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

[Dmitry Torokhov <dtor@google.com>]

On Wed, Nov 14, 2018 at 10:03 AM Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> 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.

Hmm,  instead  of disallowing access, can we switch back to USER_DS
before trying to use the user pointer?

>
>
> 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@syzkaller.appspotmail.com
> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <stable@vger.kernel.org> # v3.6+
> Cc: Jann Horn <jannh@google.com>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  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;
> +       }
> +
>         /* we need at least the "type" member of uhid_event */
>         if (count < sizeof(__u32))
>                 return -EINVAL;
> --
> 2.19.1.930.g4563a0d9d0-goog
>