linux-kernel - [PATCH 4.9] mm: page_alloc: validate buddy page before using

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Date:   Fri, 17 Jun 2022 00:19:28 +0800
From:   Xianting Tian <xianting.tian@...ux.alibaba.com>
To:     akpm@...ux-foundation.org, ziy@...dia.com,
        gregkh@...uxfoundation.org, stable@...r.kernel.org,
        guoren@...nel.org
Cc:     huanyi.xj@...baba-inc.com, guohanjun@...wei.com,
        zjb194813@...baba-inc.com, tianhu.hh@...baba-inc.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Xianting Tian <xianting.tian@...ux.alibaba.com>
Subject: [PATCH 4.9] mm: page_alloc: validate buddy page before using

Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.")
fixes a bug in 1dd214b8f21c and there is a similar bug in d9dddbf55667 that
can be fixed in a similar way too.

In addition, for RISC-V arch the first 2MB RAM could be reserved for opensbi,
so it would have pfn_base=512 and mem_map began with 512th PFN when
CONFIG_FLATMEM=y.
But __find_buddy_pfn algorithm thinks the start pfn 0, it could get 0 pfn or
less than the pfn_base value. We need page_is_buddy() to verify the buddy to
prevent accessing an invalid buddy.

Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")
Cc: stable@...r.kernel.org
Reported-by: zjb194813@...baba-inc.com
Reported-by: tianhu.hh@...baba-inc.com
Signed-off-by: Xianting Tian <xianting.tian@...ux.alibaba.com>
---
 mm/page_alloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a6e682569e5b..1c423faa4b62 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -864,6 +864,9 @@ static inline void __free_one_page(struct page *page,
 
 			buddy_idx = __find_buddy_index(page_idx, order);
 			buddy = page + (buddy_idx - page_idx);
+
+			if (!page_is_buddy(page, buddy, order))
+				goto done_merging;
 			buddy_mt = get_pageblock_migratetype(buddy);
 
 			if (migratetype != buddy_mt
-- 
2.17.1