linux-kernel - [PATCH] printk: fix integer overflow in setup_log

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Date:   Wed, 10 Oct 2018 19:38:08 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: [PATCH] printk: fix integer overflow in setup_log_buf()

The way we calculate free logbuf free space percentage
overflows signed integer:

	int free;

	free = __LOG_BUF_LEN - log_next_idx;
	pr_info("early log buf free: %u(%u%%)\n",
		free, (free * 100) / __LOG_BUF_LEN);

We support LOG_BUF_LEN of up to 2G, since setup_log_buf() is called
during early init logbuf is mostly empty, so

	__LOG_BUF_LEN - log_next_idx

is close to 2G. When we multiply it by 100, we overflow signed
integer value range.

Example, booting with log_buf_len=2G boot param:

[    0.075317] log_buf_len: -2147483648 bytes
[    0.075319] early log buf free: 33549896(-28%)

Make "free" unsigned integer.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 kernel/printk/printk.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c3f0e3927e77..c9e1d3bf695e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1083,7 +1083,7 @@ void __init setup_log_buf(int early)
 {
 	unsigned long flags;
 	char *new_log_buf;
-	int free;
+	unsigned int free;
 
 	if (log_buf != __log_buf)
 		return;
@@ -1116,8 +1116,8 @@ void __init setup_log_buf(int early)
 	memcpy(log_buf, __log_buf, __LOG_BUF_LEN);
 	logbuf_unlock_irqrestore(flags);
 
-	pr_info("log_buf_len: %d bytes\n", log_buf_len);
-	pr_info("early log buf free: %d(%d%%)\n",
+	pr_info("log_buf_len: %u bytes\n", log_buf_len);
+	pr_info("early log buf free: %u(%u%%)\n",
 		free, (free * 100) / __LOG_BUF_LEN);
 }
 
-- 
2.19.1