Re: [PATCH 2/2] x86/mm/iommu/sva: Do not allow to set FORCE_TAGGED_SVA bit from outside

[Kirill A. Shutemov]

On Mon, Apr 03, 2023 at 03:55:09PM +0200, Dmitry Vyukov wrote:
> On Mon, 3 Apr 2023 at 13:10, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >
> > arch_prctl(ARCH_FORCE_TAGGED_SVA) overrides the default and allows LAM
> > and SVA to co-exist in the process. It is expected by called by the
> > process when it knows what it is doing.
> >
> > arch_prctl() operates on the current process, but the same code is
> > reachable from ptrace where it can be called on arbitrary task.
> >
> > Make it strict and only allow to set MM_CONTEXT_FORCE_TAGGED_SVA for the
> > current process.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Fixes: 23e5d9ec2bab ("x86/mm/iommu/sva: Make LAM and SVA mutually exclusive")
> > Suggested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > ---
> >  arch/x86/kernel/process_64.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> > index c7dfd727c9ec..cefac2d3a9f6 100644
> > --- a/arch/x86/kernel/process_64.c
> > +++ b/arch/x86/kernel/process_64.c
> > @@ -885,6 +885,8 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
> >         case ARCH_ENABLE_TAGGED_ADDR:
> >                 return prctl_enable_tagged_addr(task->mm, arg2);
> >         case ARCH_FORCE_TAGGED_SVA:
> > +               if (current != task)
> > +                       return -EINVAL;
> 
> prctl_enable_tagged_addr() checks "task->mm != current->mm".
> Should we check the same here for consistency? Or also change the
> check in prctl_enable_tagged_addr().
> 
> arch_prctl() can only do task==current, so I guess "current != task"
> is a more reasonable check for prctl_enable_tagged_addr() as well.

As of now, prctl_enable_tagged_addr() doesn't have the task on hands. It
gets mm as input, so it cannot check the task directly. But functionally
it is the same check.

I would prefer to keep it this way. Unless anyone feels strongly about it.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov