[RFC 1/8] mm: add overflow and underflow checks for page->_refcount

[Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>]

The problems with page->_refcount are hard to debug, because usually
when they are detected, the damage has occurred a long time ago. Yet,
the problems with invalid page refcount may be catastrophic and lead to
memory corruptions.

Reduce the scope of when the _refcount problems manifest themselves by
adding checks for underflows and overflows into functions that modify
_refcount.

Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 include/linux/page_ref.h | 61 ++++++++++++++++++++++++++++++++--------
 1 file changed, 49 insertions(+), 12 deletions(-)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 7ad46f45df39..b3ec2b231fc7 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -90,21 +90,35 @@ static inline void init_page_count(struct page *page)
 
 static inline void page_ref_add(struct page *page, int nr)
 {
-	atomic_add(nr, &page->_refcount);
+	int ret;
+
+	VM_BUG_ON(nr <= 0);
+	ret = atomic_add_return(nr, &page->_refcount);
+	VM_BUG_ON_PAGE(ret <= 0, page);
+
 	if (page_ref_tracepoint_active(page_ref_mod))
 		__page_ref_mod(page, nr);
 }
 
 static inline void page_ref_sub(struct page *page, int nr)
 {
-	atomic_sub(nr, &page->_refcount);
+	int ret;
+
+	VM_BUG_ON(nr <= 0);
+	ret = atomic_sub_return(nr, &page->_refcount);
+	VM_BUG_ON_PAGE(ret < 0, page);
+
 	if (page_ref_tracepoint_active(page_ref_mod))
 		__page_ref_mod(page, -nr);
 }
 
 static inline int page_ref_sub_return(struct page *page, int nr)
 {
-	int ret = atomic_sub_return(nr, &page->_refcount);
+	int ret;
+
+	VM_BUG_ON(nr <= 0);
+	ret = atomic_sub_return(nr, &page->_refcount);
+	VM_BUG_ON_PAGE(ret < 0, page);
 
 	if (page_ref_tracepoint_active(page_ref_mod_and_return))
 		__page_ref_mod_and_return(page, -nr, ret);
@@ -113,31 +127,43 @@ static inline int page_ref_sub_return(struct page *page, int nr)
 
 static inline void page_ref_inc(struct page *page)
 {
-	atomic_inc(&page->_refcount);
+	int ret = atomic_inc_return(&page->_refcount);
+
+	VM_BUG_ON_PAGE(ret <= 0, page);
+
 	if (page_ref_tracepoint_active(page_ref_mod))
 		__page_ref_mod(page, 1);
 }
 
 static inline void page_ref_dec(struct page *page)
 {
-	atomic_dec(&page->_refcount);
+	int ret = atomic_dec_return(&page->_refcount);
+
+	VM_BUG_ON_PAGE(ret < 0, page);
+
 	if (page_ref_tracepoint_active(page_ref_mod))
 		__page_ref_mod(page, -1);
 }
 
 static inline int page_ref_sub_and_test(struct page *page, int nr)
 {
-	int ret = atomic_sub_and_test(nr, &page->_refcount);
+	int ret;
+
+	VM_BUG_ON(nr <= 0);
+	ret = atomic_sub_return(nr, &page->_refcount);
+	VM_BUG_ON_PAGE(ret < 0, page);
 
 	if (page_ref_tracepoint_active(page_ref_mod_and_test))
 		__page_ref_mod_and_test(page, -nr, ret);
-	return ret;
+	return ret == 0;
 }
 
 static inline int page_ref_inc_return(struct page *page)
 {
 	int ret = atomic_inc_return(&page->_refcount);
 
+	VM_BUG_ON_PAGE(ret <= 0, page);
+
 	if (page_ref_tracepoint_active(page_ref_mod_and_return))
 		__page_ref_mod_and_return(page, 1, ret);
 	return ret;
@@ -145,17 +171,21 @@ static inline int page_ref_inc_return(struct page *page)
 
 static inline int page_ref_dec_and_test(struct page *page)
 {
-	int ret = atomic_dec_and_test(&page->_refcount);
+	int ret = atomic_dec_return(&page->_refcount);
+
+	VM_BUG_ON_PAGE(ret < 0, page);
 
 	if (page_ref_tracepoint_active(page_ref_mod_and_test))
 		__page_ref_mod_and_test(page, -1, ret);
-	return ret;
+	return ret == 0;
 }
 
 static inline int page_ref_dec_return(struct page *page)
 {
 	int ret = atomic_dec_return(&page->_refcount);
 
+	VM_BUG_ON_PAGE(ret < 0, page);
+
 	if (page_ref_tracepoint_active(page_ref_mod_and_return))
 		__page_ref_mod_and_return(page, -1, ret);
 	return ret;
@@ -163,16 +193,23 @@ static inline int page_ref_dec_return(struct page *page)
 
 static inline int page_ref_add_unless(struct page *page, int nr, int u)
 {
-	int ret = atomic_add_unless(&page->_refcount, nr, u);
+	int ret;
+
+	VM_BUG_ON(nr <= 0 || u < 0);
+	ret = atomic_fetch_add_unless(&page->_refcount, nr, u);
+	VM_BUG_ON_PAGE(ret < 0, page);
 
 	if (page_ref_tracepoint_active(page_ref_mod_unless))
 		__page_ref_mod_unless(page, nr, ret);
-	return ret;
+	return ret != u;
 }
 
 static inline int page_ref_freeze(struct page *page, int count)
 {
-	int ret = likely(atomic_cmpxchg(&page->_refcount, count, 0) == count);
+	int ret;
+
+	VM_BUG_ON(count <= 0);
+	ret = likely(atomic_cmpxchg(&page->_refcount, count, 0) == count);
 
 	if (page_ref_tracepoint_active(page_ref_freeze))
 		__page_ref_freeze(page, count, ret);
-- 
2.33.0.1079.g6e70778dc9-goog