Date:   Fri, 23 Sep 2016 11:28:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jann@...jh.net>,
        Tycho Andersen <tycho.andersen@...onical.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.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 Fri, Sep 23, 2016 at 12:43 AM, Jann Horn <jann@...jh.net> wrote:
> 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.

Does CRIU use this? I wouldn't expect so, since they're using ptrace,
IIUC, to freeze/restore.

-Kees

-- 
Kees Cook
Nexus Security

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