Re: [PATCH] mm/swap: fix race on swap_info reuse between swapoffand swapon

[Andrew Morton]

On Thu, 09 Jan 2014 13:39:55 +0800 Weijie Yang <weijie.yang@xxxxxxxxxxx> wrote:

> swapoff clear swap_info's SWP_USED flag prematurely and free its resources
> after that. A concurrent swapon will reuse this swap_info while its previous
> resources are not cleared completely.
> 
> These late freed resources are:
> - p->percpu_cluster
> - swap_cgroup_ctrl[type]
> - block_device setting
> - inode->i_flags &= ~S_SWAPFILE
> 
> This patch clear SWP_USED flag after all its resources freed, so that swapon
> can reuse this swap_info by alloc_swap_info() safely.
> 
> ...
>
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1922,7 +1922,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  	p->swap_map = NULL;
>  	cluster_info = p->cluster_info;
>  	p->cluster_info = NULL;
> -	p->flags = 0;
>  	frontswap_map = frontswap_map_get(p);
>  	spin_unlock(&p->lock);
>  	spin_unlock(&swap_lock);
> @@ -1948,6 +1947,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  		mutex_unlock(&inode->i_mutex);
>  	}
>  	filp_close(swap_file, NULL);
> +
> +	/*
> +	* clear SWP_USED flag after all resources freed
> +	* so that swapon can reuse this swap_info in alloc_swap_info() safely
> +	* it is ok to not hold p->lock after we cleared its SWP_WRITEOK
> +	*/
> +	spin_lock(&swap_lock);
> +	p->flags = 0;
> +	spin_unlock(&swap_lock);
> +
>  	err = 0;
>  	atomic_inc(&proc_poll_event);
>  	wake_up_interruptible(&proc_poll_wait);

I'm scratching my head over the swap_lock use here.  Is it being used
appropriately, is it the correct lock, etc.

swap_start() and friends are playing with SWP_USED, but they're using
swapon_mutex.  I wonder if a well-timed read of /proc/swaps could cause
problems.

The swapfile.c code does not make for pleasant reading :(
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/