Re: [PATCH 0/3] Add memremap executable mapping and extend drivers/misc/sram.c

[Dave Gerlach]

On 11/07/2016 11:43 AM, Tony Lindgren wrote:
> * Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> [161107 04:05]:
> > On Thu, Oct 27, 2016 at 01:56:09PM -0500, Dave Gerlach wrote:
> > > There are several instances when one would want to execute out of on-chip
> > > SRAM, such as PM code on ARM platforms, so once again revisiting this
> > > series to allow that in a generic manner. Seems that having a solution for
> > > allowing SRAM to be mapped as executable will help clean up PM code on several
> > > ARM platforms that are using ARM internal __arm_ioremap_exec API
> > > and also open the door for PM support on new platforms like TI AM335x and
> > > AM437x. This was last sent as RFC here [1] and based on comments from Russell
> > > King and Arnd Bergmann has been rewritten to use memremap API rather than
> > > ioremap API, as executable iomem does not really make sense.
> >
> > This is better, as it avoids the issue that I pointed out last time
> > around, but I'm still left wondering about the approach.
> >
> > Sure, having executable SRAM mappings sounds nice and easy, but we're
> > creating WX mappings.  Folk have spent a while improving the security of
> > the kernel by ensuring that there are no WX mappings, and this series
> > reintroduces them.  The sad thing is that any WX mapping which appears
> > at a known address can be exploited.
> >
> > "A known address" can be something that appears to be random, but ends
> > up being the same across the same device type... or can be discovered
> > by some means.  Eg, consider if the WX mapping is dynamically allocated,
> > but occurs at exactly the same point at boot - and if this happens with
> > android phones, consider how many of those are out there.  Or if the
> > address of the WX mapping is available via some hardware register.
> > Or...
> >
> > See Kees Cook's slides at last years kernel summit -
> > 	https://outflux.net/slides/2015/ks/security.pdf
> >
> > So, I think avoiding WX mappings - mappings should be either W or X but
> > not both simultaneously (see page 19.)
> >
> > I guess what I'm angling at is that we don't want memremap_exec(), but
> > we need an API which changes the permissions of a SRAM mapping between
> > allowing writes and allowing execution.
>
> That should work just fine. So first copy the code to SRAM,
> then set it read-only and exectuable. Note that we need to
> restore the state of SRAM every time when returning from
> off mode during idle on some SoCs.

Thanks for all the comments. This seems like a reasonable concern. I do 
agree that we would need to be able to briefly restore write permission 
for things like Tony has described.

In fact, I suppose we would need this ability for every copy so that  we 
switch from exec to write and do the copy, then switch back to read-only 
executable. We have situations where we copy multiple things, from 
drivers that don't necessarily have knowledge of one another to the SRAM 
space at different times.

Any opinions on where this API should live?

Regards,
Dave

> Regards,
>
> Tony