Re: [PATCH] proc: restrict kernel stack dumps to root

[Laura Abbott <labbott@redhat.com>]

On 09/12/2018 03:27 PM, Kees Cook wrote:
> On Wed, Sep 12, 2018 at 8:29 AM, Jann Horn <jannh@google.com> wrote:
>> +linux-api, I guess
>>
>> On Tue, Sep 11, 2018 at 8:39 PM Jann Horn <jannh@google.com> wrote:
>>>
>>> Restrict the ability to inspect kernel stacks of arbitrary tasks to root
>>> in order to prevent a local attacker from exploiting racy stack unwinding
>>> to leak kernel task stack contents.
>>> See the added comment for a longer rationale.
>>>
>>> There don't seem to be any users of this userspace API that can't
>>> gracefully bail out if reading from the file fails. Therefore, I believe
>>> that this change is unlikely to break things.
>>> In the case that this patch does end up needing a revert, the next-best
>>> solution might be to fake a single-entry stack based on wchan.
>>>
>>> Fixes: 2ec220e27f50 ("proc: add /proc/*/stack")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Jann Horn <jannh@google.com>
>>> ---
>>>   fs/proc/base.c | 14 ++++++++++++++
>>>   1 file changed, 14 insertions(+)
>>>
>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>> index ccf86f16d9f0..7e9f07bf260d 100644
>>> --- a/fs/proc/base.c
>>> +++ b/fs/proc/base.c
>>> @@ -407,6 +407,20 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns,
>>>          unsigned long *entries;
>>>          int err;
>>>
>>> +       /*
>>> +        * The ability to racily run the kernel stack unwinder on a running task
>>> +        * and then observe the unwinder output is scary; while it is useful for
>>> +        * debugging kernel issues, it can also allow an attacker to leak kernel
>>> +        * stack contents.
>>> +        * Doing this in a manner that is at least safe from races would require
>>> +        * some work to ensure that the remote task can not be scheduled; and
>>> +        * even then, this would still expose the unwinder as local attack
>>> +        * surface.
>>> +        * Therefore, this interface is restricted to root.
>>> +        */
>>> +       if (!file_ns_capable(m->file, &init_user_ns, CAP_SYS_ADMIN))
>>> +               return -EACCES;
> 
> In the past, we've avoided hard errors like this in favor of just
> censoring the output. Do we want to be more cautious here? (i.e.
> return 0 or a fuller seq_printf(m, "[<0>] privileged\n"); return 0;)
> 

The -EACCES is a strong hint to run with root privileges which is
nice from an end user perspective. If we don't want to return an
actual error, I think the "privileged" message would be okay.

Laura

>>> +
>>>          entries = kmalloc_array(MAX_STACK_TRACE_DEPTH, sizeof(*entries),
>>>                                  GFP_KERNEL);
>>>          if (!entries)
>>> --
>>> 2.19.0.rc2.392.g5ba43deb5a-goog
>>>
> 
> -Kees
>