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 @@ -735,6 +735,8 @@ _request_firmware(const struct firmware 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 +753,18 @@ _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);
/* Only full reads can support decompression, platform, and sysfs. */ @@ -776,6 +790,9 @@ _request_firmware(const struct firmware } else ret = assign_fw(fw, device);
+ revert_creds(old_cred); + put_cred(kern_cred); + out: if (ret < 0) { fw_abort_batch_reqs(fw);