[RFC PATCH] powerpc/mm: Implement STRICT_MODULE_RWX

[Russell Currey]

On Wed, 2019-05-15 at 06:20 +0000, Christophe Leroy wrote:
> Strict module RWX is just like strict kernel RWX, but for modules -
> so
> loadable modules aren't marked both writable and executable at the
> same
> time.  This is handled by the generic code in kernel/module.c, and
> simply requires the architecture to implement the set_memory() set of
> functions, declared with ARCH_HAS_SET_MEMORY.
> 
> There's nothing other than these functions required to turn
> ARCH_HAS_STRICT_MODULE_RWX on, so turn that on too.
> 
> With STRICT_MODULE_RWX enabled, there are as many W+X pages at
> runtime
> as there are with CONFIG_MODULES=n (none), so in Russel's testing it
> works
> well on both Hash and Radix book3s64.
> 
> There's a TODO in the code for also applying the page permission
> changes
> to the backing pages in the linear mapping: this is pretty simple for
> Radix and (seemingly) a lot harder for Hash, so I've left it for now
> since there's still a notable security benefit for the patch as-is.
> 
> Technically can be enabled without STRICT_KERNEL_RWX, but
> that doesn't gets you a whole lot, so we should leave it off by
> default
> until we can get STRICT_KERNEL_RWX to the point where it's enabled by
> default.
> 
> Signed-off-by: Russell Currey <ruscur at russell.cc>
> Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
> ---

Thanks for this, I figured you'd know how to make this work on 32bit
too.  I'll test on my end today.

Note that there are two Ls in my name!  To quote the great Rusty, "This
Russel disease must be stamped out before it becomes widespread".