FAILED: patch "[PATCH] printk: prevent userland from spoofing kernel messages" failed to apply to 3.14-stable tree

The patch below does not apply to the 3.14-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Mathias Krause <[email protected]>
Date: Fri, 6 Nov 2015 16:30:38 -0800
Subject: [PATCH] printk: prevent userland from spoofing kernel messages

The following statement of ABI/testing/dev-kmsg is not quite right:

It is not possible to inject messages from userspace with the facility number LOG_KERN (0), to make sure that the origin of the messages can always be reliably determined.

Userland actually can inject messages with a facility of 0 by abusing the fact that the facility is stored in a u8 data type. By using a facility which is a multiple of 256 the assignment of msg->facility in log_store() implicitly truncates it to 0, i.e. LOG_KERN, allowing users of /dev/kmsg to spoof kernel messages as shown below:

The following call...

   # printf '<%d>Kernel panic - not syncing: beer empty\n' 0 >/dev/kmsg
...leads to the following log entry (dmesg -x | tail -n 1):
   user  :emerg : [   66.137758] Kernel panic - not syncing: beer empty

However, this call...

   # printf '<%d>Kernel panic - not syncing: beer empty\n' 0x800 >/dev/kmsg
...leads to the slightly different log entry (note the kernel facility):
   kern  :emerg : [   74.177343] Kernel panic - not syncing: beer empty

Fix that by limiting the user provided facility to 8 bit right from the beginning and catch the truncation early.

index b16f35487b67..2ce8826f1053 100644
@@ -269,6 +269,9 @@ static u32 clear_idx;
 #define PREFIX_MAX		32
 #define LOG_LINE_MAX		(1024 - PREFIX_MAX)
 
+#define LOG_LEVEL(v)		((v) & 0x07)
+#define LOG_FACILITY(v)		((v) >> 3 & 0xff)
+
 /* record buffer */
 #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
 #define LOG_ALIGN 4

@@ -612,7 +615,6 @@ struct devkmsg_user {
 static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
 {
 	char *buf, *line;
-	int i;
 	int level = default_message_loglevel;
 	int facility = 1;	/* LOG_USER */
 	size_t len = iov_iter_count(from);

@@ -642,12 +644,13 @@ static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
 	line = buf;
 	if (line[0] == '<') {
 		char *endp = NULL;
+		unsigned int u;
 
-		i = simple_strtoul(line+1, &endp, 10);
+		u = simple_strtoul(line + 1, &endp, 10);
 		if (endp && endp[0] == '>') {
-			level = i & 7;
-			if (i >> 3)
-				facility = i >> 3;
+			level = LOG_LEVEL(u);
+			if (LOG_FACILITY(u) != 0)
+				facility = LOG_FACILITY(u);
 			endp++;
 			len -= endp - line;
 			line = endp;