[PATCH] mm: prevent page_frag_alloc() from corrupting the memory

[Maurizio Lombardi <mlombard@redhat.com>]

A number of drivers call page_frag_alloc() with a
fragment's size > PAGE_SIZE.
In low memory conditions, __page_frag_cache_refill() may fail the order 3
cache allocation and fall back to order 0;
If this happens, the cache will be smaller than the fragment, causing
memory corruptions.

Prevent this from happening by checking if the newly allocated cache
is large enough for the fragment; if not, the allocation will fail
and page_frag_alloc() will return NULL.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
---
 mm/page_alloc.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e008a3df0485..7fb000d7e90c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5611,12 +5611,17 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
 		/* if size can vary use size else just use PAGE_SIZE */
 		size = nc->size;
 #endif
-		/* OK, page count is 0, we can safely set it */
-		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
-
 		/* reset page count bias and offset to start of new frag */
 		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
 		offset = size - fragsz;
+		if (unlikely(offset < 0)) {
+			free_the_page(page, compound_order(page));
+			nc->va = NULL;
+			return NULL;
+		}
+
+		/* OK, page count is 0, we can safely set it */
+		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
 	}
 
 	nc->pagecnt_bias--;
-- 
2.31.1