Re: [PATCH] swap: fix set_blocksize race during swapon/swapoff

[Krzysztof Kozlowski]

On Fri, 2013-10-11 at 11:55 -0700, Andrew Morton wrote:
> On Fri, 11 Oct 2013 11:54:22 +0200 Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx> wrote:
> 
> > Swapoff used old_block_size from swap_info which could be overwritten by
> > concurrent swapon.
> 
> Better changelogs, please.  What were the user-visible effects of the
> bug, and how is it triggered?
Let me update a little the changelog:
--------
Fix race between swapoff and swapon. Swapoff used old_block_size from
swap_info outside of swapon_mutex so it could be overwritten by
concurrent swapon.

The race has visible effect only if more than one swap block device
exists with different block sizes (e.g. /dev/sda1 with block size 4096
and /dev/sdb1 with 512). In such case it leads to setting the blocksize
of swapped off device with wrong blocksize.

The bug can be triggered with multiple concurrent swapoff and swapon:
0. Swap for some device is on.
1. swapoff:
First the swapoff is called on this device and "struct swap_info_struct
*p" is assigned. This is done under swap_lock however this lock is
released for the call try_to_unuse().

2. swapon:
After the assignment above (and before acquiring swapon_mutex &
swap_lock by swapoff) the swapon is called on the same device.
The p->old_block_size is assigned to the value of block_size the device.
This block size should be the same as previous but sometimes it is not.
The swapon ends successfully.

3. swapoff:
Swapoff resumes, grabs the locks and mutex and continues to disable this
swap device. Now it sets the block size to value taken from swap_info
which was overwritten by swapon in 2.
--------

Best regards,
Krzysztof

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