Re: 3.6rc6 slab corruption.

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 09/20/2012 03:19 AM, David Rientjes wrote:
> On Wed, 19 Sep 2012, David Rientjes wrote:
>
>>>  From 0806b133b5b28081adf23d0d04a99636ed3b861b Mon Sep 17 00:00:00 2001
>>> From: Konrad Rzeszutek Wilk<konrad.wilk@oracle.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@redhat.com>
>>> Suggested-by: Linus Torvalds<torvalds@linux-foundation.org>
>>
>> Tested-by: David Rientjes<rientjes@google.com>
>>
>
> An alternative to this, though, might be to never test for *ppos == 0 in
> u32_array_read() and do the format_array_alloc() in u32_array_open() to
> initialize file->private_data.  If that allocation fails, just return
> -ENOMEM.  Then you never need to add a mutex in the read path.
>
> Any reason we can't do this?
> ---
>   fs/debugfs/file.c |   33 +++++++++++----------------------
>   1 file changed, 11 insertions(+), 22 deletions(-)
>
> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> --- a/fs/debugfs/file.c
> +++ b/fs/debugfs/file.c
> @@ -526,12 +526,6 @@ struct array_data {
>   	u32 elements;
>   };
>
> -static int u32_array_open(struct inode *inode, struct file *file)
> -{
> -	file->private_data = NULL;
> -	return nonseekable_open(inode, file);
> -}
> -
>   static size_t format_array(char *buf, size_t bufsize, const char *fmt,
>   			   u32 *array, u32 array_size)
>   {
> @@ -573,26 +567,21 @@ static char *format_array_alloc(const char *fmt, u32 *array,
>   	return ret;
>   }
>
> -static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
> -			      loff_t *ppos)
> +static int u32_array_open(struct inode *inode, struct file *file)
>   {
> -	struct inode *inode = file->f_path.dentry->d_inode;
>   	struct array_data *data = inode->i_private;
> -	size_t size;
>
> -	if (*ppos == 0) {
> -		if (file->private_data) {
> -			kfree(file->private_data);
> -			file->private_data = NULL;
> -		}
> -
> -		file->private_data = format_array_alloc("%u", data->array,
> -							      data->elements);
> -	}
> +	file->private_data = format_array_alloc("%u", data->array,
> +						      data->elements);
> +	if (!file->private_data)
> +		return -ENOMEM;
> +	return nonseekable_open(inode, file);
> +}
>
> -	size = 0;
> -	if (file->private_data)
> -		size = strlen(file->private_data);
> +static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
> +			      loff_t *ppos)
> +{
> +	size_t size = strlen(file->private_data);
>
>   	return simple_read_from_buffer(buf, len, ppos,
>   					file->private_data, size);
>
>

Only problem, I find is histogram data expands dynamically (because it
changes). I think having static allocation of 352 bytes as suggested
Linus is a good idea.