From: Andrew Morton <firstname.lastname@example.org>
To: Jann Horn <email@example.com>
Cc: Kees Cook <firstname.lastname@example.org>,
Alexey Dobriyan <email@example.com>,
Ken Chen <firstname.lastname@example.org>,
kernel list <email@example.com>,
Will Deacon <firstname.lastname@example.org>,
Laura Abbott <email@example.com>,
Andy Lutomirski <firstname.lastname@example.org>,
Security Officers <email@example.com>,
Catalin Marinas <firstname.lastname@example.org>,
Josh Poimboeuf <email@example.com>,
Thomas Gleixner <firstname.lastname@example.org>,
Ingo Molnar <email@example.com>, "H . Peter Anvin" <firstname.lastname@example.org>,
Linux API <email@example.com>
Subject: Re: [PATCH resend] proc: restrict kernel stack dumps to root
Date: Thu, 27 Sep 2018 15:29:08 -0700 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
On Thu, 27 Sep 2018 17:33:16 +0200 Jann Horn <email@example.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: firstname.lastname@example.org
> Signed-off-by: Jann Horn <email@example.com>
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.
next prev parent reply other threads:[~2018-09-28 4:49 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-27 15:33 [PATCH resend] proc: restrict kernel stack dumps to root Jann Horn
2018-09-27 18:29 ` Kees Cook
2018-09-27 22:29 ` Andrew Morton [this message]
2018-09-27 22:39 ` Jann Horn
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