Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

[Dmitry Torokhov <dtor@google.com>]

On Wed, Nov 14, 2018 at 2:38 PM Jann Horn <jannh@google.com> wrote:
>
> On Wed, Nov 14, 2018 at 11:29 PM Dmitry Torokhov <dtor@google.com> wrote:
> > On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@google.com> wrote:
> > > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > > > When a UHID_CREATE command is written to the uhid char device, a
> > > > copy_from_user() is done from a user pointer embedded in the command.
> > > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > > sys_sendfile(), this can read from kernel memory.  Alternatively,
> > > > information can be leaked from a setuid binary that is tricked to write
> > > > to the file descriptor.  Therefore, forbid UHID_CREATE in these cases.
> > > >
> > > > No other commands in uhid_char_write() are affected by this bug and
> > > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > > UHID_CREATE only rather than to uhid_char_write() entirely.
> [...]
> > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> [...]
> > > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > > >
> > > >         switch (uhid->input_buf.type) {
> > > >         case UHID_CREATE:
> > > > +               /*
> > > > +                * 'struct uhid_create_req' contains a __user pointer which is
> > > > +                * copied from, so it's unsafe to allow this with elevated
> > > > +                * privileges (e.g. from a setuid binary) or via kernel_write().
> > > > +                */
> >
> > uhid is a privileged interface so we would go from root to less
> > privileged (if at all). If non-privileged process can open uhid it can
> > construct virtual keyboard and inject whatever keystrokes it wants.
> >
> > Also, instead of disallowing access, can we ensure that we switch back
> > to USER_DS before trying to load data from the user pointer?
>
> Does that even make sense? You are using some deprecated legacy
> interface; you interact with it by splicing a request from something
> like a file or a pipe into the uhid device; but the request you're
> splicing through contains a pointer into userspace memory? Do you know
> of anyone who is actually doing that? If not, anyone who does want to
> do this for some reason in the future can just go use UHID_CREATE2
> instead.

I do not know if anyone is still using UHID_CREATE with sendpage and
neither do you really. It is all about not breaking userspace without
good reason and here ensuring that we switch to USER_DS and then back
to whatever it was does not seem too hard.

>
> > > > +               if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > > +                       pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > > +                                   task_tgid_vnr(current), current->comm);
> > > > +                       ret = -EACCES;
> > > > +                       goto unlock;
> > > > +               }
> > > >                 ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > >                 break;
> > > >         case UHID_CREATE2: