[PATCH AUTOSEL 4.14 002/150] printk: fix integer overflow in setup_log_buf()

Sasha Levin
2019-11-16T16:15:33Z

From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

[ Upstream commit d2130e82e9454304e9b91ba9da551b5989af8c27 ]

The way we calculate 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 1<<25 bytes. Since setup_log_buf() is
called during early init, logbuf is mostly empty, so

__LOG_BUF_LEN - log_next_idx

is close to 1<<25. Thus when we multiply it by 100, we overflow signed
integer value range: 100 is 2^6 + 2^5 + 2^2.

Example, booting with LOG_BUF_LEN 1<<25 and 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 and use appropriate printk() specifier.

Link: http://lkml.kernel.org/r/20181010113308.9337-1-sergey.senozhatsky@gmail.com
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/printk/printk.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5aa96098c64d3..e3dd5dcabf3e6 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1093,7 +1093,7 @@ 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;
--
2.20.1