Re: [PATCH v5 2/5] efi: Add embedded peripheral firmware support

Mimi Zohar
2018-05-01T19:27:27Z

On Tue, 2018-05-01 at 21:11 +0200, Hans de Goede wrote:
> Hi,
>
> On 01-05-18 16:36, Mimi Zohar wrote:
> > [Cc'ing linux-security]
> >
> > On Sun, 2018-04-29 at 11:35 +0200, Hans de Goede wrote:
> > [...]
> >> diff --git a/drivers/base/firmware_loader/fallback_efi.c b/drivers/base/firmware_loader/fallback_efi.c
> >> new file mode 100644
> >> index 000000000000..82ba82f48a79
> >> --- /dev/null
> >> +++ b/drivers/base/firmware_loader/fallback_efi.c
> >> @@ -0,0 +1,51 @@
> >> +// SPDX-License-Identifier: GPL-2.0
> >> +
> >> +#include <linux/efi_embedded_fw.h>
> >> +#include <linux/property.h>
> >> +#include <linux/security.h>
> >> +#include <linux/vmalloc.h>
> >> +
> >> +#include "fallback.h"
> >> +#include "firmware.h"
> >> +
> >> +int fw_get_efi_embedded_fw(struct device *dev, struct fw_priv *fw_priv,
> >> + enum fw_opt *opt_flags, int ret)
> >> +{
> >> + enum kernel_read_file_id id = READING_FIRMWARE;
> >
> > Please define a new kernel_read_file_id for this (eg.
> > READING_FIRMWARE_EFI_EMBEDDED).
>
> Are you sure, I wonder how useful it is to add a new
> kernel_read_file_id every time a new way to get firmware
> comes up?
>
> I especially wonder about the sense in adding a new id
> given that the quite old FIRMWARE_PREALLOC_BUFFER is
> still not supported / checked properly by the security code.

I posted patches earlier today[1], which address this.  Patch 5/6 just
makes it equivalent to READING_FIRMWARE.  Patch 6/6 questions whether
the device has access to the pre-allocated buffer *before* the
signature has been verified.

[1] kernsec.org/pipermail/linux-security-module-archive/2018-May/006639.html

>
> Anyways I can add a new id if you want me to, what about
> when fw_get_efi_embedded_fw is reading into a driver allocated
> buffer, do you want a separate EADING_FIRMWARE_EFI_EMBEDDED_PREALLOC_BUFFER
> for that ?

Without the kernel being able to verify the firmware's signature, I'm
not sure it makes much of a difference.

>
> >
> >> + size_t size, max = INT_MAX;
> >> + int rc;
> >> +
> >> + if (!dev)
> >> + return ret;
> >> +
> >> + if (!device_property_read_bool(dev, "efi-embedded-firmware"))
> >> + return ret;
> >
> > Instead of calling security_kernel_post_read_file(), either in
> > device_property_read_bool() or here call security_kernel_read_file().
> >
> > The pre read call is for deciding whether to allow this call
> > independent of the firmware being loaded, whereas the post security
> > call is currently being used by IMA-appraisal for verifying a
> > signature.  There might be other LSMs using the post hook as well.  As
> > there is no kernel signature associated with this firmware, use the
> > security pre read_file hook.
>
> Only the pre hook? I believe the post-hook should still be called too,
> right? So that we've hashes of all loaded firmwares in the IMA core.

Good catch!  Right, if IMA-measurement is enabled, then we would want
to add the measurement.

Mimi