linux-kernel - [PATCH 6/6] mm/hugetlb: make detecting shared pte more reliable

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

Message-ID: <20220816130553.31406-7-linmiaohe@huawei.com>
Date:   Tue, 16 Aug 2022 21:05:53 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     <akpm@...ux-foundation.org>, <mike.kravetz@...cle.com>,
        <songmuchun@...edance.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <linmiaohe@...wei.com>
Subject: [PATCH 6/6] mm/hugetlb: make detecting shared pte more reliable

If the pagetables are shared, we shouldn't copy or take references. Since
src could have unshared and dst shares with another vma, huge_pte_none()
is thus used to determine whether dst_pte is shared. But this check isn't
reliable. A shared pte could have pte none in pagetable in fact. The page
count of ptep page should be checked here in order to reliably determine
whether pte is shared.

Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
---
 mm/hugetlb.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e1356ad57087..25db6d07479e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4795,15 +4795,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 
 		/*
 		 * If the pagetables are shared don't copy or take references.
-		 * dst_pte == src_pte is the common case of src/dest sharing.
 		 *
+		 * dst_pte == src_pte is the common case of src/dest sharing.
 		 * However, src could have 'unshared' and dst shares with
-		 * another vma.  If dst_pte !none, this implies sharing.
-		 * Check here before taking page table lock, and once again
-		 * after taking the lock below.
+		 * another vma. So page_count of ptep page is checked instead
+		 * to reliably determine whether pte is shared.
 		 */
-		dst_entry = huge_ptep_get(dst_pte);
-		if ((dst_pte == src_pte) || !huge_pte_none(dst_entry)) {
+		if (page_count(virt_to_page(dst_pte)) > 1) {
 			addr |= last_addr_mask;
 			continue;
 		}
@@ -4814,11 +4812,9 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 		entry = huge_ptep_get(src_pte);
 		dst_entry = huge_ptep_get(dst_pte);
 again:
-		if (huge_pte_none(entry) || !huge_pte_none(dst_entry)) {
+		if (huge_pte_none(entry)) {
 			/*
-			 * Skip if src entry none.  Also, skip in the
-			 * unlikely case dst entry !none as this implies
-			 * sharing with another vma.
+			 * Skip if src entry none.
 			 */
 			;
 		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry))) {
-- 
2.23.0