linux-kernel - [ 128/128] printk: fix buffer overflow when calling log_prefix function from call_console

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Message-Id: <20130203144655.379237534@decadent.org.uk>
Date:	Sun, 03 Feb 2013 15:48:52 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org,
	Alexandre SIMON <Alexandre.Simon@...v-lorraine.fr>
Subject: [ 128/128] printk: fix buffer overflow when  calling log_prefix function from call_console_drivers

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Alexandre SIMON <Alexandre.Simon@...v-lorraine.fr>

This patch corrects a buffer overflow in kernels from 3.0 to 3.4 when calling
log_prefix() function from call_console_drivers().

This bug existed in previous releases but has been revealed with commit
162a7e7500f9664636e649ba59defe541b7c2c60 (2.6.39 => 3.0) that made changes
about how to allocate memory for early printk buffer (use of memblock_alloc).
It disappears with commit 7ff9554bb578ba02166071d2d487b7fc7d860d62 (3.4 => 3.5)
that does a refactoring of printk buffer management.

In log_prefix(), the access to "p[0]", "p[1]", "p[2]" or
"simple_strtoul(&p[1], &endp, 10)" may cause a buffer overflow as this
function is called from call_console_drivers by passing "&LOG_BUF(cur_index)"
where the index must be masked to do not exceed the buffer's boundary.

The trick is to prepare in call_console_drivers() a buffer with the necessary
data (PRI field of syslog message) to be safely evaluated in log_prefix().

This patch can be applied to stable kernel branches 3.0.y, 3.2.y and 3.4.y.

Without this patch, one can freeze a server running this loop from shell :
  $ export DUMMY=`cat /dev/urandom | tr -dc '12345AZERTYUIOPQSDFGHJKLMWXCVBNazertyuiopqsdfghjklmwxcvbn' | head -c255`
  $ while true do ; echo $DUMMY > /dev/kmsg ; done

The "server freeze" depends on where memblock_alloc does allocate printk buffer :
if the buffer overflow is inside another kernel allocation the problem may not
be revealed, else the server may hangs up.

Signed-off-by: Alexandre SIMON <Alexandre.Simon@...v-lorraine.fr>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/linux/syslog.h |    6 ++++++
 kernel/printk.c        |   13 ++++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/syslog.h b/include/linux/syslog.h
index 3891139..ce4c665 100644
--- a/include/linux/syslog.h
+++ b/include/linux/syslog.h
@@ -47,6 +47,12 @@
 #define SYSLOG_FROM_CALL 0
 #define SYSLOG_FROM_FILE 1

+/*
+ * Syslog priority (PRI) maximum length in char : '<[0-9]{1,3}>'
+ * See RFC5424 for details
+*/
+#define SYSLOG_PRI_MAX_LENGTH 5
+
 int do_syslog(int type, char __user *buf, int count, bool from_file);

 #endif /* _LINUX_SYSLOG_H */
diff --git a/kernel/printk.c b/kernel/printk.c
index 3fc4708..6edc4e89 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -633,8 +633,19 @@ static void call_console_drivers(unsigned start, unsigned end)
 	start_print = start;
 	while (cur_index != end) {
 		if (msg_level < 0 && ((end - cur_index) > 2)) {
+			/*
+			 * prepare buf_prefix, as a contiguous array,
+			 * to be processed by log_prefix function
+			 */
+			char buf_prefix[SYSLOG_PRI_MAX_LENGTH+1];
+			unsigned i;
+			for (i = 0; i < ((end - cur_index)) && (i < SYSLOG_PRI_MAX_LENGTH); i++) {
+				buf_prefix[i] = LOG_BUF(cur_index + i);
+			}
+			buf_prefix[i] = '\0'; /* force '\0' as last string character */
+
 			/* strip log prefix */
-			cur_index += log_prefix(&LOG_BUF(cur_index), &msg_level, NULL);
+			cur_index += log_prefix((const char *)&buf_prefix, &msg_level, NULL);
 			start_print = cur_index;
 		}
 		while (cur_index != end) {

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