Re: [RFC PATCH v1 5/6] powerpc/mm: Add a framework for Kernel Userspace Access Protection

[Russell Currey <ruscur@russell.cc>]

On Wed, 2018-11-07 at 16:56 +0000, Christophe Leroy wrote:
> This patch implements a framework for Kernel Userspace Access
> Protection.
> 
> Then subarches will have to possibility to provide their own
> implementation
> by providing setup_kuap(), and lock/unlock_user_rd/wr_access
> 
> We separate read and write accesses because some subarches like
> book3s32 might only support write access protection.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

Separating read and writes does have a performance impact, I'm doing
some benchmarking to find out exactly how much - but at least for radix
it means we have to do a RMW instead of just a write.  It does add some
amount of security, though.

The other issue I have is that you're just locking everything here
(like I was), and not doing anything different for just reads or
writes.  In theory, wouldn't someone assume that they could (for
example) unlock reads, lock writes, then attempt to read?  At which
point the read would fail, because the lock actually locks both.

I would think we either need to bundle read/write locking/unlocking
together, or only implement this on platforms that can do one at a
time, unless there's a cleaner way to handle this.  Glancing at the
values you use for 8xx, this doesn't seem possible there, and it's a
definite performance hit for radix.

At the same time, as you say, it would suck for book3s32 that can only
do writes, but maybe just doing both at the same time and if
implemented for that platform it could just have a warning that it only
applies to writes on init?

Curious for people's thoughts on this.

- Russell