Re: [PATCH 0/4] Fix misused kernel_read_file() enums

Kees Cook
2020-07-09T02:00:56Z

On Wed, Jul 08, 2020 at 01:30:04PM +0000, Luis Chamberlain wrote:
> On Wed, Jul 08, 2020 at 01:58:47PM +0200, Hans de Goede wrote:
> > Hi,
> >
> > On 7/8/20 1:55 PM, Luis Chamberlain wrote:
> > > On Wed, Jul 08, 2020 at 01:37:41PM +0200, Hans de Goede wrote:
> > > > Hi,
> > > >
> > > > On 7/8/20 1:01 PM, Hans de Goede wrote:
> > > > > Hi,
> > > > >
> > > > > On 7/7/20 10:19 AM, Kees Cook wrote:
> > > > > > Hi,
> > > > > >
> > > > > > In looking for closely at the additions that got made to the
> > > > > > kernel_read_file() enums, I noticed that FIRMWARE_PREALLOC_BUFFER
> > > > > > and FIRMWARE_EFI_EMBEDDED were added, but they are not appropriate
> > > > > > *kinds* of files for the LSM to reason about. They are a "how" and
> > > > > > "where", respectively. Remove these improper aliases and refactor the
> > > > > > code to adapt to the changes.
> > > > > >
> > > > > > Additionally adds in missing calls to security_kernel_post_read_file()
> > > > > > in the platform firmware fallback path (to match the sysfs firmware
> > > > > > fallback path) and in module loading. I considered entirely removing
> > > > > > security_kernel_post_read_file() hook since it is technically unused,
> > > > > > but IMA probably wants to be able to measure EFI-stored firmware images,
> > > > > > so I wired it up and matched it for modules, in case anyone wants to
> > > > > > move the module signature checks out of the module core and into an LSM
> > > > > > to avoid the current layering violations.
> > > > > >
> > > > > > This touches several trees, and I suspect it would be best to go through
> > > > > > James's LSM tree.
> > > > > >
> > > > > > Thanks!
> > > > >
> > > > >
> > > > > I've done some quick tests on this series to make sure that
> > > > > the efi embedded-firmware support did not regress.
> > > > > That still works fine, so this series is;
> > > > >
> > > > > Tested-by: Hans de Goede <hdegoede@redhat.com>
> > > >
> > > > I made a mistake during testing I was not actually running the
> > > > kernel with the patches added.
> > > >
> > > > After fixing that I did find a problem, patch 4/4:
> > > > "module: Add hook for security_kernel_post_read_file()"
> > > >
> > > > Breaks module-loading for me. This is with the 4 patches
> > > > on top of 5.8.0-rc4, so this might just be because I'm
> > > > not using the right base.
> > > >
> > > > With patch 4/4 reverted things work fine for me.
> > > >
> > > > So, please only add my Tested-by to patches 1-3.
> > >
> > > BTW is there any testing covered by the selftests for the firmware
> > > laoder which would have caputured this? If not can you extend
> > > it with something to capture this case you ran into?
> >
> > This was not a firmware-loading issue. For me in my tests,
> > which were limited to 1 device, patch 4/4, which only touches
> > the module-loading code, stopped module loading from working.
> >
> > Since my test device has / on an eMMC and the kernel config
> > I'm using has mmc-block as a module, things just hung in the
> > initrd since no modules could be loaded, so I did not debug
> > this any further. Dropping patch 4/4 from my local tree
> > solved this.
>
> Thanks Hans!
>
> Kees, would test_kmod.c and the respective selftest would have picked
> this issue up?

I need to check -- I got a (possibly related) 0day report on it too.

Since I have to clean it up further based on Mimi's comments, and adapt
it a bit for Scott's series, I'll need to get a v2 spun for sure. :)

--
Kees Cook