Re: [PATCH v2] mm/swap_state: update zswap LRU's protection range with the folio locked

[Johannes Weiner]

On Tue, Feb 06, 2024 at 10:08:55AM -0800, Nhat Pham wrote:
> When a folio is swapped in, the protection size of the corresponding
> zswap LRU is incremented, so that the zswap shrinker is more
> conservative with its reclaiming action. This field is embedded within
> the struct lruvec, so updating it requires looking up the folio's memcg
> and lruvec. However, currently this lookup can happen after the folio is
> unlocked, for instance if a new folio is allocated, and
> swap_read_folio() unlocks the folio before returning. In this scenario,
> there is no stability guarantee for the binding between a folio and its
> memcg and lruvec:
> 
> * A folio's memcg and lruvec can be freed between the lookup and the
>   update, leading to a UAF.
> * Folio migration can clear the now-unlocked folio's memcg_data, which
>   directs the zswap LRU protection size update towards the root memcg
>   instead of the original memcg. This was recently picked up by the
>   syzbot thanks to a warning in the inlined folio_lruvec() call.
> 
> Move the zswap LRU protection range update above the swap_read_folio()
> call, and only when a new page is allocated, to prevent this.
> 
> Reported-by: syzbot+17a611d10af7d18a7092@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/000000000000ae47f90610803260@xxxxxxxxxx/
> Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>

With the fixlet applied,

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>