linux-kernel - Re: [PATCH 2/2] arm: use fixmap for text patching when text is RO

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Date:	Thu, 10 Apr 2014 21:34:46 +0200
From:	Rabin Vincent <rabin@....in>
To:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	Laura Abbott <lauraa@...eaurora.org>
Subject: Re: [PATCH 2/2] arm: use fixmap for text patching when text is RO

On Mon, Apr 07, 2014 at 02:57:51PM +0100, Jon Medhurst (Tixy) wrote:
> How does fixmap cope with cache colouring? Looking at the implementation
> it looks like it doesn't and so fixmap use on ARM is possibly buggy.
> 
> For the text patching case where we know there are no writeable mappings
> [1] this should be OK if we used set_fixmap_nocache here, so long as we
> also invalidated the dcache later for the proper virtual address.

OK.  The dcache invalidation for the proper virtual address is btw
already there via the call to flush_icache_range().

> [1] Can we know there are no writeable mappings though, the ftrace code
> modifying patches from Kees Cook have there own way of modifying text
> code permissions.

The ftrace patches does the modifications and the cache cleaning in
stop_machine(), so there should not be any dirty cache lines from those
writable mappings when we set up and write to these fixmaps.  Do you
still see a problem?
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