From: Mathias Krause <minipli@xxxxxxxxxxxxxx>
Subject: 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@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>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/printk/printk.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)
diff -puN kernel/printk/printk.c~printk-prevent-userland-from-spoofing-kernel-messages kernel/printk/printk.c
--- a/kernel/printk/printk.c~printk-prevent-userland-from-spoofing-kernel-messages
+++ a/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 kioc
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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