[PATCH V2] mm: prevent page_frag_alloc() from corrupting the memory

[PATCH V2] mm: prevent page_frag_alloc() from corrupting the memory

[Maurizio Lombardi]

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.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
---
 mm/page_alloc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e008a3df0485..b1407254a826 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5617,6 +5617,8 @@ 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))
+			return NULL;
 	}
 
 	nc->pagecnt_bias--;
-- 
2.31.1

Re: [PATCH V2] mm: prevent page_frag_alloc() from corrupting the memory

[Alexander Duyck]

On Wed, Jul 13, 2022 at 8:01 AM 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.
>
> Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
> ---
>  mm/page_alloc.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e008a3df0485..b1407254a826 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5617,6 +5617,8 @@ 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))
> +                       return NULL;
>         }
>
>         nc->pagecnt_bias--;

This works for me. If I am not mistaken it should be only adding one
conditional jump that isn't taken to the fast path based on a
calculation we were already doing.

Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>

Re: [PATCH V2] mm: prevent page_frag_alloc() from corrupting the memory

[Andrew Morton]

On Wed, 13 Jul 2022 17:01:43 +0200 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.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5617,6 +5617,8 @@ 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))
> +			return NULL;
>  	}
>  
>  	nc->pagecnt_bias--;

I think we should have a comment here explaining (at least) why we'd
bale after a successful allocation and explaining why we don't call
free_the_page().