linux-kernel - [PATCH/RFC] mm: do not drop unused pages when userfaultd is running

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20180628123916.96106-1-borntraeger@de.ibm.com>
Date:   Thu, 28 Jun 2018 14:39:16 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     linux-mm@...ck.org, linux-s390@...r.kernel.org
Cc:     kvm@...r.kernel.org, Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: [PATCH/RFC] mm: do not drop unused pages when userfaultd is running

KVM guests on s390 can notify the host of unused pages. This can result
in pte_unused callbacks to be true for KVM guest memory.

If a page is unused (checked with pte_unused) we might drop this page
instead of paging it. This can have side-effects on userfaultd, when the
page in question was already migrated:

The next access of that page will trigger a fault and a user fault
instead of faulting in a new and empty zero page. As QEMU does not
expect a userfault on an already migrated page this migration will fail.

The most straightforward solution is to ignore the pte_unused hint if a
userfault context is active for this VMA.

Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: stable@...r.kernel.org
Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
---
 mm/rmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 6db729dc4c50..3f3a72aa99f2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1481,7 +1481,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 				set_pte_at(mm, address, pvmw.pte, pteval);
 			}

-		} else if (pte_unused(pteval)) {
+		} else if (pte_unused(pteval) && !vma->vm_userfaultfd_ctx.ctx) {
 			/*
 			 * The guest indicated that the page content is of no
 			 * interest anymore. Simply discard the pte, vmscan
-- 
2.17.0