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Message-ID: <4AF8A2C0.5080700@zytor.com>
Date: Mon, 09 Nov 2009 15:16:16 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Kees Cook <kees.cook@...onical.com>
CC: Arjan van de Ven <arjan@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
Pekka Enberg <penberg@...helsinki.fi>,
Jan Beulich <jbeulich@...ell.com>,
Vegard Nossum <vegardno@....uio.no>,
Yinghai Lu <yinghai@...nel.org>,
Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] [x86] detect and report lack of NX protections
On 11/09/2009 02:10 PM, Kees Cook wrote:
> It is possible for x86_64 systems to lack the NX bit (see check_efer())
> either due to the hardware lacking support or the BIOS having turned
> off the CPU capability, so NX status should be reported. Additionally,
> anyone booting NX-capable CPUs in 32bit mode without PAE will lack NX
> functionality, so this change provides feedback for that case as well.
>
> v2: use "Alert:" instead of "Warning:" to avoid confusion with WARN_ON()
> v3: use "Notice:" instead of "Alert:" to avoid confusion with KERN_ALERT,
> and switch to KERN_NOTICE, in keeping with its use for "normal but
> significant condition" messages.
> v4: check that _NX_PAGE is non-zero to avoid setting nx_enabled accidentally.
>
> Signed-off-by: Kees Cook <kees.cook@...onical.com>
> ---
> arch/x86/mm/init.c | 10 ++++++++++
> arch/x86/mm/setup_nx.c | 3 +++
> 2 files changed, 13 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index 73ffd55..d98b43a 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -149,6 +149,16 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
> set_nx();
> if (nx_enabled)
> printk(KERN_INFO "NX (Execute Disable) protection: active\n");
> + else if (cpu_has_pae)
> +#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
> + /* PAE kernel, PAE CPU, without NX */
> + printk(KERN_NOTICE "Notice: NX (Execute Disable) protection "
> + "missing in CPU or disabled in BIOS!\n");
> +#else
> + /* 32bit non-PAE kernel, PAE CPU */
> + printk(KERN_NOTICE "Notice: NX (Execute Disable) protection "
> + "cannot be enabled: non-PAE kernel!\n");
> +#endif
>
> /* Enable PSE if available */
> if (cpu_has_pse)
> diff --git a/arch/x86/mm/setup_nx.c b/arch/x86/mm/setup_nx.c
> index 513d8ed..1b93231 100644
> --- a/arch/x86/mm/setup_nx.c
> +++ b/arch/x86/mm/setup_nx.c
> @@ -53,6 +53,9 @@ void __init set_nx(void)
> #else
> void set_nx(void)
> {
> + /* notice if _PAGE_NX exists and was removed during check_efer() */
> + if (_PAGE_NX && ((__supported_pte_mask & _PAGE_NX) == _PAGE_NX))
> + nx_enabled = 1;
> }
> #endif
>
The second clause can only get executed if CONFIG_X86_PAE is unset,
which in turn means _PAGE_NX == 0... so that piece of code is meaningless.
It also looks to me that there is no message distinguishing the case
when nx_enabled == 1 but disable_nx == 1, and instead we say NX is
"active" when in fact it is disabled in the kernel.
-hpa
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