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.

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@mail.gmail.com/

Link: https://lkml.kernel.org/r/20240126-zswap-writeback-race-v2-2-b10479847099@bytedance.comSigned-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Chris Li <chriscli@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>