| Message ID | 20220404054642.3095732-1-tweek@google.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Paul Moore |
| Headers | show |
| Series | firmware_loader: use kernel credentials when reading firmware | expand |
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(+) Is this a bugfix? if so, what commit does this fix? If not, how has this never been a problem in the past (i.e. what changed to cause problems?) thanks, greg k-h
On Mon, Apr 4, 2022 at 6:33 PM Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote: > > 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(+) > > Is this a bugfix? if so, what commit does this fix? If not, how has > this never been a problem in the past (i.e. what changed to cause > problems?) Not a bug fix. I think this is the combination of 3 factors that surfaces the issue: 1. As mentioned in the last paragraph of the commit, previously, the userspace fallback was used (that is, udev/ueventd would read the firmware file so only this process would need the correct permission to access the files). More devices are using firmware_class.path= which causes the kernel to try to load the firmware directly[1]. 2. Drivers are calling request_firmware when handling a userspace request (such as open() on a device file). Historically, request_firmware seems to have been called when probe() was invoked by the kernel. 3. The precise MAC policy on Android (as opposed to DAC where the firmware files are own by root and a root process accesses the device file). Thanks, [1] https://www.kernel.org/doc/html/v5.17/driver-api/firmware/fw_search_path.html > thanks, > > greg k-h
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
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> I recently hit a problem in AOSP: Normally on the device gralloc is the first processes to open the dri device, which then trips firmware loading. This worked ok because the firmware is tagged as vendor_firmware, and gralloc runs in vendor context and the sepolicy allows that. I had made a change to gralloc, so it no longer opened the dri device, making SurfaceFlinger the first processes to open the dri device. Unfortunately, this caused firmware loading to fail due to the sepolicy blocking SurfaceFlinger from accessing vendor_firmware tagged files. My initial workaround was to add two silly lines to open and close the dri device in the gralloc initialization, just to ensure the first access was in the right context and the firmware would load properly. But Greg pointed me to this patch, which exactly described the problem and resolved it (much more cleanly then my workaround). I can't comment on the change itself, but wanted to say thanks! Tested-by: John Stultz <jstultz@google.com> -john
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); + ret = fw_get_filesystem_firmware(device, fw->priv, "", NULL); /* Only full reads can support decompression, platform, and sysfs. */ @@ -776,6 +785,8 @@ _request_firmware(const struct firmware **firmware_p, const char *name, } else ret = assign_fw(fw, device); + revert_creds(old_cred); + out: if (ret < 0) { fw_abort_batch_reqs(fw);
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(+)