Re: [PATCH v18 07/15] fs/namespace: untag user pointers in copy_mount_options

Andrey Konovalov
2019-07-15T16:00:04Z

On Mon, Jun 24, 2019 at 7:50 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Mon, Jun 24, 2019 at 04:32:52PM +0200, Andrey Konovalov wrote:
> > This patch is a part of a series that extends 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: Khalid Aziz <khalid.aziz@oracle.com>
> > Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> > ---
> > fs/namespace.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index 7660c2749c96..ec78f7223917 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -2994,7 +2994,7 @@ 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;
>
> I think this patch needs an ack from Al Viro (cc'ed).
>
> --
> Catalin

Hi Al,

Could you take a look and give your acked-by?

Thanks!