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

[Nicholas Piggin <npiggin@gmail.com>]

On Tue, 05 Dec 2017 14:04:42 +1100
Michael Ellerman <mpe@ellerman.id.au> wrote:

> Hi Nick,
> 
> Sorry I didn't reply sooner.
> 
> Nicholas Piggin <npiggin@gmail.com> writes:
> 
> > 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