linux-kernel - Re: 3.6rc6 slab corruption.

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Message-ID: <alpine.DEB.2.00.1209191427460.6273@chino.kir.corp.google.com>
Date:	Wed, 19 Sep 2012 14:29:01 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Konrad Rzeszutek Wilk <konrad@...nel.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Suzuki Poulose <suzuki@...ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: 3.6rc6 slab corruption.

On Wed, 19 Sep 2012, Konrad Rzeszutek Wilk wrote:

> From 0806b133b5b28081adf23d0d04a99636ed3b861b Mon Sep 17 00:00:00 2001
> From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Date: Wed, 19 Sep 2012 11:23:01 -0400
> Subject: [PATCH 1/2] debugfs: Add lock for u32_array_read
> 
> Dave Jones spotted that the u32_array_read was doing something funny:
> 
> =============================================================================
> BUG kmalloc-64 (Not tainted): Redzone overwritten
> -----------------------------------------------------------------------------
> 
> INFO: 0xffff88001f4b4970-0xffff88001f4b4977. First byte 0xbb instead of 0xcc
> INFO: Allocated in u32_array_read+0xd1/0x110 age=0 cpu=6 pid=32767
>         __slab_alloc+0x516/0x5a5
>         __kmalloc+0x213/0x2c0
>         u32_array_read+0xd1/0x110
> .. snip..
> INFO: Freed in u32_array_read+0x99/0x110 age=0 cpu=0 pid=32749
>         __slab_free+0x3f/0x3bf
>         kfree+0x2d5/0x310
>         u32_array_read+0x99/0x110
> 
> Linus tracked it down and found out that "debugfs is racy for that case
> [read calls in parallel on the debugfs]. At least the file->private_data
> accesses are, for the case of that "u32_array" case.
> 
> In fact it is racy in ...  the whole "file->private_data" access ..
> If you have multiple readers on the same file, the whole
> 
> 	if (file->private_data) {
> 		kfree(file->private_data);
> 		file->private_data = NULL;
> 	}
> 
> 	file->private_data = format_array_alloc("%u", data->array,
>                                                               data->elements);
> 
> thing is just a disaster waiting to happen." He suggested
> putting a lock which this patch does.
> 

Since these are non-seekable files, it must also race to find *ppos == 0.

> The consequence of this is that it will trigger more spinlock usage,
> as this particular debugfs is used to provide a histogram of spinlock
> contention. But memory corruption is a worst offender then that.
> 
> Reported-by: Dave Jones <davej@...hat.com>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>

Tested-by: David Rientjes <rientjes@...gle.com>
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