linux-kernel - Re: [PATCH 7/7] x86/uaccess: OOPS or warn on a fault with KERNEL_DS and !pagefault

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Date:	Wed, 25 May 2016 17:33:01 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 7/7] x86/uaccess: OOPS or warn on a fault with KERNEL_DS
 and !pagefault_disabled()

On Tue, May 24, 2016 at 03:48:44PM -0700, Andy Lutomirski wrote:
> +	if (unlikely(!is_user_ds && !pagefault_disabled())) {
> +		if (extra < TASK_SIZE_MAX) {
> +			/*
> +			 * Accessing user address under KERNEL_DS.  This is a
> +			 * bug and should be fixed, but OOPSing is not helpful
> +			 * for exploit mitigation.
> +			 */
> +			WARN_ONCE(1, "BUG: uaccess fault at 0x%lx with KERNEL_DS\n",

			WARN and BUG?

Also, let's have this string and the one below differ for finding out
where we are during debugging.

> +				  extra);
> +		} else {
> +			/*
> +			 * If a bug that allows user-controlled KERNEL_DS
> +			 * access exists, this will prevent it from being used
> +			 * to trivially bypass kASLR.
> +			 */
> +			pr_crit("BUG: uaccess fault at 0x%lx with KERNEL_DS\n",
> +				extra);

-- 
Regards/Gruss,
    Boris.

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