Re: [PATCH] mm: zswap: fix potential memory corruption on duplicate store

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Sep 22, 2023 at 07:22:11PM +0200, Domenico Cerasuolo wrote:
> While stress-testing zswap a memory corruption was happening when writing
> back pages. __frontswap_store used to check for duplicate entries before
> attempting to store a page in zswap, this was because if the store fails
> the old entry isn't removed from the tree. This change removes duplicate
> entries in zswap_store before the actual attempt.
> 
> Based on commit ce9ecca0238b ("Linux 6.6-rc2")
> 
> Fixes: 42c06a0e8ebe ("mm: kill frontswap")
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

> @@ -1218,6 +1218,19 @@ bool zswap_store(struct folio *folio)
>  	if (!zswap_enabled || !tree)
>  		return false;
>  
> +	/*
> +	 * If this is a duplicate, it must be removed before attempting to store
> +	 * it, otherwise, if the store fails the old page won't be removed from
> +	 * the tree, and it might be written back overriding the new data.
> +	 */
> +	spin_lock(&tree->lock);
> +	dupentry = zswap_rb_search(&tree->rbroot, offset);
> +	if (dupentry) {
> +		zswap_duplicate_entry++;
> +		zswap_invalidate_entry(tree, dupentry);
> +	}
> +	spin_unlock(&tree->lock);

Do we still need the dupe handling at the end of the function then?

The dupe store happens because a page that's already in swapcache has
changed and we're trying to swap_writepage() it again with new data.

But the page is locked at this point, pinning the swap entry. So even
after the tree lock is dropped I don't see how *another* store to the
tree at this offset could occur while we're compressing.