Re: [PATCH v16 08/16] fs, arm64: untag user pointers in copy_mount_options

Andrey Konovalov
2019-06-11T14:38:42Z

On Sat, Jun 8, 2019 at 6:02 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, Jun 03, 2019 at 06:55:10PM +0200, Andrey Konovalov wrote:
> > This patch is a part of a series that extends arm64 kernel ABI to allow to
> > pass tagged user pointers (with the top byte set to something else other
> > than 0x00) as syscall arguments.
> >
> > In copy_mount_options a user address is being subtracted from TASK_SIZE.
> > If the address is lower than TASK_SIZE, the size is calculated to not
> > allow the exact_copy_from_user() call to cross TASK_SIZE boundary.
> > However if the address is tagged, then the size will be calculated
> > incorrectly.
> >
> > Untag the address before subtracting.
> >
> > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>
> One thing I just noticed in the commit titles... "arm64" is in the
> prefix, but these are arch-indep areas. Should the ", arm64" be left
> out?
>
> I would expect, instead:
>
> fs/namespace: untag user pointers in copy_mount_options

Hm, I've added the arm64 tag in all of the patches because they are
related to changes in arm64 kernel ABI. I can remove it from all the
patches that only touch common code if you think that it makes sense.

Thanks!

>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> -Kees
>
> > ---
> > fs/namespace.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index b26778bdc236..2e85712a19ed 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -2993,7 +2993,7 @@ void *copy_mount_options(const void __user * data)
> > * the remainder of the page.
> > */
> > /* copy_from_user cannot cross TASK_SIZE ! */
> > - size = TASK_SIZE - (unsigned long)data;
> > + size = TASK_SIZE - (unsigned long)untagged_addr(data);
> > if (size > PAGE_SIZE)
> > size = PAGE_SIZE;
> >
> > --
> > 2.22.0.rc1.311.g5d7573a151-goog
> >
>
> --
> Kees Cook