linux-kernel - [PATCH v3 1/6] mm, gup: Remove broken VM_BUG_ON

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]

Date:   Mon, 22 May 2017 14:35:59 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     akpm@...ux-foundation.org
Cc:     Will Deacon <will.deacon@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        catalin.marinas@....com, n-horiguchi@...jp.nec.com,
        kirill.shutemov@...ux.intel.com, mike.kravetz@...cle.com,
        steve.capper@....com, mark.rutland@....com,
        hillf.zj@...baba-inc.com, linux-arch@...r.kernel.org,
        aneesh.kumar@...ux.vnet.ibm.com,
        Punit Agrawal <punit.agrawal@....com>
Subject: [PATCH v3 1/6] mm, gup: Remove broken VM_BUG_ON_PAGE compound check for hugepages

From: Will Deacon <will.deacon@....com>

When operating on hugepages with DEBUG_VM enabled, the GUP code checks the
compound head for each tail page prior to calling page_cache_add_speculative.
This is broken, because on the fast-GUP path (where we don't hold any page
table locks) we can be racing with a concurrent invocation of
split_huge_page_to_list.

split_huge_page_to_list deals with this race by using page_ref_freeze to
freeze the page and force concurrent GUPs to fail whilst the component
pages are modified. This modification includes clearing the compound_head
field for the tail pages, so checking this prior to a successful call
to page_cache_add_speculative can lead to false positives: In fact,
page_cache_add_speculative *already* has this check once the page refcount
has been successfully updated, so we can simply remove the broken calls
to VM_BUG_ON_PAGE.

Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
Signed-off-by: Will Deacon <will.deacon@....com>
Acked-by: Steve Capper <steve.capper@....com>
Signed-off-by: Punit Agrawal <punit.agrawal@....com>
---
 mm/gup.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index d9e6fddcc51f..ccf8cb38234f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1361,7 +1361,6 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	head = pmd_page(orig);
 	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	do {
-		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;
 		page++;
@@ -1400,7 +1399,6 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 	head = pud_page(orig);
 	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 	do {
-		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;
 		page++;
@@ -1438,7 +1436,6 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
 	head = pgd_page(orig);
 	page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
 	do {
-		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;
 		page++;
-- 
2.11.0