Re: [PATCH] crypto: testmgr - allocate buffers with __GFP_COMP

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Apr 11, 2019 at 10:32 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Apr 11, 2019 at 12:31 PM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > From: Eric Biggers <ebiggers@google.com>
> >
> > This is needed so that CONFIG_HARDENED_USERCOPY_PAGESPAN=y doesn't
> > incorrectly report a buffer overflow when the destination of
> > copy_from_iter() spans the page boundary in the 2-page buffer.
> >
> > Fixes: 3f47a03df6e8 ("crypto: testmgr - add testvec_config struct and helper functions")
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> > ---
> >  crypto/testmgr.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/crypto/testmgr.c b/crypto/testmgr.c
> > index 0f6bfb6ce6a46..3522c0bed2492 100644
> > --- a/crypto/testmgr.c
> > +++ b/crypto/testmgr.c
> > @@ -156,7 +156,8 @@ static int __testmgr_alloc_buf(char *buf[XBUFSIZE], int order)
> >         int i;
> >
> >         for (i = 0; i < XBUFSIZE; i++) {
> > -               buf[i] = (char *)__get_free_pages(GFP_KERNEL, order);
> > +               buf[i] = (char *)__get_free_pages(GFP_KERNEL | __GFP_COMP,
> > +                                                 order);
>
> Is there a reason __GFP_COMP isn't automatically included in all page
> allocations? (Or rather, it seems like the exception is when things
> should NOT be considered part of the same allocation, so something
> like __GFP_SINGLE should exist?.)

It would be reasonable if __get_free_pages would automatically mark
consecutive pages as consecutive.
When these should not be considered part of the same allocation? Is it
possible to free them separately? Will that BUG with __GFP_COMP mark?
I understand that there can be a weak "these are actually the same
allocation, but I would like to think about them as separate". But
potentially we can ignore such cases.

> -Kees
>
> >                 if (!buf[i])
> >                         goto err_free_buf;
> >         }
> > --
> > 2.21.0.392.gf8f6787159e-goog
> >
>
>
> --
> Kees Cook