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Commit 3824657c authored by Mathias Krause's avatar Mathias Krause Committed by Linus Torvalds
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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: 7ff9554b ("printk: convert byte-buffer to variable-length...")
Signed-off-by: default avatarMathias 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent d7ec9a05

kernel/printk/printk.c

+8 −5
Original line number Diff line number Diff line
@@ -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;