Re: [PATCH 09/12] x86/process: Pin the target stack in get_wchan()

[Andy Lutomirski]

On Fri, Sep 16, 2016 at 7:00 PM, Jann Horn <jann@xxxxxxxxx> wrote:
> On Tue, Sep 13, 2016 at 02:29:29PM -0700, Andy Lutomirski wrote:
>> This will prevent a crash if get_wchan() runs after the task stack
>> is freed.
>
> I think I found some more stuff. Have a look at KSTK_EIP() and KSTK_ESP(), I think
> they read from the saved userspace registers area at the top of the kernel stack?
>
> Used on remote processes in:
>   vma_is_stack_for_task() (via /proc/$pid/maps)

This isn't used in /proc/$pid/maps -- it's only used in
/proc/$pid/task/$tid/maps.  I wonder if anyone actually cares about it
-- it certainly won't work reliably.

I could pin the stack in vma_is_stack_for_task, but it seems
potentially better to me to change it to vma_is_stack_for_current()
and remove the offending caller in /proc, replacing it with "return
0".  Thoughts?

>   do_task_stat() (/proc/$pid/stat)

Like this:

        mm = get_task_mm(task);
        if (mm) {
                vsize = task_vsize(mm);
                if (permitted) {
                        eip = KSTK_EIP(task);
                        esp = KSTK_ESP(task);
                }
        }

Can we just delete this outright?  It seems somewhere between mostly
and entirely useless, and it also seems dangerous.  Until very
recently, on x86_64, this would have been a potential info leak, as
SYSCALL followed closely by a hardware interrupt would cause *kernel*
values to land in task_pt_regs().  I don't even want to think about
what this code does if the task is in vm86 mode.  I wouldn't be at all
surprised if non-x86 architectures have all kinds of interesting
thinks happen if you do this to a task that isn't running normal
non-atomic kernel code at the time.

I would advocate for unconditionally returning zeros in these two stat fields.