[PATCH v2] mm/ksm: validate KSM rmap items before hwpoison kill

[Longlong Xia]

From: Longlong Xia <xialonglong@xxxxxxxxxx>

collect_procs_ksm() walks the stable-node rmap list and queues an
early kill for every task whose mm appears on the anon_vma chain.

That rmap item can be stale by the time memory failure handles the
poisoned KSM page. A VMA may have been split, unmapped or remapped
after the rmap item was recorded, so matching only vma->vm_mm can send
SIGBUS with an address that no longer maps the poisoned page.

Factor page_mapped_in_vma_at_address() out of page_mapped_in_vma() so
callers that already know the virtual address can validate it directly.
This avoids deriving the address from page_pgoff(), which is invalid
for KSM pages. Use the address saved in the KSM rmap item to check that
it still belongs to the VMA and that page_vma_mapped_walk() still finds
the poisoned page before adding the task to the kill list.

Fixes: 4248d0083ec5 ("mm: ksm: support hwpoison for ksm page")
Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Signed-off-by: Longlong Xia <xialonglong@xxxxxxxxxx>
---
 mm/internal.h        |  2 ++
 mm/ksm.c             | 10 +++++++---
 mm/page_vma_mapped.c | 36 +++++++++++++++++++++++-------------
 3 files changed, 32 insertions(+), 16 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 181e79f1d6a2..4c9e601b2d95 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1424,6 +1424,8 @@ void add_to_kill_ksm(struct task_struct *tsk, const struct page *p,
 		     unsigned long ksm_addr);
 unsigned long page_mapped_in_vma(const struct page *page,
 		struct vm_area_struct *vma);
+unsigned long page_mapped_in_vma_at_address(const struct page *page,
+		struct vm_area_struct *vma, unsigned long addr);
 
 #else
 static inline int unmap_poisoned_folio(struct folio *folio, unsigned long pfn, bool must_kill)
diff --git a/mm/ksm.c b/mm/ksm.c
index 7d5b76478f0b..5104e442fcb2 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -3237,13 +3237,13 @@ void collect_procs_ksm(const struct folio *folio, const struct page *page,
 	if (!stable_node)
 		return;
 	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
+		const unsigned long addr = rmap_item->address & PAGE_MASK;
 		struct anon_vma *av = rmap_item->anon_vma;
 
 		anon_vma_lock_read(av);
 		rcu_read_lock();
 		for_each_process(tsk) {
 			struct anon_vma_chain *vmac;
-			unsigned long addr;
 			struct task_struct *t =
 				task_early_kill(tsk, force_early);
 			if (!t)
@@ -3253,9 +3253,13 @@ void collect_procs_ksm(const struct folio *folio, const struct page *page,
 			{
 				vma = vmac->vma;
 				if (vma->vm_mm == t->mm) {
-					addr = rmap_item->address & PAGE_MASK;
+					const unsigned long mapped_addr =
+						page_mapped_in_vma_at_address(page, vma, addr);
+
+					if (mapped_addr == -EFAULT)
+						continue;
 					add_to_kill_ksm(t, page, vma, to_kill,
-							addr);
+							mapped_addr);
 				}
 			}
 		}
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index bac2eb5de63d..f7c5dc9248bc 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -336,6 +336,26 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 }
 
 #ifdef CONFIG_MEMORY_FAILURE
+unsigned long page_mapped_in_vma_at_address(const struct page *page,
+		struct vm_area_struct *vma, unsigned long addr)
+{
+	struct page_vma_mapped_walk pvmw = {
+		.pfn = page_to_pfn(page),
+		.nr_pages = 1,
+		.vma = vma,
+		.address = addr,
+		.flags = PVMW_SYNC,
+	};
+
+	if (addr < vma->vm_start || addr >= vma->vm_end)
+		return -EFAULT;
+	if (!page_vma_mapped_walk(&pvmw))
+		return -EFAULT;
+	page_vma_mapped_walk_done(&pvmw);
+
+	return pvmw.address;
+}
+
 /**
  * page_mapped_in_vma - check whether a page is really mapped in a VMA
  * @page: the page to test
@@ -350,20 +370,10 @@ unsigned long page_mapped_in_vma(const struct page *page,
 		struct vm_area_struct *vma)
 {
 	const struct folio *folio = page_folio(page);
-	struct page_vma_mapped_walk pvmw = {
-		.pfn = page_to_pfn(page),
-		.nr_pages = 1,
-		.vma = vma,
-		.flags = PVMW_SYNC,
-	};
+	const unsigned long addr = vma_address(vma, page_pgoff(folio, page), 1);
 
-	pvmw.address = vma_address(vma, page_pgoff(folio, page), 1);
-	if (pvmw.address == -EFAULT)
-		goto out;
-	if (!page_vma_mapped_walk(&pvmw))
+	if (addr == -EFAULT)
 		return -EFAULT;
-	page_vma_mapped_walk_done(&pvmw);
-out:
-	return pvmw.address;
+	return page_mapped_in_vma_at_address(page, vma, addr);
 }
 #endif
-- 
2.43.0