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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
To:     pasha.tatashin@soleen.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-m68k@lists.linux-m68k.org,
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Subject: [PATCH v3 1/9] mm: add overflow and underflow checks for page->_refcount
Date:   Wed, 26 Jan 2022 18:34:21 +0000
Message-Id: <20220126183429.1840447-2-pasha.tatashin@soleen.com>
In-Reply-To: <20220126183429.1840447-1-pasha.tatashin@soleen.com>
References: <20220126183429.1840447-1-pasha.tatashin@soleen.com>
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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.

Use atomic_fetch_* functions to get the old values of the _refcount,
and use it to check for overflow/underflow.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/page_ref.h | 59 +++++++++++++++++++++++++++++-----------
 1 file changed, 43 insertions(+), 16 deletions(-)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 2e677e6ad09f..fe4864f7f69c 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -117,7 +117,10 @@ 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 old_val = atomic_fetch_add(nr, &page->_refcount);
+	int new_val = old_val + nr;
+
+	VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod))
 		__page_ref_mod(page, nr);
 }
@@ -129,7 +132,10 @@ static inline void folio_ref_add(struct folio *folio, int nr)
 
 static inline void page_ref_sub(struct page *page, int nr)
 {
-	atomic_sub(nr, &page->_refcount);
+	int old_val = atomic_fetch_sub(nr, &page->_refcount);
+	int new_val = old_val - nr;
+
+	VM_BUG_ON_PAGE((unsigned int)new_val > (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod))
 		__page_ref_mod(page, -nr);
 }
@@ -141,11 +147,13 @@ static inline void folio_ref_sub(struct folio *folio, int nr)
 
 static inline int page_ref_sub_return(struct page *page, int nr)
 {
-	int ret = atomic_sub_return(nr, &page->_refcount);
+	int old_val = atomic_fetch_sub(nr, &page->_refcount);
+	int new_val = old_val - nr;
 
+	VM_BUG_ON_PAGE((unsigned int)new_val > (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod_and_return))
-		__page_ref_mod_and_return(page, -nr, ret);
-	return ret;
+		__page_ref_mod_and_return(page, -nr, new_val);
+	return new_val;
 }
 
 static inline int folio_ref_sub_return(struct folio *folio, int nr)
@@ -155,7 +163,10 @@ static inline int folio_ref_sub_return(struct folio *folio, int nr)
 
 static inline void page_ref_inc(struct page *page)
 {
-	atomic_inc(&page->_refcount);
+	int old_val = atomic_fetch_inc(&page->_refcount);
+	int new_val = old_val + 1;
+
+	VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod))
 		__page_ref_mod(page, 1);
 }
@@ -167,7 +178,10 @@ static inline void folio_ref_inc(struct folio *folio)
 
 static inline void page_ref_dec(struct page *page)
 {
-	atomic_dec(&page->_refcount);
+	int old_val = atomic_fetch_dec(&page->_refcount);
+	int new_val = old_val - 1;
+
+	VM_BUG_ON_PAGE((unsigned int)new_val > (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod))
 		__page_ref_mod(page, -1);
 }
@@ -179,8 +193,11 @@ static inline void folio_ref_dec(struct folio *folio)
 
 static inline int page_ref_sub_and_test(struct page *page, int nr)
 {
-	int ret = atomic_sub_and_test(nr, &page->_refcount);
+	int old_val = atomic_fetch_sub(nr, &page->_refcount);
+	int new_val = old_val - nr;
+	int ret = new_val == 0;
 
+	VM_BUG_ON_PAGE((unsigned int)new_val > (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod_and_test))
 		__page_ref_mod_and_test(page, -nr, ret);
 	return ret;
@@ -193,11 +210,13 @@ static inline int folio_ref_sub_and_test(struct folio *folio, int nr)
 
 static inline int page_ref_inc_return(struct page *page)
 {
-	int ret = atomic_inc_return(&page->_refcount);
+	int old_val = atomic_fetch_inc(&page->_refcount);
+	int new_val = old_val + 1;
 
+	VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod_and_return))
-		__page_ref_mod_and_return(page, 1, ret);
-	return ret;
+		__page_ref_mod_and_return(page, 1, new_val);
+	return new_val;
 }
 
 static inline int folio_ref_inc_return(struct folio *folio)
@@ -207,8 +226,11 @@ static inline int folio_ref_inc_return(struct folio *folio)
 
 static inline int page_ref_dec_and_test(struct page *page)
 {
-	int ret = atomic_dec_and_test(&page->_refcount);
+	int old_val = atomic_fetch_dec(&page->_refcount);
+	int new_val = old_val - 1;
+	int ret = new_val == 0;
 
+	VM_BUG_ON_PAGE((unsigned int)new_val > (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod_and_test))
 		__page_ref_mod_and_test(page, -1, ret);
 	return ret;
@@ -221,11 +243,13 @@ static inline int folio_ref_dec_and_test(struct folio *folio)
 
 static inline int page_ref_dec_return(struct page *page)
 {
-	int ret = atomic_dec_return(&page->_refcount);
+	int old_val = atomic_fetch_dec(&page->_refcount);
+	int new_val = old_val - 1;
 
+	VM_BUG_ON_PAGE((unsigned int)new_val > (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod_and_return))
-		__page_ref_mod_and_return(page, -1, ret);
-	return ret;
+		__page_ref_mod_and_return(page, -1, new_val);
+	return new_val;
 }
 
 static inline int folio_ref_dec_return(struct folio *folio)
@@ -235,8 +259,11 @@ static inline int folio_ref_dec_return(struct folio *folio)
 
 static inline bool page_ref_add_unless(struct page *page, int nr, int u)
 {
-	bool ret = atomic_add_unless(&page->_refcount, nr, u);
+	int old_val = atomic_fetch_add_unless(&page->_refcount, nr, u);
+	int new_val = old_val + nr;
+	int ret = old_val != u;
 
+	VM_BUG_ON_PAGE(ret && (unsigned int)new_val < (unsigned int)old_val, page);
 	if (page_ref_tracepoint_active(page_ref_mod_unless))
 		__page_ref_mod_unless(page, nr, ret);
 	return ret;
-- 
2.35.0.rc0.227.g00780c9af4-goog