From 24b2ec21192c963c17a1b687b6171e95e8b59c06 Mon Sep 17 00:00:00 2001
From: Alexey Dobriyan <adobriyan@gmail.com>
Date: Tue, 10 Apr 2018 16:31:30 -0700
Subject: [PATCH] proc: check permissions earlier for /proc/*/wchan

get_wchan() accesses stack page before permissions are checked, let's
not play this game.

Link: http://lkml.kernel.org/r/20180217071923.GA16074@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 fs/proc/base.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index d53246863cfb..d8b5a1653444 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -388,14 +388,17 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
 	unsigned long wchan;
 	char symname[KSYM_NAME_LEN];
 
-	wchan = get_wchan(task);
+	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
+		goto print0;
 
-	if (wchan && ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)
-			&& !lookup_symbol_name(wchan, symname))
+	wchan = get_wchan(task);
+	if (wchan && !lookup_symbol_name(wchan, symname)) {
 		seq_printf(m, "%s", symname);
-	else
-		seq_putc(m, '0');
+		return 0;
+	}
 
+print0:
+	seq_putc(m, '0');
 	return 0;
 }
 #endif /* CONFIG_KALLSYMS */
-- 
2.11.0