On Tue, Feb 6, 2024 at 7:15 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Mon, Feb 05, 2024 at 03:24:42PM -0800, Nhat Pham wrote: > > Move the zswap LRU protection range update above the swap_read_folio() > > call, and only when a new page is allocated. This is the case where > > (z)swapin could happen, which is a signal that the zswap shrinker should > > be more conservative with its reclaiming action. > > > > It also prevents a race, in which folio migration can clear the > > memcg_data of the now unlocked folio, resulting in a warning in the > > inlined folio_lruvec() call. > > The warning is the most probable outcome, and it will cause the update > to go against the root cgroup which is safe at least. > > But AFAICS there is no ordering guarantee to rule out a UAF if the > lookup succeeds but the memcg and lruvec get freed before the update. Ah nice. I didn't consider that. IIUC, having the folio locked should prevent this too. Based on the documentation: * For a non-kmem folio any of the following ensures folio and memcg binding * stability: * * - the folio lock I'll rework the commit log to include this, and make this more prominent :) > > I think that part should be more prominent in the changelog. It's more > important than the first paragraph. Consider somebody scrolling > through the git log and trying to decide whether to backport or not; > it's helpful to describe the bug and its impact first thing, then put > the explanation of the fix after. > > > 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> > > Would it make sense to add > > VM_WARN_ON_ONCE(!folio_test_locked(folio)); > > to zswap_folio_swapin() as well?
