Re: [PATCH 1/1] mm: slub: fix corrupted freechain in deactivate_slab()

[Andrew Morton]

On Mon, 30 Mar 2020 20:14:50 -0700 Dongli Zhang <dongli.zhang@xxxxxxxxxx> wrote:

> The slub_debug is able to fix the corrupted slab freelist/page. However,
> alloc_debug_processing() only checks the validity of current and next
> freepointer during allocation path. As a result, once some objects have
> their freepointers corrupted, deactivate_slab() may lead to page fault.
> 
> Below is from a test kernel module when
> 'slub_debug=PUF,kmalloc-128 slub_nomerge'. The test kernel corrupts the
> freepointer of one free object on purpose. Unfortunately, deactivate_slab()
> does not detect it when iterating the freechain.
> 
> ...
>
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2082,6 +2082,20 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>  		void *prior;
>  		unsigned long counters;
>  
> +		if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
> +		    !check_valid_pointer(s, page, nextfree)) {
> +			/*
> +			 * If 'nextfree' is invalid, it is possible that
> +			 * the object at 'freelist' is already corrupted.
> +			 * Therefore, all objects starting at 'freelist'
> +			 * are isolated.
> +			 */
> +			object_err(s, page, freelist, "Freechain corrupt");
> +			freelist = NULL;
> +			slab_fix(s, "Isolate corrupted freechain");
> +			break;
> +		}
> +
>  		do {
>  			prior = page->freelist;
>  			counters = page->counters;

We could do it this way:

--- a/mm/slub.c~mm-slub-fix-corrupted-freechain-in-deactivate_slab-fix
+++ a/mm/slub.c
@@ -2083,6 +2083,7 @@ static void deactivate_slab(struct kmem_
 		void *prior;
 		unsigned long counters;
 
+#ifdef CONFIG_SLAB_DEBUG
 		if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
 		    !check_valid_pointer(s, page, nextfree)) {
 			/*
@@ -2096,6 +2097,7 @@ static void deactivate_slab(struct kmem_
 			slab_fix(s, "Isolate corrupted freechain");
 			break;
 		}
+#endif
 
 		do {
 			prior = page->freelist;

But it's a bit ugly.  How about this?

--- a/mm/slub.c~mm-slub-fix-corrupted-freechain-in-deactivate_slab-fix
+++ a/mm/slub.c
@@ -650,6 +650,20 @@ static void slab_bug(struct kmem_cache *
 	va_end(args);
 }
 
+static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
+			       void *freelist, void *nextfree)
+{
+	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
+	    !check_valid_pointer(s, page, nextfree)) {
+		object_err(s, page, freelist, "Freechain corrupt");
+		freelist = NULL;
+		slab_fix(s, "Isolate corrupted freechain");
+		return true;
+	}
+
+	return false;
+}
+
 static void slab_fix(struct kmem_cache *s, char *fmt, ...)
 {
 	struct va_format vaf;
@@ -1400,6 +1414,11 @@ static inline void inc_slabs_node(struct
 static inline void dec_slabs_node(struct kmem_cache *s, int node,
 							int objects) {}
 
+static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
+			       void *freelist, void *nextfree)
+{
+	return false;
+}
 #endif /* CONFIG_SLUB_DEBUG */
 
 /*
@@ -2083,19 +2102,13 @@ static void deactivate_slab(struct kmem_
 		void *prior;
 		unsigned long counters;
 
-		if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
-		    !check_valid_pointer(s, page, nextfree)) {
-			/*
-			 * If 'nextfree' is invalid, it is possible that
-			 * the object at 'freelist' is already corrupted.
-			 * Therefore, all objects starting at 'freelist'
-			 * are isolated.
-			 */
-			object_err(s, page, freelist, "Freechain corrupt");
-			freelist = NULL;
-			slab_fix(s, "Isolate corrupted freechain");
+		/*
+		 * If 'nextfree' is invalid, it is possible that the object at
+		 * 'freelist' is already corrupted.  So isolate all objects
+		 * starting at 'freelist'.
+		 */
+		if (freelist_corrupted(s, page, freelist, nextfree))
 			break;
-		}
 
 		do {
 			prior = page->freelist;
_