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

[Eric Biggers]

Hi Dmitry,

On Wed, Nov 14, 2018 at 02:28:56PM -0800, 'Dmitry Torokhov' via syzkaller-bugs wrote:
> 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?
> 

Actually uhid doesn't have any capability checks, so it's up to userspace to
assign permissions to the device node.  I think it's best not to make
assumptions about how the interface will be used and to be consistent with how
other ->write() methods in the kernel handle the misfeature where a __user
pointer in the write() or read() payload is dereferenced.  Temporarily switching
to USER_DS would only avoid one of the two problems.

Do you think the proposed restrictions would actually break anything?

- Eric

> > > +               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:
> > > --