printk: fix buffer overflow when calling log_prefix function from call_console_drivers

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@univ-lorraine.fr>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/include/linux/syslog.h b/include/linux/syslog.h
index 38911391a139a37376e2053008d6e7bb49559cd5..ce4c6655605937c62d7048d6ee0b7130d47d703c 100644 (file)
--- 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 7982a0a841eaf082fe929e24a1f4bd5aefd4f015..c0d12ea31c86e7e6fd52a7da9e51f88f1633da45 100644 (file)
--- 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) {