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

[Jann Horn]

On Thu, Sep 13, 2018 at 4:39 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, Sep 13, 2018 at 4:55 AM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> > On Thu, Sep 13, 2018 at 12:28 AM Kees Cook <keescook@xxxxxxxxxxxx> 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;)
> >
> > In my mind, this is different because it's a place where we don't have
> > to selectively censor output while preserving parts of it, and it's a
> > place where, as Laura said, it's useful to make lack of privileges
> > clearly visible because that informs users that they may have to retry
> > with more privileges.
> >
> > Of course, if you have an example of software that actually breaks due
> > to this, I'll change it. But I looked at the three things in Debian
> > codesearch that seem to use it, and from what I can tell, they all
> > bail out cleanly when the read fails.
>
> I prefer -EACCESS too, but I thought I'd mention the alternative. So, I guess:
>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

What do I need to do to get this merged? Oh, I think I misread
MAINTAINERS - Alexey Dobriyan is not a maintainer, just a reviewer. So
I guess this has to go via Andrew Morton? Should I resend the patch
with Andrew in the recipient list?