[PATCH v4 2/2] lib/vsprintf: Limit the returning size to INT_MAX

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

The return value of vsnprintf() can overflow INT_MAX and return
a minus value. In the @size is checked input overflow, but it does
not check the output, which is expected required size.

This should never happen but it should be checked and limited.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 Changes in v4:
  - Add Petr's reviewed-by. (Thanks!)
 Changes in v3:
  - Use local variable for better readability.
---
 lib/vsprintf.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 5fa8f69030be..351b6f8e4796 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2859,6 +2859,7 @@ static unsigned long long convert_num_spec(unsigned int val, int size, struct pr
 int vsnprintf(char *buf, size_t size, const char *fmt_str, va_list args)
 {
 	char *str, *end;
+	size_t ret_size;
 	struct printf_spec spec = {0};
 	struct fmt fmt = {
 		.str = fmt_str,
@@ -2978,8 +2979,12 @@ int vsnprintf(char *buf, size_t size, const char *fmt_str, va_list args)
 	}
 
 	/* the trailing null byte doesn't count towards the total */
-	return str-buf;
+	ret_size = str - buf;
 
+	/* Make sure the return value is within the positive integer range */
+	if (WARN_ON_ONCE(ret_size > INT_MAX))
+		ret_size = INT_MAX;
+	return ret_size;
 }
 EXPORT_SYMBOL(vsnprintf);