[PATCH] fork, vmalloc: KASAN-poison backing pages of vmapped stacks

[Jann Horn <jannh@google.com>]

KASAN (except in HW_TAGS mode) tracks memory state based on virtual
addresses. The mappings of kernel stack pages in the linear mapping are
currently marked as fully accessible.
Since stack corruption issues can cause some very gnarly errors, let's be
extra careful and tell KASAN to forbid accesses to stack memory through the
linear mapping.

Signed-off-by: Jann Horn <jannh@google.com>
---
I wrote this after seeing
https://lore.kernel.org/all/Y8W5rjKdZ9erIF14@casper.infradead.org/
and wondering about possible ways that this kind of stack corruption
could be sneaking past KASAN.
That's proooobably not the explanation, but still...

 include/linux/vmalloc.h |  6 ++++++
 kernel/fork.c           | 10 ++++++++++
 mm/vmalloc.c            | 24 ++++++++++++++++++++++++
 3 files changed, 40 insertions(+)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 096d48aa3437..bfb50178e5e3 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -297,4 +297,10 @@ bool vmalloc_dump_obj(void *object);
 static inline bool vmalloc_dump_obj(void *object) { return false; }
 #endif
 
+#if defined(CONFIG_MMU) && (defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS))
+void vmalloc_poison_backing_pages(const void *addr);
+#else
+static inline void vmalloc_poison_backing_pages(const void *addr) {}
+#endif
+
 #endif /* _LINUX_VMALLOC_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index 9f7fe3541897..5c8c103a3597 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -321,6 +321,16 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 		vfree(stack);
 		return -ENOMEM;
 	}
+
+	/*
+	 * A virtually-allocated stack's memory should only be accessed through
+	 * the vmalloc area, not through the linear mapping.
+	 * Inform KASAN that all accesses through the linear mapping should be
+	 * reported (instead of permitting all accesses through the linear
+	 * mapping).
+	 */
+	vmalloc_poison_backing_pages(stack);
+
 	/*
 	 * We can't call find_vm_area() in interrupt context, and
 	 * free_thread_stack() can be called in interrupt context,
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ca71de7c9d77..10c79c53cf5c 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4042,6 +4042,30 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
 }
 #endif	/* CONFIG_SMP */
 
+#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
+/*
+ * Poison the KASAN shadow for the linear mapping of the pages used as stack
+ * memory.
+ * NOTE: This makes no sense in HW_TAGS mode because HW_TAGS marks physical
+ * memory, not virtual memory.
+ */
+void vmalloc_poison_backing_pages(const void *addr)
+{
+	struct vm_struct *area;
+	int i;
+
+	if (WARN(!PAGE_ALIGNED(addr), "bad address (%p)\n", addr))
+		return;
+
+	area = find_vm_area(addr);
+	if (WARN(!area, "nonexistent vm area (%p)\n", addr))
+		return;
+
+	for (i = 0; i < area->nr_pages; i++)
+		kasan_poison_pages(area->pages[i], 0, false);
+}
+#endif
+
 #ifdef CONFIG_PRINTK
 bool vmalloc_dump_obj(void *object)
 {

base-commit: 5dc4c995db9eb45f6373a956eb1f69460e69e6d4
-- 
2.39.0.314.g84b9a713c41-goog