Re: [PATCH] firmware_loader: use kernel credentials when reading firmware

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Apr 04, 2022 at 03:46:42PM +1000, Thiébaud Weksteen wrote:
> Device drivers may decide to not load firmware when probed to avoid
> slowing down the boot process should the firmware filesystem not be
> available yet. In this case, the firmware loading request may be done
> when a device file associated with the driver is first accessed. The
> credentials of the userspace process accessing the device file may be
> used to validate access to the firmware files requested by the driver.
> Ensure that the kernel assumes the responsibility of reading the
> firmware.
> 
> This was observed on Android for a graphic driver loading their firmware
> when the device file (e.g. /dev/mali0) was first opened by userspace
> (i.e. surfaceflinger). The security context of surfaceflinger was used
> to validate the access to the firmware file (e.g.
> /vendor/firmware/mali.bin).
> 
> Because previous configurations were relying on the userspace fallback
> mechanism, the security context of the userspace daemon (i.e. ueventd)
> was consistently used to read firmware files. More devices are found to
> use the command line argument firmware_class.path which gives the kernel
> the opportunity to read the firmware directly, hence surfacing this
> misattribution.
> 
> Signed-off-by: Thiébaud Weksteen <tweek@google.com>
> ---
>  drivers/base/firmware_loader/main.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
> index 94d1789a233e..416ee3cc6584 100644
> --- a/drivers/base/firmware_loader/main.c
> +++ b/drivers/base/firmware_loader/main.c
> @@ -735,6 +735,8 @@ _request_firmware(const struct firmware **firmware_p, const char *name,
>  		  size_t offset, u32 opt_flags)
>  {
>  	struct firmware *fw = NULL;
> +	struct cred *kern_cred = NULL;
> +	const struct cred *old_cred;
>  	bool nondirect = false;
>  	int ret;
>  
> @@ -751,6 +753,13 @@ _request_firmware(const struct firmware **firmware_p, const char *name,
>  	if (ret <= 0) /* error or already assigned */
>  		goto out;
>  
> +	kern_cred = prepare_kernel_cred(NULL);
> +	if (!kern_cred) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +	old_cred = override_creds(kern_cred);

Can you add a comment here before the call to prepare_kernel_cred() to
say why you are doing this and what it is for?  Otherwise it is not
obvious at all.

thanks,

greg k-h