Re: [PATCH] HID: uhid: prevent uhid_char_write() under KERNEL_DS

[Eric Biggers]

On Wed, Nov 14, 2018 at 07:18:39PM +0100, 'Jann Horn' via syzkaller-bugs wrote:
> +cc Andy
> 
> On Wed, Nov 14, 2018 at 7:03 PM Eric Biggers <ebiggers@xxxxxxxxxx> 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
> > sendfile(), this can read from kernel memory.  Therefore, UHID_CREATE
> > must not be allowed in this case.
> >
> > For consistency and to make sure all current and future uhid commands
> > are covered, apply the restriction to uhid_char_write() as a whole
> > rather than to UHID_CREATE specifically.
> >
> > 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
> 
> Wheeeee, it found something! :)
> 
> > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > Cc: <stable@xxxxxxxxxxxxxxx> # v3.6+
> > Cc: Jann Horn <jannh@xxxxxxxxxx>
> > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> > ---
> >  drivers/hid/uhid.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > index 3c55073136064..e94c5e248b56e 100644
> > --- a/drivers/hid/uhid.c
> > +++ b/drivers/hid/uhid.c
> > @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> >         int ret;
> >         size_t len;
> >
> > +       if (uaccess_kernel()) { /* payload may contain a __user pointer */
> > +               pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n",
> > +                           __func__, task_tgid_vnr(current), current->comm);
> > +               return -EACCES;
> > +       }
> 
> If this file can conceivably be opened by a process that doesn't have
> root privileges, this check should be something along the lines of
> ib_safe_file_access() or sg_check_file_access().
> 
> Checking for uaccess_kernel() prevents the symptom that syzkaller
> notices - a user being able to cause a kernel memory access -, but it
> doesn't deal with the case where a user opens a file descriptor to
> this device and tricks a more privileged process into writing into it
> (e.g. by passing it to a suid binary as stdout or stderr).
> 

Yep, I'll do that.

> Looking closer, I wonder whether this kind of behavior is limited to
> the UHID_CREATE request, which has a comment on it saying "/*
> Obsolete! Use UHID_CREATE2. */". If we could keep this kind of ugly
> kludge away from the code paths you're supposed to be using, that
> would be nice...
> 

I wanted to be careful, but yes AFAICS it can be limited to UHID_CREATE only,
so I'll do that instead.

- Eric