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

From: Pasha Tatashin
Date: Tue Oct 26 2021 - 13:38:35 EST


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