Reply-To: <20171030173216.72y673m32gra4b55@hirez.programming.kicks-ass.net>

Hello Peter,

Thanks for your reply.

On 30.10.2017 20:32, Peter Zijlstra wrote:
> On Mon, Oct 30, 2017 at 07:51:33PM +0300, Alexander Popov wrote:
>> When the thread stack is exhausted, this BUG() is hit. But do_error_trap(),
>> which handles the exception, calls track_stack() itself again (since it is
>> instrumented by the gcc plugin). So this recursion proceeds with exhausting the
>> thread stack.
> 
> Add a __attribute__((nostacktrack)) on it?

Yes, I already tried some blacklisting in the plugin, but it didn't really help,
because:

1. there are other (more than 5) instrumented functions, that are called during
BUG() handling too;

2. decreasing CONFIG_STACKLEAK_TRACK_MIN_SIZE would add more instrumented
functions, which should be manually blacklisted (not good).

I guess handling BUG() in another stack would be a solution. For example, Andy
Lutomirski calls handle_stack_overflow in the DOUBLEFAULT_STACK
(arch/x86/mm/fault.c). Should I do something similar?

Thanks!

Best regards,
Alexander