[RFC PATCH] arm64: deactivate saved ttbr when mm is deactivated

[will.deacon@arm.com (Will Deacon)]

On Tue, Dec 05, 2017 at 11:06:20AM +0000, Mark Rutland wrote:
> On Tue, Dec 05, 2017 at 10:30:40AM +0530, Vinayak Menon wrote:
> > On 12/4/2017 11:30 PM, Mark Rutland wrote:
> > > On Mon, Dec 04, 2017 at 04:55:33PM +0000, Will Deacon wrote:
> > >> On Mon, Dec 04, 2017 at 09:53:26PM +0530, Vinayak Menon wrote:
> > >>> A case is observed where a wrong physical address is read,
> > >>> resulting in a bus error and that happens soon after TTBR0 is
> > >>> set to the saved ttbr by uaccess_ttbr0_enable. This is always
> > >>> seen to happen in the exit path of the task.
> > >>>
> > >>> exception
> > >>> __arch_copy_from_user
> > >>> __copy_from_user
> > >>> probe_kernel_read
> > >>> get_freepointer_safe
> > >>> slab_alloc_node
> > >>> slab_alloc
> > >>> kmem_cache_alloc
> > >>> kmem_cache_zalloc
> > >>> fill_pool
> > >>> __debug_object_init
> > >>> debug_object_init
> > >>> rcuhead_fixup_activate
> > >>> debug_object_fixup
> > >>> debug_object_activate
> > >>> debug_rcu_head_queue
> > >>> __call_rcu
> > >>> ep_remove
> > >>> eventpoll_release_file
> > >>> __fput
> > >>> ____fput
> > >>> task_work_run
> > >>> do_exit
> > >>>
> > >>> The mm has been released and the pgd is freed, but probe_kernel_read
> > >>> invoked from slub results in call to __arch_copy_from_user. At the
> > >>> entry to __arch_copy_from_user, when SW PAN is enabled, this results
> > >>> in stale value being set to ttbr0. May be a speculative fetch aftwerwards
> > >>> is resulting in invalid physical address access.
> 
> > > I think the problem here is that switch_mm() avoids updating the saved ttbr
> > > value when the next mm is init_mm.
> 
> > For this switch to happen, the schedule() in do_task_dead at the end
> > of do_exit() need to be called, right ?  The issue is happening soon
> > after exit_mm (probably from exit_files).
> 
> I'd assumed that we'd switch_mm() away from the task's mm prior to the
> final mmput(). Otherwise, I can't see why we don't have issues in the
> non SW PAN case (as that would leave the HW TTBR0 stale).
> 
> However, I can't see exactly where we do that, so I'll go diggging.
> Something doesn't seem quite right.
> 
> Do you have a reproducer for the issue?

I'd be very interested in that, or just more details about how this was
observed. What was the workload? Kernel version? Hardware? .config? Do you
know for sure that it was a page table walk that triggered the abort?

In the report above, Vinayak claims that "The mm has been released and the
pgd is freed" but that really shouldn't happen in the do_exit path. We free
the other levels of page table in free_pgtables, but deliberately keep the
mm and the pgd around until we've switched away in finish_task_switch.

I'm quite prepared to believe that the ttbr0 stashing by the SW PAN code
isn't bulletproof, but I'm struggling to see how the backtrace above
can happen.

Will