Re: [Bug 216489] New: Machine freezes due to memory lock

[Kees Cook <keescook@chromium.org>]

On Fri, Sep 16, 2022 at 09:38:33AM +0100, Matthew Wilcox wrote:
> On Thu, Sep 15, 2022 at 05:59:56PM -0600, Yu Zhao wrote:
> > I think this is a manifest of the lockdep warning I reported a couple
> > of weeks ago:
> > https://lore.kernel.org/r/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@mail.gmail.com/
> 
> That would certainly match the symptoms.
> 
> Turning vmap_lock into an NMI-safe lock would be bad.  I don't even know
> if we have primitives for that (it's not like you can disable an NMI
> ...)
> 
> I don't quite have time to write a patch right now.  Perhaps something
> like:
> 
> struct vmap_area *find_vmap_area_nmi(unsigned long addr)
> {
>         struct vmap_area *va;
> 
>         if (spin_trylock(&vmap_area_lock))
> 		return NULL;
>         va = __find_vmap_area(addr, &vmap_area_root);
>         spin_unlock(&vmap_area_lock);
> 
>         return va;
> }
> 
> and then call find_vmap_area_nmi() in check_heap_object().  I may have
> the polarity of the return value of spin_trylock() incorrect.

I think we'll need something slightly tweaked, since this would
return NULL under any contention (and a NULL return is fatal in
check_heap_object()). It seems like we need to explicitly check
for being in nmi context in check_heap_object() to deal with it?
Like this (only build tested):


diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 096d48aa3437..c8a00f181a11 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -216,6 +216,7 @@ 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..9f943c29e7ec 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -173,7 +173,16 @@ 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;
+
+		if (!in_nmi()) {
+			area = find_vmap_area(addr);
+		} else {
+			area = find_vmap_area_try(addr);
+			/* Give up under NMI to avoid deadlocks. */
+			if (!area)
+				return;
+		}
 
 		if (!area)
 			usercopy_abort("vmalloc", "no area", to_user, 0, n);
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index dd6cdb201195..f14f1902c2f6 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1840,6 +1840,17 @@ struct vmap_area *find_vmap_area(unsigned long addr)
 	return va;
 }
 
+struct vmap_area *find_vmap_area_try(unsigned long addr)
+{
+	struct vmap_area *va = NULL;
+
+	if (spin_trylock(&vmap_area_lock)) {
+		va = __find_vmap_area(addr, &vmap_area_root);
+		spin_unlock(&vmap_area_lock);
+	}
+	return va;
+}
+
 /*** Per cpu kva allocator ***/
 
 /*

-- 
Kees Cook