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

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Date:   Thu, 27 Sep 2018 11:29:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Ken Chen <kenchen@...gle.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Laura Abbott <labbott@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Security Officers <security@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH resend] proc: restrict kernel stack dumps to root

On Thu, Sep 27, 2018 at 8:33 AM, Jann Horn <jannh@...gle.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@...r.kernel.org
> Signed-off-by: Jann Horn <jannh@...gle.com>

Acked-by: Kees Cook <keescook@...omium.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