Re: [PATCH 5.15] mm/filemap: fix UAF in find_lock_entries

[Liu Shixin <liushixin2@huawei.com>]

On 2022/7/6 11:21, Matthew Wilcox wrote:
> On Wed, Jul 06, 2022 at 11:24:34AM +0800, Liu Shixin wrote:
>> Release refcount after xas_set to fix UAF which may cause panic like this:
> I think we can do better.  How about this?
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 00e391e75880..11ae38cc4fd3 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2090,7 +2090,9 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
>  
>  	rcu_read_lock();
>  	while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
> +		unsigned long next_idx = xas.xa_index + 1;
>  		if (!xa_is_value(page)) {
> +			next_idx = page->index + thp_nr_pages(page);
I noticed that there was a VM_BUG_ON_PAGE before which was deleted by patch 6560e8cd869b ("mm/filemap.c: remove bogus VM_BUG_ON")
It seems that page->index and xas.xa_index are not guaranteed to be equal. Therefore, I conservatively retained
the PageTransHuge to keep consistent with the original logic :)

@@ -2090,7 +2090,11 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
 
        rcu_read_lock();
        while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
+               unsigned long next_idx = xas.xa_index;
+
                if (!xa_is_value(page)) {
+                       if (PageTransHuge(page))
+                               next_idx = page->index + thp_nr_pages(page);
                        if (page->index < start)
                                goto put;
                        if (page->index + thp_nr_pages(page) - 1 > end)

>  			if (page->index < start)
>  				goto put;
>  			if (page->index + thp_nr_pages(page) - 1 > end)
> @@ -2111,14 +2113,11 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
>  put:
>  		put_page(page);
>  next:
> -		if (!xa_is_value(page) && PageTransHuge(page)) {
> -			unsigned int nr_pages = thp_nr_pages(page);
> -
> -			/* Final THP may cross MAX_LFS_FILESIZE on 32-bit */
> -			xas_set(&xas, page->index + nr_pages);
> -			if (xas.xa_index < nr_pages)
> -				break;
> -		}
> +		/* Final THP may cross MAX_LFS_FILESIZE on 32-bit */
> +		if (next_idx < xas.xa_index)
> +			break;
> +		if (next_idx != xas.xa_index + 1)
> +			xas_set(&xas, next_idx);
>  	}
>  	rcu_read_unlock();
>  
>
> .
>