[U-Boot] [RFC, PATCH] omap: Invalidate first page to avoid speculation

[Albert ARIBAUD <albert.u.boot@aribaud.net>]

Hi Vincent,

On Fri, 16 Nov 2012 14:36:29 +0100, Vincent Stehl? <v-stehle@ti.com>
wrote:

> 
> Hello u-boot list,
> 
> Here is a "request for comments" on the best way to solve a little
> "speculation" issue on recent OMAPs. Any guidance/feedback on the way to go
> would be greatly appreciated, please.
> 
> I am using u-boot on an OMAP5 HS device (with security, that is), and I am
> experiencing "security violations" due to speculative accesses done by the
> Cortex-A15 processor to the region near address zero. This region is a secure
> region, where non-secure accesses are forbidden and reported by the security
> firmware on an OMAP HS device. On an OMAP GP device, those accesses may very
> well exist, but are silently ignored by the firmware. Note that the speculative
> accesses are not actual functional accesses, so their being aborted does not
> harm the functionality of u-boot as it is.
>   A quick (and dirty) solution is to mark the region near address zero as being
> invalid, which prevents the processor from doing speculative accesses there
> (see patch).
>   This patch as it is has a number of issues: it impacts all ARM devices and it
> unmaps too large a region. I am not sure how to cleanly rework the patch so
> that it would be made OMAP-only cleanly. Also, unmapping a smaller region to
> better fit the hardware characteristics would require using second level
> descriptors, and I do not know if this is recommended. To make this worse,
> chips in the OMAP family have differences in their secure rom boundaries.
> 
> Does the u-boot community feels this issue needs to be addressed? What would be
> the best way to solve this?
> 
> Best regards,
> 
> V.
> 
> 
> Signed-off-by: Vincent Stehl? <v-stehle@ti.com>
> ---
>  arch/arm/lib/cache-cp15.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/lib/cache-cp15.c b/arch/arm/lib/cache-cp15.c
> index 939de10..57e1974 100644
> --- a/arch/arm/lib/cache-cp15.c
> +++ b/arch/arm/lib/cache-cp15.c
> @@ -72,8 +72,13 @@ static inline void mmu_setup(void)
>  	u32 reg;
>  
>  	arm_init_before_mmu();
> +
> +	/* First page (starting at 0x0) is made invalid to avoid
> +	 * speculative accesses in secure rom. */
> +	page_table[0] = 0;
> +
>  	/* Set up an identity-mapping for all 4GB, rw for everyone */
> -	for (i = 0; i < 4096; i++)
> +	for (i = 1; i < 4096; i++)
>  		page_table[i] = i << 20 | (3 << 10) | 0x12;
>  
>  	for (i = 0; i < CONFIG_NR_DRAM_BANKS; i++) {

To make this affect only some CPUs or even boards, you can define and
use a weak function which would handle filling the page-table; the weak,
default, function would fill table[0] like others, while OMAP5 would
have a strong version which would clear table[0].

Amicalement,
-- 
Albert.