Re: [PATCH] vmap(): don't allow invalid pages

[Mark Rutland <mark.rutland@arm.com>]

Hi,

I replied ot the original RFC before spotting this; duplicating those comments
here because I think they apply regardless of the mechanism used to work around
this.

On Tue, Jan 18, 2022 at 03:52:44PM -0800, Yury Norov wrote:
> vmap() takes struct page *pages as one of arguments, and user may provide
> an invalid pointer which would lead to DABT at address translation later. 
>
> Currently, kernel checks the pages against NULL. In my case, however, the
> address was not NULL, and was big enough so that the hardware generated
> Address Size Abort on arm64.

Can you give an example of when this might happen? It sounds like you're
actually hitting this, so a backtrace would be nice.

I'm a bit confused as to when why we'd try to vmap() pages that we
didn't have a legitimate struct page for -- where did these addresses
come from?

It sounds like this is going wrong at a higher level, and we're passing
entirely bogus struct page pointers around. This seems like the sort of
thing DEBUG_VIRTUAL or similar should check when we initially generate
the struct page pointer.

> Interestingly, this abort happens even if copy_from_kernel_nofault() is
> used, which is quite inconvenient for debugging purposes. 

I can go take a look at this, but TBH we never expect to take an address size
fault to begin with, so this is arguably correct -- it's an internal
consistency problem.

> This patch adds a pfn_valid() check into vmap() path, so that invalid
> mapping will not be created.
> 
> RFC: https://lkml.org/lkml/2022/1/18/815
> v1: use pfn_valid() instead of adding an arch-specific
>     arch_vmap_page_valid(). Thanks to Matthew Wilcox for the hint.
> 
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> ---
>  mm/vmalloc.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d2a00ad4e1dd..a4134ee56b10 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -477,6 +477,8 @@ static int vmap_pages_pte_range(pmd_t *pmd, unsigned long addr,
>  			return -EBUSY;
>  		if (WARN_ON(!page))
>  			return -ENOMEM;
> +		if (WARN_ON(!pfn_valid(page_to_pfn(page))))
> +			return -EINVAL;

My fear here is that for this to fire, we've already passed a bogus struct page
pointer around the intermediate infrastructure, and any of that might try to
use it in unsafe ways (in future even if we don't use it today).

I think the fundamental issue here is that we generate a bogus struct page
pointer at all, and knowing where that came from would help to fix that.

Thanks,
Mark.

>  		set_pte_at(&init_mm, addr, pte, mk_pte(page, prot));
>  		(*nr)++;
>  	} while (pte++, addr += PAGE_SIZE, addr != end);
> -- 
> 2.30.2
>