linux-kernel - Re: [PATCH V2] x86/efi: Fix kernel panic when CONFIG_DEBUG

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Date:	Sat, 17 Oct 2015 11:47:38 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	linux-efi@...r.kernel.org, matt@...eblueprint.co.uk,
	linux-kernel@...r.kernel.org, dave.hansen@...el.com,
	Matt Fleming <matt.fleming@...el.com>,
	Ricardo Neri <ricardo.neri@...el.com>,
	Glenn P Williamson <glenn.p.williamson@...el.com>,
	Ravi Shankar <ravi.v.shankar@...el.com>
Subject: Re: [PATCH V2] x86/efi: Fix kernel panic when CONFIG_DEBUG_VIRTUAL
 is enabled

On Fri, Oct 16, 2015 at 04:20:27PM -0700, Sai Praneeth Prakhya wrote:
> From: Sai Praneeth <sai.praneeth.prakhya@...el.com>
> 
> When CONFIG_DEBUG_VIRTUAL is enabled, all accesses to __pa(address) are
> monitored to see whether address falls in direct mapping or kernel text
> mapping (see Documentation/x86/x86_64/mm.txt for details), if it does
> not kernel panics.

At least a comma and a "the" here:

"..., if it does not, the kernel panics."

> During 1:1 mapping of EFI runtime services we access
> virtual addresses which are == physical addresses, thus the 1:1 mapping
> and these addresses donot fall in either of the above two regions and

		     do not

> hence when passed as arguments to __pa() kernel panics as reported by
> Dave Hansen here https://lkml.kernel.org/r/5462999A.7090706@intel.com.
> So, before calling __pa() virtual addresses should be validated which
> results in skipping call to split_page_count() and that should be fine
> because it is used to keep track of everything *but* 1:1 mappings.
> 
> Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
> Reported-by: Dave Hansen <dave.hansen@...el.com>
> Cc: Matt Fleming <matt.fleming@...el.com>
> Cc: Ricardo Neri <ricardo.neri@...el.com>
> Cc: Glenn P Williamson <glenn.p.williamson@...el.com>
> Cc: Ravi Shankar <ravi.v.shankar@...el.com>
> 
> Changes since v1:
> Made commit message more clear by adding reference to
> Documentation/x86/x86_64/mm.txt, instead of addresses "below 4G"
> changed it to "virtual addresses == physical addresses" quoted
> mail with k.org redirector and changed last line "to keep track of
> evrything *but* 1:1 mappings." Also made code more readable by adding a
> variable.
> ---
>  arch/x86/mm/pageattr.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> index 727158cb3b3c..9abe0c9b1098 100644
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -648,9 +648,12 @@ __split_large_page(struct cpa_data *cpa, pte_t *kpte, unsigned long address,
>  	for (i = 0; i < PTRS_PER_PTE; i++, pfn += pfninc)
>  		set_pte(&pbase[i], pfn_pte(pfn, canon_pgprot(ref_prot)));
>  
> -	if (pfn_range_is_mapped(PFN_DOWN(__pa(address)),
> -				PFN_DOWN(__pa(address)) + 1))
> -		split_page_count(level);
> +	if (virt_addr_valid(address)) {
> +		unsigned long pfn = PFN_DOWN(__pa(address));
> +
> +		if (pfn_range_is_mapped(pfn, pfn + 1))
> +			split_page_count(level);
> +	}
>  
>  	/*
>  	 * Install the new, split up pagetable.
> -- 

Looks ok to me, these minor nitpicks can be adjusted when applying, no
need to send another version.

Which brings me to the next question: Matt, are you picking this up or
should I?

If you, then

Reviewed-by: Borislav Petkov <bp@...e.de>

Thanks!

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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