linux-kernel - Re: [PATCH 1/9] mm: Hardened usercopy

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Date:	Thu, 7 Jul 2016 09:42:17 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Kees Cook <keescook@...omium.org>
cc:	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Casey Schaufler <casey@...aufler-ca.com>,
	PaX Team <pageexec@...email.hu>,
	Brad Spengler <spender@...ecurity.net>,
	Russell King <linux@...linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"David S. Miller" <davem@...emloft.net>, x86@...nel.org,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...e.de>,
	Mathias Krause <minipli@...glemail.com>,
	Jan Kara <jack@...e.cz>, Vitaly Wool <vitalywool@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Laura Abbott <labbott@...oraproject.org>,
	linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 1/9] mm: Hardened usercopy

On Wed, 6 Jul 2016, Kees Cook wrote:
> +
> +#if defined(CONFIG_FRAME_POINTER) && defined(CONFIG_X86)
> +	const void *frame = NULL;
> +	const void *oldframe;
> +#endif

That's ugly

> +
> +	/* Object is not on the stack at all. */
> +	if (obj + len <= stack || stackend <= obj)
> +		return 0;
> +
> +	/*
> +	 * Reject: object partially overlaps the stack (passing the
> +	 * the check above means at least one end is within the stack,
> +	 * so if this check fails, the other end is outside the stack).
> +	 */
> +	if (obj < stack || stackend < obj + len)
> +		return -1;
> +
> +#if defined(CONFIG_FRAME_POINTER) && defined(CONFIG_X86)
> +	oldframe = __builtin_frame_address(1);
> +	if (oldframe)
> +		frame = __builtin_frame_address(2);
> +	/*
> +	 * low ----------------------------------------------> high
> +	 * [saved bp][saved ip][args][local vars][saved bp][saved ip]
> +	 *		     ^----------------^
> +	 *             allow copies only within here
> +	 */
> +	while (stack <= frame && frame < stackend) {
> +		/*
> +		 * If obj + len extends past the last frame, this
> +		 * check won't pass and the next frame will be 0,
> +		 * causing us to bail out and correctly report
> +		 * the copy as invalid.
> +		 */
> +		if (obj + len <= frame)
> +			return obj >= oldframe + 2 * sizeof(void *) ? 2 : -1;
> +		oldframe = frame;
> +		frame = *(const void * const *)frame;
> +	}
> +	return -1;
> +#else
> +	return 1;
> +#endif

I'd rather make that a weak function returning 1 which can be replaced by
x86 for CONFIG_FRAME_POINTER=y. That also allows other architectures to
implement their specific frame checks.

Thanks,

	tglx