Re: [PATCHv4 3/8] mm: Pass down mm_struct to untagged_addr()

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Thu, Jul 07, 2022 at 10:56:53AM +0200, Alexander Potapenko wrote:
> On Thu, Jul 7, 2022 at 1:14 AM Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
> >
> > On Tue, Jul 05, 2022 at 05:42:21PM +0200, Alexander Potapenko wrote:
> > > Kirill,
> > >
> > >
> > > > diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> > > > index feeb935a2299..abc096a68f05 100644
> > > > --- a/lib/strnlen_user.c
> > > > +++ b/lib/strnlen_user.c
> > > > @@ -97,7 +97,7 @@ long strnlen_user(const char __user *str, long count)
> > > >                 return 0;
> > > >
> > > >         max_addr = TASK_SIZE_MAX;
> > > > -       src_addr = (unsigned long)untagged_addr(str);
> > > > +       src_addr = (unsigned long)untagged_addr(current->mm, str);
> > >
> > > In a downstream kernel with LAM disabled I'm seeing current->mm being
> > > NULL at this point, because strnlen_user() is being called by
> > > kdevtmpfs.
> > > IIUC current->mm is only guaranteed to be non-NULL in the userspace
> > > process context, whereas untagged_addr() may get called in random
> > > places.
> > >
> > > Am I missing something?
> >
> > Hm. Could you show a traceback?
> >
> > As strnlen_user() intended to be used on an user string I expected it to
> > be called from a process context. I guess I'm wrong, but I don't yet
> > understand why.
> 
> Oh, I see now. The old implementation of devtmpfsd()
> (https://elixir.bootlin.com/linux/v5.4/source/drivers/base/devtmpfs.c#L397)
> uses ksys_mount(), which assumes that the strings must be copied from
> the userspace, whereas they are actually constants in kernel .rodata
> 
> Wonder if the validity of mm->current for userspace accesses is
> actually enforced anyhow in newer kernels.

I think it is.

See 967747bbc084 and how it changes strnlen_user(). With max_addr equal to
TASK_SIZE_MAX, strnlen_user() will always fail on a kernel string.

-- 
 Kirill A. Shutemov