register_console: prevent adding the same console twice

This patch guards the console_drivers list to be corrupted. The
for_each_console() macro insist on a strictly forward list ended by NULL:

 con0->next->con1->next->NULL

Without this patch it may happen easily to destroy this list for example by
adding 'earlyprintk' twice, especially on embedded devices where the early
console is often a single static instance.  This will result in the following
list:

 con0->next->con0

This in turn will result in an endless loop in console_unlock() later on by
printing the first __log_buf line endlessly.

Signed-off-by: Andreas Bießmann <andreas@biessmann.de>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5b5a7080e2a5..b4e8500afdb3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2226,6 +2226,13 @@ void register_console(struct console *newcon)
         struct console *bcon = NULL;
         struct console_cmdline *c;
 
+        if (console_drivers)
+                for_each_console(bcon)
+                        if (WARN(bcon == newcon,
+                                        "console '%s%d' already registered\n",
+                                        bcon->name, bcon->index))
+                                return;
+
         /*
          * before we register a new CON_BOOT console, make sure we don't
          * already have a valid console