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Subject[PATCH 50/50] printk: do not mangle valid userspace syslog prefixes
Date
From: Kay Sievers <kay.sievers@vrfy.org>

printk: do not mangle valid userspace syslog prefixes with /dev/kmsg

Log messages passed to the kernel log by using /dev/kmsg or /dev/ttyprintk
might contain a syslog prefix including the syslog facility value.

This makes printk to recognize these headers properly, extract the real log
level from it to use, and add the prefix as a proper prefix to the
log buffer, instead of wrongly printing it as the log message text.

Before:
$ echo '<14>text' > /dev/kmsg
$ dmesg -r
<4>[135159.594810] <14>text

After:
$ echo '<14>text' > /dev/kmsg
$ dmesg -r
<14>[ 50.750654] text

Cc: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
kernel/printk.c | 138 +++++++++++++++++++++++++++++++++++++++++--------------
1 files changed, 104 insertions(+), 34 deletions(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index 2ddbdc7..5e3d042 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -499,6 +499,71 @@ static void _call_console_drivers(unsigned start,
}

/*
+ * Parse the syslog header <[0-9]*>. The decimal value represents 32bit, the
+ * lower 3 bit are the log level, the rest are the log facility. In case
+ * userspace passes usual userspace syslog messages to /dev/kmsg or
+ * /dev/ttyprintk, the log prefix might contain the facility. Printk needs
+ * to extract the correct log level for in-kernel processing, and not mangle
+ * the original value.
+ *
+ * If a prefix is found, the length of the prefix is returned. If 'level' is
+ * passed, it will be filled in with the log level without a possible facility
+ * value. If 'special' is passed, the special printk prefix chars are accepted
+ * and returned. If no valid header is found, 0 is returned and the passed
+ * variables are not touched.
+ */
+static size_t log_prefix(const char *p, unsigned int *level, char *special)
+{
+ unsigned int lev = 0;
+ char sp = '\0';
+ size_t len;
+
+ if (p[0] != '<' || !p[1])
+ return 0;
+ if (p[2] == '>') {
+ /* usual single digit level number or special char */
+ switch (p[1]) {
+ case '0' ... '7':
+ lev = p[1] - '0';
+ break;
+ case 'c': /* KERN_CONT */
+ case 'd': /* KERN_DEFAULT */
+ sp = p[1];
+ break;
+ default:
+ return 0;
+ }
+ len = 3;
+ } else {
+ /* multi digit including the level and facility number */
+ char *endp = NULL;
+
+ if (p[1] < '0' && p[1] > '9')
+ return 0;
+
+ lev = (simple_strtoul(&p[1], &endp, 10) & 7);
+ if (endp == NULL || endp[0] != '>')
+ return 0;
+ len = (endp + 1) - p;
+ }
+
+ /* do not accept special char if not asked for */
+ if (sp && !special)
+ return 0;
+
+ if (special) {
+ *special = sp;
+ /* return special char, do not touch level */
+ if (sp)
+ return len;
+ }
+
+ if (level)
+ *level = lev;
+ return len;
+}
+
+/*
* Call the console drivers, asking them to write out
* log_buf[start] to log_buf[end - 1].
* The console_lock must be held.
@@ -513,13 +578,9 @@ static void call_console_drivers(unsigned start, unsigned end)
cur_index = start;
start_print = start;
while (cur_index != end) {
- if (msg_level < 0 && ((end - cur_index) > 2) &&
- LOG_BUF(cur_index + 0) == '<' &&
- LOG_BUF(cur_index + 1) >= '0' &&
- LOG_BUF(cur_index + 1) <= '7' &&
- LOG_BUF(cur_index + 2) == '>') {
- msg_level = LOG_BUF(cur_index + 1) - '0';
- cur_index += 3;
+ if (msg_level < 0 && ((end - cur_index) > 2)) {
+ /* strip log prefix */
+ cur_index += log_prefix(&LOG_BUF(cur_index), &msg_level, NULL);
start_print = cur_index;
}
while (cur_index != end) {
@@ -717,6 +778,8 @@ asmlinkage int vprintk(const char *fmt, va_list args)
unsigned long flags;
int this_cpu;
char *p;
+ size_t plen;
+ char special;

boot_delay_msec();
printk_delay();
@@ -757,45 +820,52 @@ asmlinkage int vprintk(const char *fmt, va_list args)
printed_len += vscnprintf(printk_buf + printed_len,
sizeof(printk_buf) - printed_len, fmt, args);

-
p = printk_buf;

- /* Do we have a loglevel in the string? */
- if (p[0] == '<') {
- unsigned char c = p[1];
- if (c && p[2] == '>') {
- switch (c) {
- case '0' ... '7': /* loglevel */
- current_log_level = c - '0';
- /* Fallthrough - make sure we're on a new line */
- case 'd': /* KERN_DEFAULT */
- if (!new_text_line) {
- emit_log_char('\n');
- new_text_line = 1;
- }
- /* Fallthrough - skip the loglevel */
- case 'c': /* KERN_CONT */
- p += 3;
- break;
+ /* Read log level and handle special printk prefix */
+ plen = log_prefix(p, &current_log_level, &special);
+ if (plen) {
+ p += plen;
+
+ switch (special) {
+ case 'c': /* Strip <c> KERN_CONT, continue line */
+ plen = 0;
+ break;
+ case 'd': /* Strip <d> KERN_DEFAULT, start new line */
+ plen = 0;
+ default:
+ if (!new_text_line) {
+ emit_log_char('\n');
+ new_text_line = 1;
}
}
}

/*
- * Copy the output into log_buf. If the caller didn't provide
- * appropriate log level tags, we insert them here
+ * Copy the output into log_buf. If the caller didn't provide
+ * the appropriate log prefix, we insert them here
*/
- for ( ; *p; p++) {
+ for (; *p; p++) {
if (new_text_line) {
- /* Always output the token */
- emit_log_char('<');
- emit_log_char(current_log_level + '0');
- emit_log_char('>');
- printed_len += 3;
new_text_line = 0;

+ if (plen) {
+ /* Copy original log prefix */
+ int i;
+
+ for (i = 0; i < plen; i++)
+ emit_log_char(printk_buf[i]);
+ printed_len += plen;
+ } else {
+ /* Add log prefix */
+ emit_log_char('<');
+ emit_log_char(current_log_level + '0');
+ emit_log_char('>');
+ printed_len += 3;
+ }
+
if (printk_time) {
- /* Follow the token with the time */
+ /* Add the current time stamp */
char tbuf[50], *tp;
unsigned tlen;
unsigned long long t;
--
1.7.4.1


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