Re: [PATCH] [PATCH 4.19 STABLE] mm: fix dereference a null pointer in migrate[_huge]_page_move_mapping()

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Feb 17, 2022 at 07:54:16PM +0800, liqiong wrote:
> Upstream doesn't use radix tree any more in migrate.c, no need this patch.
> 
> The two functions look up a slot and dereference the pointer,
> If the pointer is null, the kernel would crash and dump.
> 
> The 'numad' service calls 'migrate_pages' periodically. If some slots
> being replaced (Cache Eviction), the radix_tree_lookup_slot() returns
> a null pointer that causes kernel crash.
> 
> "numad":  crash> bt
> [exception RIP: migrate_page_move_mapping+337]
> 
> Introduce pointer checking to avoid dereference a null pointer.
> 
> Cc: <stable@vger.kernel.org> # linux-4.19.y
> Signed-off-by: liqiong <liqiong@nfschina.com>
> ---
>  mm/migrate.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a69b842f95da..76f8dedc0e02 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -472,6 +472,10 @@ int migrate_page_move_mapping(struct address_space *mapping,
>  
>  	pslot = radix_tree_lookup_slot(&mapping->i_pages,
>   					page_index(page));
> +	if (pslot == NULL) {
> +		xa_unlock_irq(&mapping->i_pages);
> +		return -EAGAIN;
> +	}
>  
>  	expected_count += hpage_nr_pages(page) + page_has_private(page);
>  	if (page_count(page) != expected_count ||
> @@ -590,6 +594,10 @@ int migrate_huge_page_move_mapping(struct address_space *mapping,
>  	xa_lock_irq(&mapping->i_pages);
>  
>  	pslot = radix_tree_lookup_slot(&mapping->i_pages, page_index(page));
> +	if (pslot == NULL) {
> +		xa_unlock_irq(&mapping->i_pages);
> +		return -EAGAIN;
> +	}
>  
>  	expected_count = 2 + page_has_private(page);
>  	if (page_count(page) != expected_count ||
> -- 
> 2.25.1
> 

Sorry for the delay, now queued up.

greg k-h