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

[Dave Gerlach]

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@xxxxxx>
> ---
>  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. I originally split the sram-exec code into its own file as it 
already depends on the changes you made to set_memory_* APIs for ARM which we 
have a hard dependency on here, and not all platforms support this. So this 
allowed me to constrain the sram_exec code to platforms with the proper 
set_memory_* APIs defined, but also now this lets us directly use the fncpy 
macro in this driver. For future platforms that want to make use of sram_exec we 
set the constraint that an arch must:

* Support the required set_memory_* APIs
* Define a fncpy macro that guarantees safe movement of a function.

This seems reasonable to me and gives support for ARM right away with a path 
forward for additional architectures to support sram_exec.

Regards,
Dave

[1] https://www.spinics.net/lists/arm-kernel/msg574481.html