linux-kernel - Re: [PATCH 3/8] idr: fix NULL pointer dereference when ida_remove(unallocated

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Date:	Fri, 18 Apr 2014 13:09:35 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jean Delvare <jdelvare@...e.de>,
	Monam Agarwal <monamagarwal123@...il.com>,
	Jeff Layton <jlayton@...hat.com>,
	Andreas Gruenbacher <agruen@...bit.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH 3/8] idr: fix NULL pointer dereference when
 ida_remove(unallocated_id)

On Fri, Apr 18, 2014 at 08:49:50PM +0800, Lai Jiangshan wrote:
> If the ida has at least one existed_id, and when an special unallocated_id

		existing id or allocated id

					  when an unallocated id which
					  meets a certain condition is passed to...

> is passed to the ida_remove(), the system will crash because it hits NULL
> pointer dereference.
> 
> This special unallocated_id is that it shares the same lowest idr layer with

The condition is that the ID shares the...

> an exsited_id, but the idr slot is different(if the unallocated_id is allocated).

the existing ID,                  would be different if the unallocated ID were to be allocated.

> In this case the supposed idr slot of the unallocated_id is NULL,

                   matching          for

> It means @bitmap == NULL, and when the code dereference it, it crash the kernel.

causing @bitmap to be NULL which the function dereferences without
checking crashing the kernel.

> 
> See the test code:
> 
> static void test3(void)
> {
> 	int id;
> 	DEFINE_IDA(test_ida);
> 
> 	printk(KERN_INFO "Start test3\n");
> 	if (ida_pre_get(&test_ida, GFP_KERNEL) < 0) return;
> 	if (ida_get_new(&test_ida,  &id) < 0) return;
> 	ida_remove(&test_ida, 4000); /* bug: null deference here */
> 	printk(KERN_INFO "End of test3\n");
> }
> 
> It only happens when unallocated_id, it is caller's fault. It is not

It happens only when the caller tries to free an unallocated ID which
is the caller's fault.

> a bug. But it is better to add the proper check and complains instead

                                                  and complain rather than crashing the kernel

> of crashing the kernel.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun
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