linux-kernel - Re: [PATCH] x86: clear XD_DISABLED flag on Intel to regain NX

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4C1AA15E.3050205@zytor.com>
Date:	Thu, 17 Jun 2010 15:27:42 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <kees.cook@...onical.com>
CC:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Tim Abbott <tabbott@...lice.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Jiri Kosina <jkosina@...e.cz>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH] x86: clear XD_DISABLED flag on Intel to regain NX

On 06/17/2010 03:13 PM, Kees Cook wrote:
> This will clear the MSR_IA32_MISC_ENABLE_XD_DISABLE bit so that NX cannot
> be inappropriately controlled by the BIOS on Intel CPUs.  If NX actually
> needs to be disabled, "noexec=off" can be used.
> 
> Signed-off-by: Kees Cook <kees.cook@...onical.com>
> ---
>  arch/x86/kernel/head_32.S |   19 +++++++++++++++++++
>  arch/x86/kernel/head_64.S |   18 ++++++++++++++++++
>  arch/x86/mm/setup_nx.c    |    2 +-
>  3 files changed, 38 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index 37c3d4b..111e434 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -309,6 +309,25 @@ ENTRY(startup_32_smp)
>  	subl $0x80000001, %eax
>  	cmpl $(0x8000ffff-0x80000001), %eax
>  	ja 6f
> +
> +	/* Is this "GenuineIntel"? */
> +	movl $0x0, %eax
> +	cpuid
> +	cmpl $0x756e6547, %ebx
> +	jnz 5f
> +	cmpl $0x49656e69, %edx
> +	jnz 5f
> +	cmpl $0x6c65746e, %ecx
> +	jnz 5f
> +
> +	/* Clear MSR_IA32_MISC_ENABLE_XD_DISABLE if set */
> +	movl $MSR_IA32_MISC_ENABLE, %ecx
> +	rdmsr
> +	btrl $2, %edx
> +	jnc 5f
> +	wrmsr
> +
> +5:
>  	mov $0x80000001, %eax
>  	cpuid
>  	/* Execute Disable bit supported? */

Multiple problems with this code.

a) Not all Intel CPUs with extended CPUID levels have
   MSR_IA32_MISC_ENABLE bit 34.  Since we can't take traps here we would
   have to know positively that we aren't going to trip on anything.
b) For 64 bits, this should go into verify_cpu_64.S, and since that is
   32-bit code anyway, it would be best if we could merge the 32- and
   64-bit code into that file; it already simply returns a value
   that could be ignored on 32 bits.

	-hpa
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/