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

From: Jann Horn <jannh@google.com>
To: Andrew Morton <akpm@linux-foundation.org>, jannh@google.com
Cc: Kees Cook <keescook@chromium.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: [PATCH resend] proc: restrict kernel stack dumps to root
Date: Thu, 27 Sep 2018 17:33:16 +0200	[thread overview]
Message-ID: <20180927153316.200286-1-jannh@google.com> (raw)

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>
---
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

