linux-kernel - Re: [PATCH] misc: sram-exec: Use aligned fncpy instead of memcpy

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Date:   Thu, 6 Apr 2017 20:07:47 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Dave Gerlach <d-gerlach@...com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Tony Lindgren <tony@...mide.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-omap@...r.kernel.org, Shawn Guo <shawnguo@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Keerthy J <j-keerthy@...com>
Subject: Re: [PATCH] misc: sram-exec: Use aligned fncpy instead of memcpy

On Wed, Apr 05, 2017 at 02:22:33PM -0500, Dave Gerlach wrote:
> Russell,
> On 04/05/2017 02:21 PM, Dave Gerlach wrote:
> >Currently the sram-exec functionality, which allows allocation of
> >executable memory and provides an API to move code to it, is only
> >selected in configs for the ARM architecture. Based on commit
> >5756e9dd0de6 ("ARM: 6640/1: Thumb-2: Symbol manipulation macros for
> >function body copying") simply copying a C function pointer address
> >using memcpy without consideration of alignment and Thumb is unsafe on
> >ARM platforms.
> >
> >The aforementioned patch introduces the fncpy macro which is a safe way
> >to copy executable code on ARM platforms, so let's make use of that here
> >rather than the unsafe plain memcpy that was previously used by
> >sram_exec_copy.
> >
> >In the future, architectures hoping to make use of the sram-exec
> >functionality must define an fncpy macro just as ARM has done to
> >guarantee or check for safe copying to executable memory before allowing
> >the arch to select CONFIG_SRAM_EXEC.
> >
> >Signed-off-by: Dave Gerlach <d-gerlach@...com>
> >---
> > drivers/misc/sram-exec.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/misc/sram-exec.c b/drivers/misc/sram-exec.c
> >index ac522417c462..0057eabe5c03 100644
> >--- a/drivers/misc/sram-exec.c
> >+++ b/drivers/misc/sram-exec.c
> >@@ -19,6 +19,7 @@
> > #include <linux/sram.h>
> >
> > #include <asm/cacheflush.h>
> >+#include <asm/fncpy.h>
> >
> > #include "sram.h"
> >
> >@@ -93,7 +94,7 @@ int sram_exec_copy(struct gen_pool *pool, void *dst, void *src,
> > 	set_memory_nx((unsigned long)base, pages);
> > 	set_memory_rw((unsigned long)base, pages);
> >
> >-	memcpy(dst, src, size);
> >+	fncpy(dst, src, size);
> >
> > 	set_memory_ro((unsigned long)base, pages);
> > 	set_memory_x((unsigned long)base, pages);
> >
> 
> Does this address your concerns from here [1]? Because the only user of this
> code is ARM right now I already only build the sram-exec code in if
> CONFIG_ARM is selected.

Sorry, it does not.  Please read the comments in asm/fncpy.h.

Deviating from the proscribed usage means your code is, quite simply,
buggy.  There's no two ways about that.

-- 
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