linux-kernel - [PATCH] printk: fix possible reuse of va

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Message-Id: <201805112002.GIF21216.OFVHFOMLJtQFSO@I-love.SAKURA.ne.jp>
Date:   Fri, 11 May 2018 20:02:31 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     sergey.senozhatsky.work@...il.com
Cc:     pmladek@...e.com, dvyukov@...gle.com, sergey.senozhatsky@...il.com,
        syzkaller@...glegroups.com, rostedt@...dmis.org,
        fengguang.wu@...el.com, linux-kernel@...r.kernel.org,
        peterz@...radead.org
Subject: [PATCH] printk: fix possible reuse of va_list variable

>>From 766cf72b5fdc00d1cf5a8ca2c6b23ebb75e2b4d4 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Fri, 11 May 2018 19:54:19 +0900
Subject: [PATCH] printk: fix possible reuse of va_list variable

I noticed that there is a possibility that printk_safe_log_store() causes
kernel oops because "args" parameter is passed to vsnprintf() again when
atomic_cmpxchg() detected that we raced. Fix this by using va_copy().

Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Fixes: 42a0bb3f71383b45 ("printk/nmi: generic solution for safe printk in NMI")
Cc: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc: Petr Mladek <pmladek@...e.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
---
 kernel/printk/printk_safe.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 3e3c200..449d67e 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -82,6 +82,7 @@ static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
 {
 	int add;
 	size_t len;
+	va_list ap;
 
 again:
 	len = atomic_read(&s->len);
@@ -100,7 +101,9 @@ static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
 	if (!len)
 		smp_rmb();
 
-	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
+	va_copy(ap, args);
+	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);
+	va_end(ap);
 	if (!add)
 		return 0;
 
-- 
1.8.3.1