Re: [PATCH v3 08/14] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0

[Will Deacon <will@kernel.org>]

On Thu, Nov 19, 2020 at 05:14:48PM +0100, Peter Zijlstra wrote:
> On Fri, Nov 13, 2020 at 09:37:13AM +0000, Will Deacon wrote:
> > When exec'ing a 32-bit task on a system with mismatched support for
> > 32-bit EL0, try to ensure that it starts life on a CPU that can actually
> > run it.
> > 
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  arch/arm64/kernel/process.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 1540ab0fbf23..17b94007fed4 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -625,6 +625,16 @@ unsigned long arch_align_stack(unsigned long sp)
> >  	return sp & ~0xf;
> >  }
> >  
> > +static void adjust_compat_task_affinity(struct task_struct *p)
> > +{
> > +	const struct cpumask *mask = system_32bit_el0_cpumask();
> > +
> > +	if (restrict_cpus_allowed_ptr(p, mask))
> > +		set_cpus_allowed_ptr(p, mask);
> 
> This silently destroys user state, at the very least that ought to go
> with a WARN or something. Ideally SIGKILL though. What's to stop someone
> from doing a sched_setaffinity() right after the execve, same problem.
> So why bother..

It's no different to CPU hot-unplug though, is it? From the perspective of
the 32-bit task, the 64-bit-only cores were hot-unplugged at the point of
execve(). Calls to sched_setaffinity() for 32-bit tasks will reject attempts
to include 64-bit-only cores.

I initially wanted to punt this all to userspace, but one of the big
problems with that is when a 64-bit task is running on a CPU only capable
of running 64-bit tasks and it execve()s a 32-bit task. At the point, we
have to do something because we can't even run the new task for it to do
a sched_affinity() call (and we also can't deliver SIGILL).

Will