linux-hardening - [PATCH RFC 4/9] mm: Explicitly zero page table lock ptr

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Date:   Tue,  4 May 2021 17:30:27 -0700
From:   Rick Edgecombe <rick.p.edgecombe@...el.com>
To:     dave.hansen@...el.com, luto@...nel.org, peterz@...radead.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        linux-hardening@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Cc:     ira.weiny@...el.com, rppt@...nel.org, dan.j.williams@...el.com,
        linux-kernel@...r.kernel.org,
        Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: [PATCH RFC 4/9] mm: Explicitly zero page table lock ptr

In ptlock_init() there is a VM_BUG_ON_PAGE() check on the page table lock
pointer. Explicitly zero the lock in ptlock_free() so a page table lock
can be re-initialized without triggering the BUG_ON().

It appears this doesn't normally trigger because the private field
shares the same space in struct page as ptl and page tables always
return to the buddy allocator before being re-initialized as new page
tables. When the page returns to the buddy allocator, private gets
used to store the page order, so it inadvertently clears ptl as well.
In future patches, pages will get re-initialized as page tables without
returning to the buddy allocator so this is needed.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
---
 mm/memory.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/memory.c b/mm/memory.c
index 5efa07fb6cdc..130f8c1e380a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5225,5 +5225,6 @@ bool ptlock_alloc(struct page *page)
 void ptlock_free(struct page *page)
 {
 	kmem_cache_free(page_ptl_cachep, page->ptl);
+	page->ptl = 0;
 }
 #endif
-- 
2.30.2