From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1AD0A86337; Wed, 4 Jun 2025 19:27:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749065257; cv=none; b=PNenTRQoK9eZWZsTvAkVbblarLD/muIlSmhEdqS5fOMlLoX7tSGWWfEZ4lG4spRvdiGTggBAbZaFLBA+oqfuWUo6QMTd8vW7bR147rS+ZAe/j6rRew0BbNxsxlCzTlW026Uf4DLvLm0Mg/CI/Ez/m1+oHcUHDFpVhAawn5Wp2jk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749065257; c=relaxed/simple; bh=nMgqY2jkFUb9NnbRKeB474icfBmRN1AyIJSk4BLPkJ0=; h=Date:To:From:Subject:Message-Id; b=bW0Cr37Ik3DIMUlzOSXF0MGpyKNYh9vgpi/ULY8ub7yYKlobCx1RZW4vDtoN2WVU4n0SoHGrLV2Zg2vDPPgzvuezT/WjUxPVeV7ic1jsG3F1tCgTSiM/K2gMpRukMshLIWHQZ8hVc2vdoS9IJQOkgKowwToJVTSLN1xNCjUVuYU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=oFqqzIS+; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="oFqqzIS+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 58647C4CEE4; Wed, 4 Jun 2025 19:27:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1749065255; bh=nMgqY2jkFUb9NnbRKeB474icfBmRN1AyIJSk4BLPkJ0=; h=Date:To:From:Subject:From; b=oFqqzIS+NYZNjXBRqhgg9cZRxJ1BimblPXwzXNQgySTpFXyQA047f2WLsp5LGTX5k 0leNGjh5vu6Wjrxif7QGQKeoQtLpsoKcPxITsU8t18cSkiNARE3ICuZ6+i6KpbbutR OZ2zUceXM3OVZ+4HX4s/dIkOsY4VpapqJUJKwc8g= Date: Wed, 04 Jun 2025 12:27:34 -0700 To: mm-commits@vger.kernel.org,surenb@google.com,stable@vger.kernel.org,peterx@redhat.com,lokeshgidra@google.com,david@redhat.com,aarcange@redhat.com,21cnbao@gmail.com,kasong@tencent.com,akpm@linux-foundation.org From: Andrew Morton Subject: + mm-userfaultfd-fix-race-of-userfaultfd_move-and-swap-cache.patch added to mm-hotfixes-unstable branch Message-Id: <20250604192735.58647C4CEE4@smtp.kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: mm: userfaultfd: fix race of userfaultfd_move and swap cache has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-userfaultfd-fix-race-of-userfaultfd_move-and-swap-cache.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-userfaultfd-fix-race-of-userfaultfd_move-and-swap-cache.patch This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: Kairui Song Subject: mm: userfaultfd: fix race of userfaultfd_move and swap cache Date: Wed, 4 Jun 2025 23:10:38 +0800 On seeing a swap entry PTE, userfaultfd_move does a lockless swap cache lookup, and tries to move the found folio to the faulting vma. Currently, it relies on checking the PTE value to ensure that the moved folio still belongs to the src swap entry and that no new folio has been added to the swap cache, which turns out to be unreliable. While working and reviewing the swap table series with Barry, following existing races are observed and reproduced [1]: In the example below, move_pages_pte is moving src_pte to dst_pte, where src_pte is a swap entry PTE holding swap entry S1, and S1 is not in the swap cache: CPU1 CPU2 userfaultfd_move move_pages_pte() entry = pte_to_swp_entry(orig_src_pte); // Here it got entry = S1 ... < interrupted> ... // folio A is a new allocated folio // and get installed into src_pte // 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. // folio B is a folio in another VMA. // S1 is freed, folio B can use it // for swap out with no problem. ... folio = filemap_get_folio(S1) // Got folio B here !!! ... < interrupted again> ... // Now S1 is free to be used again. // 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 easily. This can be fixed by checking if the folio returned by filemap is the valid swap cache folio after acquiring the folio lock. Another similar race is possible: filemap_get_folio may return NULL, but folio (A) could be swapped in and then swapped out again using the same swap entry after the lookup. In such a case, folio (A) may remain in the swap cache, so it must be moved too: CPU1 CPU2 userfaultfd_move