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printk: prevent userland from spoofing kernel messages

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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: 7ff9554bb5 ("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>
This commit is contained in:
Mathias Krause 2015-11-06 16:30:38 -08:00 committed by Linus Torvalds
parent d7ec9a05d6
commit 3824657c52
1 changed files with 8 additions and 5 deletions
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13
kernel/printk/printk.c
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@ -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;