On Thu, 27 Sep 2018 17:33:16 +0200 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>
It's a bit worrisome cc'ing stable on a patch which might need a revert.
> --- 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.
> + */
The /proc file is 0400 so the user can only read owned-by-self stacks,
yes? In what way could exposure of one's own kernel stack contents
lead to plausible attacks? I guess maybe post-setuid, perhaps?
I do think we're owed considerably more explanation of the present risk
before considering a somewhat dangerous -stable backport, please.
