[PATCH] ARM: mm: harden branch predictor before opening interrupts during fault

[Lecopzer Chen]

> On 2021-01-26 10:59:32 [+0000], Russell King - ARM Linux admin wrote:
> > On Tue, Jan 26, 2021 at 05:17:08PM +0800, Lecopzer Chen wrote:
> > > Hi all,
> > > 
> > > I don't see any fix for this issue now(maybe I missed it..?),
> > > could we fix this if there is better solution?
> > > This issue exists almost two years.
> > 
> > I don't think anyone provided an acceptable patch.
> > 
> > The first patch moved the hardening out of the translation/section
> > fault handling. Since the kernel is mapped with sections, these
> > are above TASK_SIZE, and the whole point of the branch prediction
> > hardening is to prevent the prediction in the kernel being exploited,
> > missing the hardening effectively makes the mitigation useless.
> > 
> > The discussion in February 2019 never concluded from what I can see.
> 
> My memory is that I never got a reply which I understood.
> Let me try again this week with the information above.


NOTE:
Before sending this mail, I had searched the relative threads and
there are two solutions in general:
    1. Add get_pcpu()/put_cpu() https://lkml.org/lkml/2019/6/3/426
       Reject by Marc:
       > The right fix would be to move the call to a point where we haven't
       > enabled preemption yet.

    2. Move out like the patch from Sebastian:
       This seems follow the concept of 1.
       (move the call to a point where we haven't enabled preemption yet).
       But I can't find any reply in the thread.

Now the CONFIG_HARDEN_BRANCH_PREDICTOR has already backported to LTS,
and after upgrading ARM CONFIG_CPU_V7 products to latest LTS, the
CONFIG_HARDEN_BRANCH_PREDICTOR will be default y and this issue makes
our devices panic and we have to either disable HARDEN_BRANCH_PREDICTOR
or hack in-house to avoid the kernel panic.


I'll wait for anyone have acceptable idea and patching(maybe Sebastian could? :)),
and I'll also take some time to find other solution too.


Thanks,
Lecopzer