printk: do not mangle valid userspace syslog prefixes

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>

1 files changed, 104 insertions, 34 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 2ddbdc73aade..5e3d042e7001 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;