Re: [PATCH 3/3] usercopy: Add find_vmap_area_try() to avoid deadlocks

[Matthew Wilcox <willy@infradead.org>]

On Fri, Sep 16, 2022 at 08:09:16AM -0700, Kees Cook wrote:
> On Fri, Sep 16, 2022 at 03:46:07PM +0100, Matthew Wilcox wrote:
> > On Fri, Sep 16, 2022 at 06:59:57AM -0700, Kees Cook wrote:
> > > The check_object_size() checks under CONFIG_HARDENED_USERCOPY need to be
> > > more defensive against running from interrupt context. Use a best-effort
> > > check for VMAP areas when running in interrupt context
> > 
> > I had something more like this in mind:
> 
> Yeah, I like -EAGAIN. I'd like to keep the interrupt test to choose lock
> vs trylock, otherwise it's trivial to bypass the hardening test by having
> all the other CPUs beating on the spinlock.

I was thinking about this:

+++ b/mm/vmalloc.c
@@ -1844,12 +1844,19 @@
 {
 	struct vmap_area *va;

-	if (!spin_lock(&vmap_area_lock))
-		return ERR_PTR(-EAGAIN);
+	/*
+	 * It's safe to walk the rbtree under the RCU lock, but we may
+	 * incorrectly find no vmap_area if the tree is being modified.
+	 */
+	rcu_read_lock();
 	va = __find_vmap_area(addr, &vmap_area_root);
-	spin_unlock(&vmap_area_lock);
+	if (!va && in_interrupt())
+		va = ERR_PTR(-EAGAIN);
+	rcu_read_unlock();

-	return va;
+	if (va)
+		return va;
+	return find_vmap_area(addr);
 }

 /*** Per cpu kva allocator ***/

... but I don't think that works since vmap_areas aren't freed by RCU,
and I think they're reused without going through an RCU cycle.

So here's attempt #4, which actually compiles, and is, I think, what you
had in mind.

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 096d48aa3437..2b7c52e76856 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -215,7 +215,7 @@ extern struct vm_struct *__get_vm_area_caller(unsigned long size,
 void free_vm_area(struct vm_struct *area);
 extern struct vm_struct *remove_vm_area(const void *addr);
 extern struct vm_struct *find_vm_area(const void *addr);
-struct vmap_area *find_vmap_area(unsigned long addr);
+struct vmap_area *find_vmap_area_try(unsigned long addr);
 
 static inline bool is_vm_area_hugepages(const void *addr)
 {
diff --git a/mm/usercopy.c b/mm/usercopy.c
index c1ee15a98633..e0fb605c1b38 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -173,7 +173,11 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
 	}
 
 	if (is_vmalloc_addr(ptr)) {
-		struct vmap_area *area = find_vmap_area(addr);
+		struct vmap_area *area = find_vmap_area_try(addr);
+
+		/* We may be in NMI context */
+		if (area == ERR_PTR(-EAGAIN))
+			return;
 
 		if (!area)
 			usercopy_abort("vmalloc", "no area", to_user, 0, n);
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index dd6cdb201195..c47b3b5d1c2d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1829,7 +1829,7 @@ static void free_unmap_vmap_area(struct vmap_area *va)
 	free_vmap_area_noflush(va);
 }
 
-struct vmap_area *find_vmap_area(unsigned long addr)
+static struct vmap_area *find_vmap_area(unsigned long addr)
 {
 	struct vmap_area *va;
 
@@ -1840,6 +1840,26 @@ struct vmap_area *find_vmap_area(unsigned long addr)
 	return va;
 }
 
+/*
+ * The vmap_area_lock is not interrupt-safe, and we can end up here from
+ * NMI context, so it's not worth even trying to make it IRQ-safe.
+ */
+struct vmap_area *find_vmap_area_try(unsigned long addr)
+{
+	struct vmap_area *va;
+
+	if (in_interrupt()) {
+		if (!spin_trylock(&vmap_area_lock))
+			return ERR_PTR(-EAGAIN);
+	} else {
+		spin_lock(&vmap_area_lock);
+	}
+	va = __find_vmap_area(addr, &vmap_area_root);
+	spin_unlock(&vmap_area_lock);
+
+	return va;
+}
+
 /*** Per cpu kva allocator ***/
 
 /*