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

From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Ken Chen <kenchen@google.com>,
	kernel list <linux-kernel@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	Will Deacon <will.deacon@arm.com>,
	Laura Abbott <labbott@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Security Officers <security@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH resend] proc: restrict kernel stack dumps to root
Date: Thu, 27 Sep 2018 11:29:53 -0700	[thread overview]
Message-ID: <CAGXu5j+9=6tvtynh=KLdvt6O9TkS6cj49itwZk9n0x0qDqaShg@mail.gmail.com> (raw)
In-Reply-To: <20180927153316.200286-1-jannh@google.com>

On Thu, Sep 27, 2018 at 8:33 AM, 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>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> Resending because I forgot to send this to akpm the first time.
>
>  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;
> +
>         entries = kmalloc_array(MAX_STACK_TRACE_DEPTH, sizeof(*entries),
>                                 GFP_KERNEL);
>         if (!entries)
> --
> 2.19.0.rc2.392.g5ba43deb5a-goog
>


-- 
Kees Cook
Pixel Security