move_pages_pte() entry = pte_to_swp_entry(orig_src_pte); // Here it got entry = S1, and S1 is not in swap cache folio = filemap_get_folio(S1) // Got NULL ... < interrupted again> ... move_swap_pte double_pt_lock is_pte_pages_stable // Check passed because src_pte == S1 folio_move_anon_rmap(...) // folio A is ignored !!! Fix this by checking the swap cache again after acquiring the src_pte lock. And to avoid the filemap overhead, we check swap_map directly [2]. The SWP_SYNCHRONOUS_IO path does make the problem more complex, but so far we don't need to worry about that, since folios can only be exposed to the swap cache in the swap out path, and this is covered in this patch by checking the swap cache again after acquiring the src_pte lock. Testing with a simple C program that allocates and moves several GB of memory did not show any observable performance change. Link: https://lkml.kernel.org/r/20250604151038.21968-1-ryncsn@gmail.com Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") Signed-off-by: Kairui Song Closes: https://lore.kernel.org/linux-mm/CAMgjq7B1K=6OOrK2OUZ0-tqCzi+EJt+2_K97TPGoSt=9+JwP7Q@mail.gmail.com/ [1] Link: https://lore.kernel.org/all/CAGsJ_4yJhJBo16XhiC-nUzSheyX-V3-nFE+tAi=8Y560K8eT=A@mail.gmail.com/ [2] Reviewed-by: Lokesh Gidra Acked-by: Peter Xu Reviewed-by: Suren Baghdasaryan Cc: Andrea Arcangeli Cc: Barry Song <21cnbao@gmail.com> Cc: David Hildenbrand Cc: Kairui Song Cc: Signed-off-by: Andrew Morton --- mm/userfaultfd.c | 33 +++++++++++++++++++++++++++++++-- 1 file changed, 31 insertions(+), 2 deletions(-) --- a/mm/userfaultfd.c~mm-userfaultfd-fix-race-of-userfaultfd_move-and-swap-cache +++ a/mm/userfaultfd.c @@ -1084,8 +1084,18 @@ static int move_swap_pte(struct mm_struc pte_t orig_dst_pte, pte_t orig_src_pte, pmd_t *dst_pmd, pmd_t dst_pmdval, spinlock_t *dst_ptl, spinlock_t *src_ptl, - struct folio *src_folio) + struct folio *src_folio, + struct swap_info_struct *si, swp_entry_t entry) { + /* + * 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 (src_folio && unlikely(!folio_test_swapcache(src_folio) || + entry.val != src_folio->swap.val)) + return -EAGAIN; + double_pt_lock(dst_ptl, src_ptl); if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte, @@ -1102,6 +1112,25 @@ static int move_swap_pte(struct mm_struc if (src_folio) { folio_move_anon_rmap(src_folio, dst_vma); src_folio->index = linear_page_index(dst_vma, dst_addr); + } else { + /* + * Check if the swap entry is cached after acquiring the src_pte + * lock. Otherwise, we might miss a newly loaded swap cache folio. + * + * Check swap_map directly to minimize overhead, READ_ONCE is sufficient. + * We are trying to catch newly added swap cache, the only possible case is + * when a folio is swapped in and out again staying in swap cache, using the + * same entry before the PTE check above. The PTL is acquired and released + * twice, each time after updating the swap_map's flag. So holding + * the PTL here ensures we see the updated value. False positive is possible, + * e.g. SWP_SYNCHRONOUS_IO swapin may set the flag without touching the + * cache, or during the tiny synchronization window between swap cache and + * swap_map, but it will be gone very quickly, worst result is retry jitters. + */ + if (READ_ONCE(si->swap_map[swp_offset(entry)]) & SWAP_HAS_CACHE) { + double_pt_unlock(dst_ptl, src_ptl); + return -EAGAIN; + } } orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte); @@ -1412,7 +1441,7 @@ retry: } 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, - dst_ptl, src_ptl, src_folio); + dst_ptl, src_ptl, src_folio, si, entry); } out: _ Patches currently in -mm which might be from kasong@tencent.com are mm-userfaultfd-fix-race-of-userfaultfd_move-and-swap-cache.patch