linux-kernel - [PATCH 1/3] proc: Stop reporting eip and esp in /proc/PID/stat

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Date:   Fri, 30 Sep 2016 10:58:56 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Brian Gerst <brgerst@...il.com>,
        Borislav Petkov <bp@...en8.de>, Jann Horn <jann@...jh.net>,
        Linux API <linux-api@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Tycho Andersen <tycho.andersen@...onical.com>,
        Andy Lutomirski <luto@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: [PATCH 1/3] proc: Stop reporting eip and esp in /proc/PID/stat

Reporting these fields on a non-current task is dangerous.  If the
task is in any state other than normal kernel code, they may contain
garbage or even kernel addresses on some architectures.  (x86_64
used to do this.  I bet lots of architectures still do.)  With
CONFIG_THREAD_INFO_IN_TASK, it can OOPS, too.

As far as I know, there are no use programs that make any material
use of these fields, so just get rid of them.

Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: Tycho Andersen <tycho.andersen@...onical.com>
Cc: Kees Cook <keescook@...omium.org>
Reported-by: Jann Horn <jann@...jh.net>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 fs/proc/array.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 88c7de12197b..1bb1097e73b7 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -417,10 +417,11 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	mm = get_task_mm(task);
 	if (mm) {
 		vsize = task_vsize(mm);
-		if (permitted) {
-			eip = KSTK_EIP(task);
-			esp = KSTK_ESP(task);
-		}
+		/*
+		 * esp and eip are intentionally zeroed out.  There is no
+		 * non-racy way to read them without freezing the task.
+		 * Programs that need reliable values can use ptrace(2).
+		 */
 	}
 
 	get_task_comm(tcomm, task);
-- 
2.7.4