Re: [v3] powerpc/64s: ISAv3 initialize MMU registers before setting partition table

On Wed, 2017-12-06 at 08:21:14 UTC, Nicholas Piggin wrote:

kexec can leave MMU registers set when booting into a new kernel, PIDR
in particular. The boot sequence does not zero PIDR, so it only gets
set when CPUs first switch to a userspace processes (until then it's
running a kernel thread with effective PID = 0).

This leaves a window where a process table entry and page tables are
set up due to user processes running on other CPUs, that happen to
match with a stale PID. The CPU with that PID may cause speculative
accesses that address quadrant 0, which will result in cached
translations and PWC for that process, on a CPU which is not in the
mm_cpumask and so they will not get invalidated properly.

The most common result is the kernel hanging in infinite page fault
loops soon after kexec (usually in schedule_tail, which is usually the
first non-speculative quardant 0 access to a new PID) due to a stale
PWC. However being a stale translation error, it could result in
anything up to security and data corruption problems.

Fix this by zeroing out PIDR at boot and kexec.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/371b80447ff33ddac392c189cf884a

cheers