Re: [PATCH v2] powerpc/64s: ISAv3 initialize MMU registers before setting partition table

On Tue, 05 Dec 2017 14:04:42 +1100
Michael Ellerman [off-list ref] wrote:

Hi Nick,

Sorry I didn't reply sooner.

Nicholas Piggin [off-list ref] writes:

quoted

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 erorr, it could result in
anything up to security and data corruption errors.

Fix this by zeroing out PIDR before setting PTCR.

LPIDR is also not initialized, and may cause a similar issue with
speculative access to quadrant 1/2. This has not been observed, but
LPIDR is cleared to prevent that possibility.  

Isn't LPID initialised in __setup_cpu_power9() and __restore_cpu_power9() ?

eg:

_GLOBAL(__setup_cpu_power9)
	mflr	r11
	bl	__init_FSCR
	bl	__init_PMU
	bl	__init_hvmode_206
	mtlr	r11
	beqlr
	li	r0,0
	mtspr	SPRN_PSSCR,r0
	mtspr	SPRN_LPID,r0


Similarly, shouldn't we be doing the PID initialisation there as well?

Hmm, yes I must have missed that! Yes that would be the best place to
put it.

Thanks,
Nick