linux-kernel - Re: [kernel-hardening] [PATCH v5 07/10] arm64/mm: Don't flush the data cache if the page is unmapped by XPFO

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Message-ID: <20170812115736.GC16374@remoulade>
Date:   Sat, 12 Aug 2017 12:57:37 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Tycho Andersen <tycho@...ker.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-hardening@...ts.openwall.com,
        Marco Benatto <marco.antonio.780@...il.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        Juerg Haefliger <juerg.haefliger@....com>
Subject: Re: [kernel-hardening] [PATCH v5 07/10] arm64/mm: Don't flush the
 data cache if the page is unmapped by XPFO

On Wed, Aug 09, 2017 at 02:07:52PM -0600, Tycho Andersen wrote:
> From: Juerg Haefliger <juerg.haefliger@....com>
> 
> If the page is unmapped by XPFO, a data cache flush results in a fatal
> page fault. So don't flush in that case.

Do you have an example callchain where that happens? We might need to shuffle
things around to cater for that case.

> @@ -30,7 +31,9 @@ void sync_icache_aliases(void *kaddr, unsigned long len)
>  	unsigned long addr = (unsigned long)kaddr;
>  
>  	if (icache_is_aliasing()) {
> -		__clean_dcache_area_pou(kaddr, len);
> +		/* Don't flush if the page is unmapped by XPFO */
> +		if (!xpfo_page_is_unmapped(virt_to_page(kaddr)))
> +			__clean_dcache_area_pou(kaddr, len);
>  		__flush_icache_all();
>  	} else {
>  		flush_icache_range(addr, addr + len);

I don't think this patch is correct. If data cache maintenance is required in
the absence of XPFO, I don't see why it wouldn't be required in the presence of
XPFO.

I'm not immediately sure why the non-aliasing case misses data cache
maintenance. I couldn't spot where that happens otherwise.

On a more general note, in future it would be good to Cc the arm64 maintainers
and the linux-arm-kernel mailing list for patches affecting arm64.

Thanks,
Mark.