[PATCH] ksm: Fix potential NULL pointer dereference

[PATCH] ksm: Fix potential NULL pointer dereference

[Fushuai Wang]

The stable_tree_search() function may return an error pointer
(NULL or ERR_PTR(...)). The current code does not check for
these cases before dereferencing the returned value.

Gaosheng once added the necessary check in commit 98c3ca0015b8
("ksm: convert cmp_and_merge_page() to use a folio"), but it seems
the check was lost during the process of being merged into the
mainline.

Fixes: 98c3ca0015b8 ("ksm: convert cmp_and_merge_page() to use a folio")
Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
---
 mm/ksm.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 160787bb121c..7aa9b2829a7b 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2272,7 +2272,8 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
 
 	/* Start by searching for the folio in the stable tree */
 	kfolio = stable_tree_search(page);
-	if (&kfolio->page == page && rmap_item->head == stable_node) {
+	if (!IS_ERR_OR_NULL(kfolio) && &kfolio->page == page &&
+	    rmap_item->head == stable_node) {
 		folio_put(kfolio);
 		return;
 	}
-- 
2.36.1

Re: [PATCH] ksm: Fix potential NULL pointer dereference

[David Hildenbrand]

On 25.09.25 05:13, Fushuai Wang wrote:
> The stable_tree_search() function may return an error pointer
> (NULL or ERR_PTR(...)). The current code does not check for
> these cases before dereferencing the returned value.

We discussed this in [1] where Willy clarified that there is no 
de-referencing happening [2].

So no, this is not required.

Consider "&kfolio->page" currently being a "(struct page *) kfolio", 
which is just a pointer cast and no de-referencing.

[1] 
https://lore.kernel.org/linux-mm/20241024032300.2501949-1-cuigaosheng1@huawei.com/
[2] https://lore.kernel.org/linux-mm/Zzdrnod2zcTeI-Nt@casper.infradead.org/

-- 
Cheers

David / dhildenb