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
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