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

[Hugh Dickins]

On Fri, 11 Oct 2013, Krzysztof Kozlowski wrote:

> Swapoff used old_block_size from swap_info which could be overwritten by
> concurrent swapon.
> 
> Reported-by: Weijie Yang <weijie.yang.kh@xxxxxxxxx>
> Signed-off-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

Yes, this is straightforward: it was relying on p->old_block_size
after it had given up its hold on *p: a use-after-free (though
those slots are not freed back to a lower-level allocator).

What is not obvious is why swapon needs to use set_blocksize() at all:
if I knew once upon a time, I've forgotten now: because a bdev starts
out with blocksize 0 and someone needs to set it non-0??

> ---
>  mm/swapfile.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 3963fc2..de7c904 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1824,6 +1824,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  	struct filename *pathname;
>  	int i, type, prev;
>  	int err;
> +	unsigned int old_block_size;
>  
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
> @@ -1914,6 +1915,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  	}
>  
>  	swap_file = p->swap_file;
> +	old_block_size = p->old_block_size;
>  	p->swap_file = NULL;
>  	p->max = 0;
>  	swap_map = p->swap_map;
> @@ -1938,7 +1940,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  	inode = mapping->host;
>  	if (S_ISBLK(inode->i_mode)) {
>  		struct block_device *bdev = I_BDEV(inode);
> -		set_blocksize(bdev, p->old_block_size);
> +		set_blocksize(bdev, old_block_size);
>  		blkdev_put(bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);
>  	} else {
>  		mutex_lock(&inode->i_mutex);
> -- 
> 1.7.9.5
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