From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-vs1-f73.google.com ([209.85.217.73]:53407 "EHLO mail-vs1-f73.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728020AbeI0VwO (ORCPT ); Thu, 27 Sep 2018 17:52:14 -0400 Received: by mail-vs1-f73.google.com with SMTP id f29-v6so1048548vsl.20 for ; Thu, 27 Sep 2018 08:33:25 -0700 (PDT) Date: Thu, 27 Sep 2018 17:33:16 +0200 Message-Id: <20180927153316.200286-1-jannh@google.com> Mime-Version: 1.0 Subject: [PATCH resend] proc: restrict kernel stack dumps to root From: Jann Horn To: Andrew Morton , jannh@google.com Cc: Kees Cook , Alexey Dobriyan , Ken Chen , kernel list , "linux-fsdevel@vger.kernel.org" , Will Deacon , Laura Abbott , Andy Lutomirski , Security Officers , Catalin Marinas , Josh Poimboeuf , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Linux API Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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 --- 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