Re: [PATCH 3/3] mm/usercopy: Detect compound page overruns

[Kees Cook <keescook@chromium.org>]

On Mon, Oct 04, 2021 at 11:42:23PM +0100, Matthew Wilcox (Oracle) wrote:
> Move the compound page overrun detection out of
> CONFIG_HARDENED_USERCOPY_PAGESPAN so it's enabled for more people.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/usercopy.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 7bfc4f9ed1e4..e395462961d5 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -194,11 +194,6 @@ static inline void check_page_span(const void *ptr, unsigned long n,
>  		   ((unsigned long)end & (unsigned long)PAGE_MASK)))
>  		return;
>  
> -	/* Allow if fully inside the same compound (__GFP_COMP) page. */
> -	endpage = virt_to_head_page(end);
> -	if (likely(endpage == page))
> -		return;
> -
>  	/*
>  	 * Reject if range is entirely either Reserved (i.e. special or
>  	 * device memory), or CMA. Otherwise, reject since the object spans
> @@ -250,6 +245,10 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
>  	if (PageSlab(page)) {
>  		/* Check slab allocator for flags and size. */
>  		__check_heap_object(ptr, n, page, to_user);
> +	} else if (PageHead(page)) {
> +		/* A compound allocation */
> +		if (ptr + n > page_address(page) + page_size(page))
> +			usercopy_abort("page alloc", NULL, to_user, 0, n);

"0" could be "ptr - page_address(page)", I think? With that:

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

>  	} else {
>  		/* Verify object does not incorrectly span multiple pages. */
>  		check_page_span(ptr, n, page, to_user);
> -- 
> 2.32.0
> 

-- 
Kees Cook