linux-kernel - Re: [PATCH v6 07/11] arm64/mm, xpfo: temporarily map dcache regions

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Date:   Thu, 14 Sep 2017 19:25:56 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Tycho Andersen <tycho@...ker.com>
Cc:     linux-kernel@...r.kernel.org,
        Marco Benatto <marco.antonio.780@...il.com>,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 07/11] arm64/mm, xpfo: temporarily map dcache regions

On Thu, Sep 07, 2017 at 11:36:05AM -0600, Tycho Andersen wrote:
> From: Juerg Haefliger <juerg.haefliger@...onical.com>
> 
> If the page is unmapped by XPFO, a data cache flush results in a fatal
> page fault, so let's temporarily map the region, flush the cache, and then
> unmap it.
> 
> v6: actually flush in the face of xpfo, and temporarily map the underlying
>     memory so it can be flushed correctly
> 
> CC: linux-arm-kernel@...ts.infradead.org
> Signed-off-by: Juerg Haefliger <juerg.haefliger@...onical.com>
> Signed-off-by: Tycho Andersen <tycho@...ker.com>
> ---
>  arch/arm64/mm/flush.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
> index 21a8d828cbf4..e335e3fd4fca 100644
> --- a/arch/arm64/mm/flush.c
> +++ b/arch/arm64/mm/flush.c
> @@ -20,6 +20,7 @@
>  #include <linux/export.h>
>  #include <linux/mm.h>
>  #include <linux/pagemap.h>
> +#include <linux/xpfo.h>
>  
>  #include <asm/cacheflush.h>
>  #include <asm/cache.h>
> @@ -28,9 +29,15 @@
>  void sync_icache_aliases(void *kaddr, unsigned long len)
>  {
>  	unsigned long addr = (unsigned long)kaddr;
> +	unsigned long num_pages = XPFO_NUM_PAGES(addr, len);
> +	void *mapping[num_pages];
>  
>  	if (icache_is_aliasing()) {
> +		xpfo_temp_map(kaddr, len, mapping,
> +			      sizeof(mapping[0]) * num_pages);
>  		__clean_dcache_area_pou(kaddr, len);
> +		xpfo_temp_unmap(kaddr, len, mapping,
> +			        sizeof(mapping[0]) * num_pages);

Does this create the mapping in-place?

Can we not just kmap_atomic() an alias? Or is there a problem with that?

Thanks,
Mark.