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printk: fix buffer overflow when calling log_prefix function from cal…

…l_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
162a7e7 (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 7ff9554 (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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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1 parent 9ad3bfb commit ce0030c00f95cf9110d9cdcd41e901e1fb814417 @igit igit committed with gregkh Feb 1, 2013
Showing with 18 additions and 1 deletion.
  1. +6 −0 include/linux/syslog.h
  2. +12 −1 kernel/printk.c
View
@@ -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 */
View
@@ -638,8 +638,19 @@ 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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