[PATCH 5.4 62/68] firmware_loader: use kernel credentials when reading firmware

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

From: Thiébaud Weksteen <tweek@google.com>

commit 581dd69830341d299b0c097fc366097ab497d679 upstream.

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: Thiébaud Weksteen <tweek@google.com>
Cc: <stable@vger.kernel.org> # 5.10
Reviewed-by: Paul Moore <paul@paul-moore.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20220502004952.3970800-1-tweek@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/base/firmware_loader/main.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -761,6 +761,8 @@ _request_firmware(const struct firmware
 		  enum fw_opt opt_flags)
 {
 	struct firmware *fw = NULL;
+	struct cred *kern_cred = NULL;
+	const struct cred *old_cred;
 	int ret;
 
 	if (!firmware_p)
@@ -776,6 +778,18 @@ _request_firmware(const struct firmware
 	if (ret <= 0) /* error or already assigned */
 		goto out;
 
+	/*
+	 * We are about to try to access the firmware file. Because we may have been
+	 * called by a driver when serving an unrelated request from userland, we use
+	 * the kernel credentials to read the file.
+	 */
+	kern_cred = prepare_kernel_cred(NULL);
+	if (!kern_cred) {
+		ret = -ENOMEM;
+		goto out;
+	}
+	old_cred = override_creds(kern_cred);
+
 	ret = fw_get_filesystem_firmware(device, fw->priv, "", NULL);
 #ifdef CONFIG_FW_LOADER_COMPRESS
 	if (ret == -ENOENT)
@@ -792,6 +806,9 @@ _request_firmware(const struct firmware
 	} else
 		ret = assign_fw(fw, device, opt_flags);
 
+	revert_creds(old_cred);
+	put_cred(kern_cred);
+
  out:
 	if (ret < 0) {
 		fw_abort_batch_reqs(fw);