[PATCH 1/2] mm/kasan: fix vmalloc shadow memory (de-)population races

[Alexander Gordeev <agordeev@linux.ibm.com>]

When vmalloc shadow memory is established the modification of the
corresponding page tables is not protected by any locks. Instead,
the locking is done per-PTE. This scheme however has defects.

kasan_populate_vmalloc_pte() - while ptep_get() read is atomic the
sequence pte_none(ptep_get()) is not. Doing that outside of the
lock might lead to a concurrent PTE update and what could be
seen as a shadow memory corruption as result.

kasan_depopulate_vmalloc_pte() - by the time a page whose address
was extracted from ptep_get() read and cached in a local variable
outside of the lock is attempted to get free, could actually be
freed already.

To avoid these put ptep_get() itself and the code that manipulates
the result of the read under lock. In addition, move freeing of the
page out of the atomic context.

Fixes: 3c5c3cfb9ef4 ("kasan: support backing vmalloc space with real shadow memory")
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
---
 mm/kasan/shadow.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index d2c70cd2afb1..4d846d146d02 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -305,9 +305,6 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 	pte_t pte;
 	int index;
 
-	if (likely(!pte_none(ptep_get(ptep))))
-		return 0;
-
 	index = PFN_DOWN(addr - data->start);
 	page = data->pages[index];
 	__memset(page_to_virt(page), KASAN_VMALLOC_INVALID, PAGE_SIZE);
@@ -461,18 +458,19 @@ int kasan_populate_vmalloc(unsigned long addr, unsigned long size)
 static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 					void *unused)
 {
-	unsigned long page;
-
-	page = (unsigned long)__va(pte_pfn(ptep_get(ptep)) << PAGE_SHIFT);
+	pte_t pte;
+	int none;
 
 	spin_lock(&init_mm.page_table_lock);
-
-	if (likely(!pte_none(ptep_get(ptep)))) {
+	pte = ptep_get(ptep);
+	none = pte_none(pte);
+	if (likely(!none))
 		pte_clear(&init_mm, addr, ptep);
-		free_page(page);
-	}
 	spin_unlock(&init_mm.page_table_lock);
 
+	if (likely(!none))
+		__free_page(pfn_to_page(pte_pfn(pte)));
+
 	return 0;
 }
 
-- 
2.48.1