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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, liqiong <liqiong@nfschina.com>
Subject: [PATCH 4.19 21/57] mm: fix dereference a null pointer in migrate[_huge]_page_move_mapping()
Date:   Mon, 21 Mar 2022 14:52:02 +0100
Message-Id: <20220321133222.602567121@linuxfoundation.org>
In-Reply-To: <20220321133221.984120927@linuxfoundation.org>
References: <20220321133221.984120927@linuxfoundation.org>
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From: liqiong <liqiong@nfschina.com>

Upstream doesn't use radix tree any more in migrate.c, no need this patch.

The two functions look up a slot and dereference the pointer,
If the pointer is null, the kernel would crash and dump.

The 'numad' service calls 'migrate_pages' periodically. If some slots
being replaced (Cache Eviction), the radix_tree_lookup_slot() returns
a null pointer that causes kernel crash.

"numad":  crash> bt
[exception RIP: migrate_page_move_mapping+337]

Introduce pointer checking to avoid dereference a null pointer.

Cc: <stable@vger.kernel.org> # linux-4.19.y
Signed-off-by: liqiong <liqiong@nfschina.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 mm/migrate.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -472,6 +472,10 @@ int migrate_page_move_mapping(struct add
 
 	pslot = radix_tree_lookup_slot(&mapping->i_pages,
  					page_index(page));
+	if (pslot == NULL) {
+		xa_unlock_irq(&mapping->i_pages);
+		return -EAGAIN;
+	}
 
 	expected_count += hpage_nr_pages(page) + page_has_private(page);
 	if (page_count(page) != expected_count ||
@@ -590,6 +594,10 @@ int migrate_huge_page_move_mapping(struc
 	xa_lock_irq(&mapping->i_pages);
 
 	pslot = radix_tree_lookup_slot(&mapping->i_pages, page_index(page));
+	if (pslot == NULL) {
+		xa_unlock_irq(&mapping->i_pages);
+		return -EAGAIN;
+	}
 
 	expected_count = 2 + page_has_private(page);
 	if (page_count(page) != expected_count ||