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

[Laura Abbott]

On 09/12/2018 03:27 PM, Kees Cook wrote:
> On Wed, Sep 12, 2018 at 8:29 AM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> > +linux-api, I guess
> >
> > On Tue, Sep 11, 2018 at 8:39 PM Jann Horn <jannh@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> > > ---
> > >   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