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

Matthew Wilcox
2022-09-16T19:39:22Z

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 ***/

/*