powerpc/64s: ISAv3 initialize MMU registers before setting partition table

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.

---
v3: do the register clearing in the early CPU setup code, suggested by
Michael Ellerman, which already clears LPID.

 arch/powerpc/kernel/cpu_setup_power.S | 2 ++
 1 file changed, 2 insertions(+)

index 610955fe8b81..679bbe714e85 100644
@@ -102,6 +102,7 @@ _GLOBAL(__setup_cpu_power9)
 	li	r0,0
 	mtspr	SPRN_PSSCR,r0
 	mtspr	SPRN_LPID,r0
+	mtspr	SPRN_PID,r0
 	mfspr	r3,SPRN_LPCR
 	LOAD_REG_IMMEDIATE(r4, LPCR_PECEDH | LPCR_PECE_HVEE | LPCR_HVICE  | LPCR_HEIC)
 	or	r3, r3, r4

@@ -126,6 +127,7 @@ _GLOBAL(__restore_cpu_power9)
 	li	r0,0
 	mtspr	SPRN_PSSCR,r0
 	mtspr	SPRN_LPID,r0
+	mtspr	SPRN_PID,r0
 	mfspr   r3,SPRN_LPCR
 	LOAD_REG_IMMEDIATE(r4, LPCR_PECEDH | LPCR_PECE_HVEE | LPCR_HVICE | LPCR_HEIC)
 	or	r3, r3, r4

2.15.0

Applied to powerpc fixes, thanks.

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

cheers