Re:[PATCH V3] mm: prevent page_frag_alloc() from corrupting the memory

["Chen Lin" <chen45464546@163.com>]

At 2022-07-15 20:50:13, "Maurizio Lombardi" <mlombard@redhat.com> wrote:
>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;
>In this case, 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.
>
>V2: do not free the cache page because this could make memory pressure
>even worse, just return NULL.
>
>V3: add a comment to explain why we return NULL.
>
>Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
>---
> mm/page_alloc.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
>diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>index e008a3df0485..59c4dddf379f 100644
>--- a/mm/page_alloc.c
>+++ b/mm/page_alloc.c
>@@ -5617,6 +5617,18 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
> 		/* 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)) {
>+			/*
>+			 * The caller is trying to allocate a fragment
>+			 * with fragsz > PAGE_SIZE but the cache isn't big
>+			 * enough to satisfy the request, this may
>+			 * happen in low memory conditions.
>+			 * We don't release the cache page because
>+			 * it could make memory pressure worse
>+			 * so we simply return NULL here.
>+			 */
>+			return NULL;
>+		}
> 	}
> 
> 	nc->pagecnt_bias--;
>-- 
>2.31.1

Will  this lead to memory leak when device driver miss use this interface muti-times？

----------------------------------------
If we can accept adding a branch to this process, why not add it at the beginning like below?
The below changes are also more in line with the definition of "page fragment", 
which i mean the above changes may make the allocation of more than one page successful.

index 7a28f7d..9d09ea5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5551,6 +5551,8 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,

        offset = nc->offset - fragsz;
        if (unlikely(offset < 0)) {
+               if (unlikely(fragsz > PAGE_SIZE))
+                       return NULL;
                page = virt_to_page(nc->va);

                if (!page_ref_sub_and_test(page, nc->pagecnt_bias))