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.

Fixes: 7ff9554bb578 ("printk: convert byte-buffer to variable-length...")
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Petr Mladek <pmladek@suse.cz>
Cc: Alex Elder <elder@linaro.org>
Cc: Joe Perches <joe@perches.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b16f35487b67985275169cc0bbaa4efc6cce171d..2ce8826f1053d801f0b003f5a3e67de0fdfe8e30 100644 (file)
--- 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
@@ -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;