powerpc/tm: Avoid possible userspace r1 corruption on reclaim

[Michael Ellerman]

On Tue, 2018-09-25 at 09:36:47 UTC, Michael Neuling wrote:
> Current we store the userspace r1 to PACATMSCRATCH before finally
> saving it to the thread struct.
> 
> In theory an exception could be taken here (like a machine check or
> SLB miss) that could write PACATMSCRATCH and hence corrupt the
> userspace r1. The SLB fault currently doesn't touch PACATMSCRATCH, but
> others do.
> 
> We've never actually seen this happen but it's theoretically
> possible. Either way, the code is fragile as it is.
> 
> This patch saves r1 to the kernel stack (which can't fault) before we
> turn MSR[RI] back on. PACATMSCRATCH is still used but only with
> MSR[RI] off. We then copy r1 from the kernel stack to the thread
> struct once we have MSR[RI] back on.
> 
> Suggested-by: Breno Leitao <leitao at debian.org>
> Signed-off-by: Michael Neuling <mikey at neuling.org>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/96dc89d526ef77604376f06220e3d2

cheers