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

From: Kees Cook <[email-masked]>
Date: Mon, 22 Jul 2019 09:46:06 -0700

+Eric Biederman too, who might be able to Ack this...

On Mon, Jul 15, 2019 at 06:00:04PM +0200, Andrey Konovalov wrote:
> On Mon, Jun 24, 2019 at 7:50 PM Catalin Marinas <[email-masked]> 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 <[email-masked]>
> > > Reviewed-by: Vincenzo Frascino <[email-masked]>
> > > Reviewed-by: Kees Cook <[email-masked]>
> > > Reviewed-by: Catalin Marinas <[email-masked]>
> > > Signed-off-by: Andrey Konovalov <[email-masked]>
> > > ---
> > >  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!

-- 
Kees Cook

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