On 1/26/22 19:34, Pasha Tatashin wrote:
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@xxxxxxxxxx>
---
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);
This seems somewhat weird, as it will trigger not just on overflow, but also
if nr is negative. Which I think is valid usage, even though the function
has 'add' in name, because 'nr' is signed?
