Will Deacon <will.dea...@arm.com> 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
