linux-kernel - Re: [PATCH 09/12] x86/process: Pin the target stack in get

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Date:   Fri, 23 Sep 2016 09:43:06 +0200
From:   Jann Horn <jann@...jh.net>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 09/12] x86/process: Pin the target stack in get_wchan()

On Thu, Sep 22, 2016 at 03:44:37PM -0700, Andy Lutomirski wrote:
> On Fri, Sep 16, 2016 at 7:00 PM, Jann Horn <jann@...jh.net> 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?

I just scrolled through the debian codesearch results for "\[stack\]" -
there seem to only be 105 across all of debian's packages, many of them
duplicates - and I didn't see any that looked like they used the tid map.
So I think this might work.

( https://codesearch.debian.net/search?q=%22%5C%5Bstack%5C%5D%22 )


> >   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.

I'd like that a lot.

I guess the two things that might theoretically use it are ptrace users
and (very theoretically) sampling profiling stuff or so?

In gdb, the only code I can find that reads this is in gdb/linux-nat.c, but
it's behind an "#ifdef 0":

  #if 0   /* Don't know how architecture-dependent the rest is...
             Anyway the signal bitmap info is available from "status".  */
            if (fscanf (procfile, "%lu ", &ltmp) > 0)     /* FIXME arch?  */
              printf_filtered (_("Kernel stack pointer: 0x%lx\n"), ltmp);
            if (fscanf (procfile, "%lu ", &ltmp) > 0)     /* FIXME arch?  */
              printf_filtered (_("Kernel instr pointer: 0x%lx\n"), ltmp);
  [...]

strace and ltrace don't seem to be using it.

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