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From:   Paul Moore <paul@paul-moore.com>
Date:   Tue, 3 May 2022 13:58:09 -0400
Message-ID: <CAHC9VhQJ1xH4iwSXFdQnCobJMv5Kniam3XWG8-UL=VcmdFwEcg@mail.gmail.com>
Subject: Re: [PATCH v4] firmware_loader: use kernel credentials when reading firmware
To:     =?UTF-8?Q?Thi=C3=A9baud_Weksteen?= <tweek@google.com>
Cc:     Luis Chamberlain <mcgrof@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Qian Cai <quic_qiancai@quicinc.com>,
        John Stultz <jstultz@google.com>,
        Jeffrey Vander Stoep <jeffv@google.com>,
        Saravana Kannan <saravanak@google.com>,
        Alistair Delva <adelva@google.com>,
        Adam Shih <adamshih@google.com>, selinux@vger.kernel.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
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On Sun, May 1, 2022 at 8:50 PM Thi=C3=A9baud Weksteen <tweek@google.com> wr=
ote:
>
> 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).
>
> Previously, Android configurations were not setting up the
> firmware_class.path command line argument and were relying on the
> userspace fallback mechanism. In this case, the security context of the
> userspace daemon (i.e. ueventd) was consistently used to read firmware
> files. More Android devices are now found to set firmware_class.path
> which gives the kernel the opportunity to read the firmware directly
> (via kernel_read_file_from_path_initns). In this scenario, the current
> process credentials were used, even if unrelated to the loading of the
> firmware file.
>
> Signed-off-by: Thi=C3=A9baud Weksteen <tweek@google.com>
> Cc: <stable@vger.kernel.org> # 5.10
> ---
> v4: Add stable to Cc
> v3:
>         - Add call to put_cred to avoid a memory leak. Confirmed that no =
new
>                 memory leak occurs on a Pixel 4a.
>         - Update commit log.
> v2: Add comment
>
>  drivers/base/firmware_loader/main.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)

Reviewed-by: Paul Moore <paul@paul-moore.com>

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