Re: [PATCH 4/6] mm, hwpoison: fix possible use-after-free in mf_dax_kill_procs()

[HORIGUCHI NAOYA(堀口 直也)]

On Thu, Aug 18, 2022 at 09:00:14PM +0800, Miaohe Lin wrote:
> After kill_procs(), tk will be freed without being removed from the to_kill
> list. In the next iteration, the freed list entry in the to_kill list will
> be accessed, thus leading to use-after-free issue.

kill_procs() runs over the to_kill list and frees all listed items in each
iteration.  So just after returning from unmap_and_kill(), to_kill->next and
to_kill->prev still point to the addresses of struct to_kill which was the
first or last item (already freed!).  This is bad-manered, but
collect_procs_fsdax() in the next iteration calls list_add_tail() and
overwrites the dangling pointers with newly allocated item.  So this problem
should not be so critical?  Anyway, I agree with fixing this fragile code.

> Fix it by reinitializing
> the to_kill list after unmap_and_kill().
> 
> Fixes: c36e20249571 ("mm: introduce mf_dax_kill_procs() for fsdax case")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>

> ---
>  mm/memory-failure.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 7023c3d81273..a2f4e8b00a26 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1658,6 +1658,8 @@ int mf_dax_kill_procs(struct address_space *mapping, pgoff_t index,
>  		collect_procs_fsdax(page, mapping, index, &to_kill);
>  		unmap_and_kill(&to_kill, page_to_pfn(page), mapping,
>  				index, mf_flags);
> +		/* Reinitialize to_kill list for later resuing. */

s/resuing/reusing/ ?

> +		INIT_LIST_HEAD(&to_kill);

How about adding list_del() in kill_procs()?  Other callers now use
to_kill only once, but fixing generally looks tidier to me.

Thanks,
Naoya Horiguchi