linux-kernel - [PATCH] base/class.c: prevent ooops due to insert/remove race (v3)

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Date:	Thu, 29 Nov 2007 13:09:53 -0500
From:	Mark Lord <lkml@....ca>
To:	Greg KH <gregkh@...e.de>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-usb-devel@...ts.sourceforge.net
Subject: [PATCH] base/class.c: prevent ooops due to insert/remove race (v3)

Mark Lord wrote:
> Mark Lord wrote:
>> Mark Lord wrote:
>> ...
>> And here is a "prevented" oops, courtesy of the patch (2.6.23.8).
>> These are easy to reproduce (just jiggle the connection on an
>> attached USB multi-card reader with a CF card inserted):
> ...
>> [  347.099562] usb 5-6: USB disconnect, address 10
>> [  347.101077] BUG: unable to handle kernel NULL pointer dereference 
>> at virtual address 00000000
> 
> ...
> 
> 
> Mmmm.. that's odd.  It *did* Oops there.  I wonder how/why ?
...

Ahh.. it was the *other* call to strlen() that was crapping out.
Here's an updated patch to prevent the Oops:

* * * *

While doing insert/remove (quickly) tests on USB,
I managed to trigger an Oops on 2.6.23.8 on a call
to strlen() in make_class_name().

USB maintainers can try this themselves, by plugging in an
external USB XX-in-1 flash reader, with a CF card inserted.
Then just jiggle the connection so that the device connects
and disconnects rapidly.  This may not sound realistic,
but there's definitely a race in there, and this is a quick
way to reproduce it if need be.

The patch below prevents the oops, but still keeps the bug visible.

Signed-off-by:  Mark Lord <mlord@...ox.com> 
---

Patch applies to both 2.6.24 and 2.6.23.

--- old/drivers/base/class.c	2007-11-29 10:51:43.000000000 -0500
+++ linux/drivers/base/class.c	2007-11-29 13:00:15.000000000 -0500
@@ -352,9 +352,22 @@
 char *make_class_name(const char *name, struct kobject *kobj)
 {
 	char *class_name;
+	const char *kname;
 	int size;
 
-	size = strlen(name) + strlen(kobject_name(kobj)) + 2;
+	/* Rapidly inserting/removing a USB device (others?)
+	 * can trigger an Oops on the strlen() call.
+	 * Cause unknown yet, so prevent the Oops
+	 * but don't mask the issue.
+	 */
+	kname = kobject_name(kobj);
+	if (!kname || !name) {
+		printk(KERN_ERR "make_class_name: name=%p kname=%p\n",
+						name, kname);
+		BUG_ON(1);
+		return NULL;
+	}
+	size = strlen(name) + strlen(kname) + 2;
 
 	class_name = kmalloc(size, GFP_KERNEL);
 	if (!class_name)
-
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