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Subject[PATCH 3.16.y-ckt 125/126] printk: prevent userland from spoofing kernel messages
Date
3.16.7-ckt21 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mathias Krause <minipli@googlemail.com>

commit 3824657c522f19f85a76bd932821174a5557a382 upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[ kamal: backport to 3.13-stable: retain local 'int i' ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
kernel/printk/printk.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f7f6f7e5ff8a..5fa3f3b3b338 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -259,6 +259,9 @@ 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
@@ -547,12 +550,13 @@ static ssize_t devkmsg_writev(struct kiocb *iocb, const struct iovec *iv,
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;

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