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

[Jann Horn]

+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).

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...