[PATCH] printk: prevent userland from spoofing kernel messages

[Mathias Krause]

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.

Fixes: 7ff9554bb578 ("printk: convert byte-buffer to variable-length...")
Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxx>
Cc: Alex Elder <elder@xxxxxxxxxx>
Cc: Joe Perches <joe@xxxxxxxxxxx>
Cc: Kay Sievers <kay@xxxxxxxx>
---
Might be worth to apply to stable, too. Don't know. Prior to commit
7ff9554bb578 there was no facility so user injected messages had an implicit
facility for LOG_KERN. But commit 7ff9554bb578 explicitly mentions this
feature, so, dunno :/

 kernel/printk/printk.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8f0324ef72ab..9982616ce712 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -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
@@ -611,7 +614,6 @@ 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);
@@ -641,12 +643,13 @@ 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;
-- 
1.7.10.4

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