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Message-ID: <CA+EESO7Gck6YpjPTMSzDGcmRXjci=zG3i8F+LTt=u2Krbp_cRg@mail.gmail.com>
Date: Fri, 30 May 2025 23:10:02 -0700
From: Lokesh Gidra <lokeshgidra@...gle.com>
To: Barry Song <21cnbao@...il.com>
Cc: Kairui Song <kasong@...cent.com>, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
Suren Baghdasaryan <surenb@...gle.com>, Andrea Arcangeli <aarcange@...hat.com>,
David Hildenbrand <david@...hat.com>, stable@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: userfaultfd: fix race of userfaultfd_move and swap cache
On Fri, May 30, 2025 at 9:42 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Sat, May 31, 2025 at 4:04 PM Barry Song <21cnbao@...il.com> wrote:
> >
> > On Sat, May 31, 2025 at 8:17 AM Kairui Song <ryncsn@...il.com> wrote:
> > >
> > > From: Kairui Song <kasong@...cent.com>
> > >
> > > On seeing a swap entry PTE, userfaultfd_move does a lockless swap cache
> > > lookup, and try to move the found folio to the faulting vma when.
> > > Currently, it relies on the PTE value check to ensure the moved folio
> > > still belongs to the src swap entry, which turns out is not reliable.
> > >
> > > While working and reviewing the swap table series with Barry, following
> > > existing race is observed and reproduced [1]:
> > >
> > > ( move_pages_pte is moving src_pte to dst_pte, where src_pte is a
> > > swap entry PTE holding swap entry S1, and S1 isn't in the swap cache.)
> > >
> > > CPU1 CPU2
> > > userfaultfd_move
> > > move_pages_pte()
> > > entry = pte_to_swp_entry(orig_src_pte);
> > > // Here it got entry = S1
> > > ... < Somehow interrupted> ...
> > > <swapin src_pte, alloc and use folio A>
> > > // folio A is just a new allocated folio
> > > // and get installed into src_pte
> > > <frees swap entry S1>
> > > // src_pte now points to folio A, S1
> > > // has swap count == 0, it can be freed
> > > // by folio_swap_swap or swap
> > > // allocator's reclaim.
> > > <try to swap out another folio B>
> > > // folio B is a folio in another VMA.
> > > <put folio B to swap cache using S1 >
> > > // S1 is freed, folio B could use it
> > > // for swap out with no problem.
> > > ...
> > > folio = filemap_get_folio(S1)
> > > // Got folio B here !!!
> > > ... < Somehow interrupted again> ...
> > > <swapin folio B and free S1>
> > > // Now S1 is free to be used again.
> > > <swapout src_pte & folio A using S1>
> > > // Now src_pte is a swap entry pte
> > > // holding S1 again.
> > > folio_trylock(folio)
> > > move_swap_pte
> > > double_pt_lock
> > > is_pte_pages_stable
> > > // Check passed because src_pte == S1
> > > folio_move_anon_rmap(...)
> > > // Moved invalid folio B here !!!
> > >
> > > The race window is very short and requires multiple collisions of
> > > multiple rare events, so it's very unlikely to happen, but with a
> > > deliberately constructed reproducer and increased time window, it can be
> > > reproduced [1].
> > >
> > > It's also possible that folio (A) is swapped in, and swapped out again
> > > after the filemap_get_folio lookup, in such case folio (A) may stay in
> > > swap cache so it needs to be moved too. In this case we should also try
> > > again so kernel won't miss a folio move.
> > >
> > > Fix this by checking if the folio is the valid swap cache folio after
> > > acquiring the folio lock, and checking the swap cache again after
> > > acquiring the src_pte lock.
> > >
> > > SWP_SYNCRHONIZE_IO path does make the problem more complex, but so far
> > > we don't need to worry about that since folios only might get exposed to
> > > swap cache in the swap out path, and it's covered in this patch too by
> > > checking the swap cache again after acquiring src_pte lock.
