[PATCH, v3] isdn: Fix stack corruption in isdnloop_init()

[Ingo Molnar <mingo@elte.hu>]

* Arjan van de Ven <arjan@infradead.org> wrote:

> > -	char rev[10];
> > +	char rev[strlen(revision)+1];
> >  
> >  	if ((p = strchr(revision, ':'))) {
> >  		strcpy(rev, p + 1);
> 
> now it;s a runtime variable sized array.
> NotNice(tm)

ah, indeed - i thought GCC would figure out its constness but it 
doesnt. v3 should fix this:

-------------->
>From aa9aff8c2633cf24ad6465d8bf5aa93aa5d91f83 Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@elte.hu>
Date: Tue, 26 May 2009 21:18:22 +0200
Subject: [PATCH] isdn: Fix stack corruption in isdnloop_init()

-tip testing found this stack corruption and bootup crash
in the ISDN subsystem, reported by stackprotector:

[   25.656688] calling  isdn_init+0x0/0x2c2 @ 1
[   25.660388] ISDN subsystem Rev: 1.1.2.3/1.1.2.3/1.1.2.2/1.1.2.3/1.1.2.2/1.1.2.2
[   25.668179] initcall isdn_init+0x0/0x2c2 returned 0 after 6510 usecs
[   25.670005] calling  isdn_bsdcomp_init+0x0/0x45 @ 1
[   25.673336] PPP BSD Compression module registered
[   25.676674] initcall isdn_bsdcomp_init+0x0/0x45 returned 0 after 3255 usecs
[   25.680005] calling  isdnloop_init+0x0/0x88 @ 1
[   25.683337] isdnloop-ISDN-driver Rev 1.11.6.7
[   25.686705] isdnloop: (loop0) virtual card added
[   25.690004] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: c1de2d8b
[   25.690006]
[   25.693338] Pid: 1, comm: swapper Not tainted 2.6.31-rc8-tip-01250-geed031c-dirty #9565
[   25.696672] Call Trace:
[   25.700008]  [<c190f517>] ? printk+0x1d/0x30
[   25.703339]  [<c190f45d>] panic+0x50/0xed
[   25.706677]  [<c1059194>] __stack_chk_fail+0x1e/0x42
[   25.710005]  [<c1de2d8b>] ? isdnloop_init+0x83/0x88
[   25.713338]  [<c1de2d8b>] isdnloop_init+0x83/0x88
[   25.716674]  [<c1001056>] _stext+0x56/0x15a
[   25.720007]  [<c1da8368>] kernel_init+0x8f/0xf1
[   25.723338]  [<c1da82d9>] ? kernel_init+0x0/0xf1
[   25.726675]  [<c1025c67>] kernel_thread_helper+0x7/0x58
[   25.730005] Rebooting in 1 seconds..Press any key to enter the menu

The bug is that the temporary array:

	char rev[10];

Is sized one byte too small to store strings based on
the 'revision' string.

This is a truly ancient bug: it has been introduced in
the v2.4.2.1 kernel, ~8.5 years ago, which extended
the length of 'revision' by 1 byte.

Instead of using a fixed size temporary array, size
it based on the 'revision' string.

Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 drivers/isdn/isdnloop/isdnloop.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/isdn/isdnloop/isdnloop.c b/drivers/isdn/isdnloop/isdnloop.c
index a335c85..ea07fdd 100644
--- a/drivers/isdn/isdnloop/isdnloop.c
+++ b/drivers/isdn/isdnloop/isdnloop.c
@@ -15,7 +15,7 @@
 #include <linux/sched.h>
 #include "isdnloop.h"
 
-static char *revision = "$Revision: 1.11.6.7 $";
+static char revision[] = "$Revision: 1.11.6.7 $";
 static char *isdnloop_id = "loop0";
 
 MODULE_DESCRIPTION("ISDN4Linux: Pseudo Driver that simulates an ISDN card");
@@ -1494,7 +1494,7 @@ static int __init
 isdnloop_init(void)
 {
 	char *p;
-	char rev[10];
+	char rev[sizeof(revision)];
 
 	if ((p = strchr(revision, ':'))) {
 		strcpy(rev, p + 1);