[kernel-hardening] [PATCH v2] kernel: escape non-ASCII and control characters in printk()

[Vasiliy Kulikov <segoon@openwall.com>]

This patch escapes control characters fed to printk() except '\n' and '\t'.

There are numerous printk() instances with user supplied input as "%s"
data, and unprivileged user may craft log messages with substrings
containing control characters via these printk()s.  Control characters
might fool root viewing the logs via tty, e.g. using ^[1A to suppress
the previous log line.

On the testing Samsung Q310 laptop there are no users of chars outside
of the restricted charset. 

v2 - Allow chars with code >127.  Allow tabs.

Reported-by: Solar Designer <solar@openwall.com>
Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
---
 kernel/printk.c |   17 ++++++++++++++++-
 1 files changed, 16 insertions(+), 1 deletions(-)

---
diff --git a/kernel/printk.c b/kernel/printk.c
index 3518539..727ff7d 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -41,6 +41,7 @@
 #include <linux/cpu.h>
 #include <linux/notifier.h>
 #include <linux/rculist.h>
+#include <linux/ctype.h>
 
 #include <asm/uaccess.h>
 
@@ -671,6 +672,20 @@ static void emit_log_char(char c)
 		logged_chars++;
 }
 
+static void emit_log_char_escaped(char c)
+{
+	char buffer[8];
+	int i, len;
+
+	if (!iscntrl(c) || (c == '\n') || (c == '\t'))
+		emit_log_char(c);
+	else {
+		len = sprintf(buffer, "#x%02x", c);
+		for (i = 0; i < len; i++)
+			emit_log_char(buffer[i]);
+	}
+}
+
 /*
  * Zap console related locks when oopsing. Only zap at most once
  * every 10 seconds, to leave time for slow consoles to print a
@@ -938,7 +953,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 				break;
 		}
 
-		emit_log_char(*p);
+		emit_log_char_escaped(*p);
 		if (*p == '\n')
 			new_text_line = 1;
 	}
---