Re: [PATCH] perf: Fix oops when kthread execs user process

[Michael Ellerman]

Will Deacon <will.deacon@xxxxxxx> writes:
> On Tue, May 28, 2019 at 04:01:03PM +0200, Peter Zijlstra wrote:
>> On Tue, May 28, 2019 at 08:31:29PM +0800, Young Xiao wrote:
>> > When a kthread calls call_usermodehelper() the steps are:
>> >   1. allocate current->mm
>> >   2. load_elf_binary()
>> >   3. populate current->thread.regs
>> > 
>> > While doing this, interrupts are not disabled. If there is a perf
>> > interrupt in the middle of this process (i.e. step 1 has completed
>> > but not yet reached to step 3) and if perf tries to read userspace
>> > regs, kernel oops.
>
> This seems to be because pt_regs(current) gives NULL for kthreads on Power.

Right, we've done that since roughly forever in copy_thread():

int copy_thread(unsigned long clone_flags, unsigned long usp,
		unsigned long kthread_arg, struct task_struct *p)
{
	...
	/* Copy registers */
	sp -= sizeof(struct pt_regs);
	childregs = (struct pt_regs *) sp;
	if (unlikely(p->flags & PF_KTHREAD)) {
		/* kernel thread */
		memset(childregs, 0, sizeof(struct pt_regs));
		childregs->gpr[1] = sp + sizeof(struct pt_regs);
                ...
		p->thread.regs = NULL;	/* no user register state */

See commit from 2002:
  https://github.com/mpe/linux-fullhistory/commit/c0a96c0918d21d8a99270e94d9c4a4a322d04581#diff-edb76bfcc84905163f34d24d2aad3f3aR187

> From the initial report [1], it doesn't look like the mm isn't initialised,
> but rather than we're dereferencing a NULL pt_regs pointer somehow for the
> current task (see previous comment). I don't see how that can happen on
> arm64, given that we put the pt_regs on the kernel stack which is allocated
> during fork.

We have the regs on the stack too (see above), but we're explicitly
NULL'ing the link from task->thread.

Looks like on arm64 and x86 there is no link from task->thread, instead
you get from task to pt_regs via task_stack_page().

That actually seems potentially fishy given the comment on
task_stack_page() about the stack going away for exiting tasks. We
should probably be NULL'ing the regs pointer in free_thread_stack() or
similar. Though that race mustn't be happening because other arches
would see it.

Or are we just wrong and kthreads should have non-NULL regs? I can't
find another arch that does the same as us.

cheers