> > >
> > > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> > > Closes: https://lore.kernel.org/linux-mm/CAMgjq7B1K=6OOrK2OUZ0-tqCzi+EJt+2_K97TPGoSt=9+JwP7Q@mail.gmail.com/ [1]
> > > Signed-off-by: Kairui Song <kasong@...cent.com>
> > > ---
> > > mm/userfaultfd.c | 26 ++++++++++++++++++++++++++
> > > 1 file changed, 26 insertions(+)
> > >
> > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > > index bc473ad21202..a1564d205dfb 100644
> > > --- a/mm/userfaultfd.c
> > > +++ b/mm/userfaultfd.c
> > > @@ -15,6 +15,7 @@
> > > #include <linux/mmu_notifier.h>
> > > #include <linux/hugetlb.h>
> > > #include <linux/shmem_fs.h>
> > > +#include <linux/delay.h>
> > > #include <asm/tlbflush.h>
> > > #include <asm/tlb.h>
> > > #include "internal.h"
> > > @@ -1086,6 +1087,8 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
> > > spinlock_t *dst_ptl, spinlock_t *src_ptl,
> > > struct folio *src_folio)
> > > {
> > > + swp_entry_t entry;
> > > +
> > > double_pt_lock(dst_ptl, src_ptl);
> > >
> > > if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> > > @@ -1102,6 +1105,19 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
> > > if (src_folio) {
> > > folio_move_anon_rmap(src_folio, dst_vma);
> > > src_folio->index = linear_page_index(dst_vma, dst_addr);
> > > + } else {
> > > + /*
> > > + * Check again after acquiring the src_pte lock. Or we might
> > > + * miss a new loaded swap cache folio.
> > > + */
> > > + entry = pte_to_swp_entry(orig_src_pte);
> > > + src_folio = filemap_get_folio(swap_address_space(entry),
> > > + swap_cache_index(entry));
> > > + if (!IS_ERR_OR_NULL(src_folio)) {
> > > + double_pt_unlock(dst_ptl, src_ptl);
> > > + folio_put(src_folio);
> > > + return -EAGAIN;
> > > + }
> > > }
> >
> > step 1: src pte points to a swap entry without swapcache
> > step 2: we call move_swap_pte()
> > step 3: someone swap-in src_pte by swap_readhead() and make src_pte's swap entry
> > have swapcache again - for non-sync/non-zRAM swap device;
> > step 4: move_swap_pte() gets ptl, move src_pte to dst_pte and *clear* src_pte;
> > step 5: do_swap_page() for src_pte holds the ptl and found pte has
> > been cleared in
> > step 4; pte_same() returns false;
> > step 6: do_swap_page() won't map src_pte to the new swapcache got from step 3;
> > if the swapcache folio is dropped, it seems everything is fine.
> >
> > So the real issue is that do_swap_page() doesn’t drop the new swapcache
> > even when pte_same() returns false? That means the dst_pte swap-in
> > can still hit the swap cache entry brought in by the src_pte's swap-in?
>
> It seems also possible for the sync zRAM device.
>
> step 1: src pte points to a swap entry S without swapcache
> step 2: we call move_swap_pte()
> step 3: someone swap-in src_pte by sync path, no swapcache; swap slot
> S is freed.
> -- for zRAM;
> step 4: someone swap-out src_pte, get the exactly same swap slot S as step 1,
> adds folio to swapcache due to swapout;
> step 5: move_swap_pte() gets ptl and finds page tables are stable
> since swap-out
> happens to have the same swap slot as step1;
> step 6: we clear src_pte, move src_pte to dst_pte; but miss to move the folio.
>
> Yep, we really need to re-check pte for swapcache after holding PTL.
>
Any idea what is the overhead of filemap_get_folio()? In particular,
when no folio exists for the given entry, how costly is it?
Given how rare it is, unless filemap_get_folio() is cheap for 'no
folio' case, if there is no way to avoid calling it after holding PTL,
then we should do it only once at that point. If a folio is returned,
then like in the pte_present() case, we attempt folio_trylock() with
PTL held, otherwise do the retry dance.
> >
> > >
> > > orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
> > > @@ -1409,6 +1425,16 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> > > folio_lock(src_folio);
> > > goto retry;
> > > }
> > > + /*
> > > + * Check if the folio still belongs to the target swap entry after
> > > + * acquiring the lock. Folio can be freed in the swap cache while
> > > + * not locked.
> > > + */
> > > + if (unlikely(!folio_test_swapcache(folio) ||
> > > + entry.val != folio->swap.val)) {
> > > + err = -EAGAIN;
> > > + goto out;
> > > + }
> > > }
> > > err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
> > > orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
> > > --
> > > 2.49.0
> > >
> >
>
> Thanks
> Barry
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