[ 128/128] printk: fix buffer overflow when calling log_prefix function from call_console_drivers

Ben Hutchings
2013-02-03T15:11:19Z

From: Alexandre SIMON <Alexandre.Simon@univ-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@univ-lorraine.fr>
Signed-off-by: Ben Hutchings <ben@decadent.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) {