On Sun, Jan 28, 2024 at 01:28:50PM +0000, Chengming Zhou wrote: > LRU writeback has race problem with swapoff, as spotted by Yosry [1]: > > CPU1 CPU2 > shrink_memcg_cb swap_off > list_lru_isolate zswap_invalidate > zswap_swapoff > kfree(tree) > // UAF > spin_lock(&tree->lock) > > The problem is that the entry in lru list can't protect the tree from > being swapoff and freed, and the entry also can be invalidated and freed > concurrently after we unlock the lru lock. > > We can fix it by moving the swap cache allocation ahead before > referencing the tree, then check invalidate race with tree lock, > only after that we can safely deref the entry. Note we couldn't > deref entry or tree anymore after we unlock the folio, since we > depend on this to hold on swapoff. > > So this patch moves all tree and entry usage to zswap_writeback_entry(), > we only use the copied swpentry on the stack to allocate swap cache > and if returned with folio locked we can reference the tree safely. > Then we can check invalidate race with tree lock, the following things > is much the same like zswap_load(). > > Since we can't deref the entry after zswap_writeback_entry(), we > can't use zswap_lru_putback() anymore, instead we rotate the entry > in the beginning. And it will be unlinked and freed when invalidated > if writeback success. You are also removing one redundant lookup from the zswap writeback path to check for the invalidation race, and simplifying the code. Give yourself full credit :) > > Another change is we don't update the memcg nr_zswap_protected in > the -ENOMEM and -EEXIST cases anymore. -EEXIST case means we raced > with swapin or concurrent shrinker action, since swapin already > have memcg nr_zswap_protected updated, don't need double counts here. > For concurrent shrinker, the folio will be writeback and freed anyway. > -ENOMEM case is extremely rare and doesn't happen spuriously either, > so don't bother distinguishing this case. > > [1] https://lore.kernel.org/all/CAJD7tkasHsRnT_75-TXsEe58V9_OW6m3g6CF7Kmsvz8CKRG_EA@xxxxxxxxxxxxxx/ > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> > Acked-by: Nhat Pham <nphamcs@xxxxxxxxx> > Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> > --- > mm/zswap.c | 114 ++++++++++++++++++++++++++----------------------------------- > 1 file changed, 49 insertions(+), 65 deletions(-) > > diff --git a/mm/zswap.c b/mm/zswap.c > index 81cb3790e0dd..f5cb5a46e4d7 100644 > --- a/mm/zswap.c > +++ b/mm/zswap.c > @@ -277,7 +277,7 @@ static inline struct zswap_tree *swap_zswap_tree(swp_entry_t swp) > zpool_get_type((p)->zpools[0])) > > static int zswap_writeback_entry(struct zswap_entry *entry, > - struct zswap_tree *tree); > + swp_entry_t swpentry); > static int zswap_pool_get(struct zswap_pool *pool); > static void zswap_pool_put(struct zswap_pool *pool); > > @@ -445,27 +445,6 @@ static void zswap_lru_del(struct list_lru *list_lru, struct zswap_entry *entry) > rcu_read_unlock(); > } > > -static void zswap_lru_putback(struct list_lru *list_lru, > - struct zswap_entry *entry) > -{ > - int nid = entry_to_nid(entry); > - spinlock_t *lock = &list_lru->node[nid].lock; > - struct mem_cgroup *memcg; > - struct lruvec *lruvec; > - > - rcu_read_lock(); > - memcg = mem_cgroup_from_entry(entry); > - spin_lock(lock); > - /* we cannot use list_lru_add here, because it increments node's lru count */ > - list_lru_putback(list_lru, &entry->lru, nid, memcg); > - spin_unlock(lock); > - > - lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry_to_nid(entry))); > - /* increment the protection area to account for the LRU rotation. */ > - atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected); > - rcu_read_unlock(); > -} > - > /********************************* > * rbtree functions > **********************************/ > @@ -860,40 +839,47 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o > { > struct zswap_entry *entry = container_of(item, struct zswap_entry, lru); > bool *encountered_page_in_swapcache = (bool *)arg; > - struct zswap_tree *tree; > - pgoff_t swpoffset; > + swp_entry_t swpentry; > enum lru_status ret = LRU_REMOVED_RETRY; > int writeback_result; > > + /* > + * Rotate the entry to the tail before unlocking the LRU, > + * so that in case of an invalidation race concurrent > + * reclaimers don't waste their time on it. > + * > + * If writeback succeeds, or failure is due to the entry > + * being invalidated by the swap subsystem, the invalidation > + * will unlink and free it. > + * > + * Temporary failures, where the same entry should be tried > + * again immediately, almost never happen for this shrinker. > + * We don't do any trylocking; -ENOMEM comes closest, > + * but that's extremely rare and doesn't happen spuriously > + * either. Don't bother distinguishing this case. > + * > + * But since they do exist in theory, the entry cannot just > + * be unlinked, or we could leak it. Hence, rotate. The entry cannot be unlinked because we cannot get a ref on it without holding the tree lock, and we cannot deref the tree before we acquire a swap cache ref in zswap_writeback_entry() -- or if zswap_writeback_entry() fails. This should be called out explicitly somewhere. Perhaps we can describe this whole deref dance with added docs to shrink_memcg_cb(). We could also use a comment in zswap_writeback_entry() (or above it) to state that we only deref the tree *after* we get the swapcache ref.
