Re: [PATCH v2 3/3] mm/usercopy: Detect compound page overruns

[Kees Cook <keescook@chromium.org>]

On Wed, Oct 06, 2021 at 03:08:46PM +0100, Matthew Wilcox wrote:
> On Wed, Oct 06, 2021 at 01:42:26PM +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>
> > Acked-by: Kees Cook <keescook@chromium.org>
> > ---
> >  mm/usercopy.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/usercopy.c b/mm/usercopy.c
> > index 63476e1506e0..b825c4344917 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
> 
> Needs an extra hunk to avoid a warning with that config:

Ah yeah, good catch.

> 
> @@ -163,7 +163,6 @@ static inline void check_page_span(const void *ptr, unsigned long n,
>  {
>  #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
>         const void *end = ptr + n - 1;
> -       struct page *endpage;
>         bool is_reserved, is_cma;
> 
>         /*
> 
> I'll wait a few days and send a v3.

When you send v3, can you CC linux-hardening@vger.kernel.org too?

Thanks for poking at this!

-Kees

-- 
Kees Cook