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

[Dmitry Torokhov]

On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> >
> > From: Eric Biggers <ebiggers@xxxxxxxxxx>
> >
> > 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.
> >
> > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > helpers fault on kernel addresses"), allowing this bug to be found.
> >
> > Reported-by: syzbot+72473edc9bf4eb1c6556@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > Cc: <stable@xxxxxxxxxxxxxxx> # v3.6+
> > Cc: Jann Horn <jannh@xxxxxxxxxx>
> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
>
> > ---
> >  drivers/hid/uhid.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > index 3c55073136064..051639c09f728 100644
> > --- a/drivers/hid/uhid.c
> > +++ b/drivers/hid/uhid.c
> > @@ -12,6 +12,7 @@
> >
> >  #include <linux/atomic.h>
> >  #include <linux/compat.h>
> > +#include <linux/cred.h>
> >  #include <linux/device.h>
> >  #include <linux/fs.h>
> >  #include <linux/hid.h>
> > @@ -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?

> > +               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:
> > --
> > 2.19.1.930.g4563a0d9d0-goog
> >

Thanks.

-- 
Dmitry