RE: [RFC PATCH v7 12/16] fsverity|security: add security hooks to fsverity digest and signature

Roberto Sassu
2021-10-27T08:42:02Z

> From: Deven Bowers [mailto:deven.desai@linux.microsoft.com]
> Sent: Tuesday, October 26, 2021 9:04 PM
> On 10/22/2021 9:31 AM, Roberto Sassu wrote:
> >> From: Roberto Sassu [mailto:roberto.sassu@huawei.com]
> >> Sent: Wednesday, October 20, 2021 5:09 PM
> >>> From: Eric Biggers [mailto:ebiggers@kernel.org]
> >>> Sent: Friday, October 15, 2021 10:11 PM
> >>> On Fri, Oct 15, 2021 at 12:25:53PM -0700, Deven Bowers wrote:
> >>>> On 10/13/2021 12:24 PM, Eric Biggers wrote:
> >>>>> On Wed, Oct 13, 2021 at 12:06:31PM -0700,
> >>> deven.desai@linux.microsoft.com wrote:
> >>>>>> From: Fan Wu<wufan@linux.microsoft.com>
> >>>>>>
> >>>>>> Add security_inode_setsecurity to fsverity signature verification.
> >>>>>> This can let LSMs save the signature data and digest hashes provided
> >>>>>> by fsverity.
> >>>>> Can you elaborate on why LSMs need this information?
> >>>> The proposed LSM (IPE) of this series will be the only one to need
> >>>> this information at the  moment. IPE’s goal is to have provide
> >>>> trust-based access control. Trust and Integrity are tied together,
> >>>> as you cannot prove trust without proving integrity.
> >>> I think you mean authenticity, not integrity?
> >>>
> >>> Also how does this differ from IMA? I know that IMA doesn't support fs-
> verity
> >>> file hashes, but that could be changed. Why not extend IMA to cover your
> use
> >>> case(s)?
> >>>
> >>>> IPE needs the digest information to be able to compare a digest
> >>>> provided by the policy author, against the digest calculated by
> >>>> fsverity to make a decision on whether that specific file, represented
> >>>> by the digest is authorized for the actions specified in the policy.
> >>>>
> >>>> A more concrete example, if an IPE policy author writes:
> >>>>
> >>>>     op=EXECUTE fsverity_digest=<HexDigest > action=DENY
> >>>>
> >>>> IPE takes the digest provided by this security hook, stores it
> >>>> in IPE's security blob on the inode. If this file is later
> >>>> executed, IPE compares the digest stored in the LSM blob,
> >>>> provided by this hook, against <HexDigest> in the policy, if
> >>>> it matches, it denies the access, performing a revocation
> >>>> of that file.
> >>> Do you have a better example? This one is pretty useless since one can get
> >>> around it just by executing a file that doesn't have fs-verity enabled.
> >> I was wondering if the following use case can be supported:
> >> allow the execution of files protected with fsverity if the root
> >> digest is found among reference values (instead of providing
> >> them one by one in the policy).
> >>
> >> Something like:
> >>
> >> op=EXECUTE fsverity_digest=diglim action=ALLOW
> > Looks like it works. I modified IPE to query the root digest
> > of an fsverity-protected file in DIGLIM.
> >
> > # cat ipe-policy
> > policy_name="AllowFSVerityKmodules" policy_version=0.0.1
> > DEFAULT action=ALLOW
> > DEFAULT op=KMODULE action=DENY
> > op=KMODULE fsverity_digest=diglim action=ALLOW
> >
> > IPE setup:
> > # cat ipe-policy.p7s > /sys/kernel/security/ipe/new_policy
> > # echo -n 1 > /sys/kernel/security/ipe/policies/AllowFSVerityKmodules/active
> > # echo 1 > /sys/kernel/security/ipe/enforce
> >
> > IPE denies loading of kernel modules not protected by fsverity:
> > # insmod /lib/modules/5.15.0-rc1+/kernel/fs/fat/fat.ko
> > insmod: ERROR: could not insert module /lib/modules/5.15.0-
> rc1+/kernel/fs/fat/fat.ko: Permission denied
> >
> > Protect fat.ko with fsverity:
> > # cp /lib/modules/5.15.0-rc1+/kernel/fs/fat/fat.ko /fsverity
> > # fsverity enable /fsverity/fat.ko
> > # fsverity measure /fsverity/fat.ko
> >
> sha256:079be6d88638e58141ee24bba89813917c44faa55ada4bf5d80335efe154
> 7803 /fsverity/fat.ko
> >
> > IPE still denies the loading of fat.ko (root digest not uploaded to the kernel):
> > # insmod /fsverity/fat.ko
> > insmod: ERROR: could not insert module /fsverity/fat.ko: Permission denied
> >
> > Generate a digest list with the root digest above and upload it to the kernel:
> > # ./compact_gen -i
> 079be6d88638e58141ee24bba89813917c44faa55ada4bf5d80335efe1547803 -a
> sha256 -d test -s -t file -f
> > # echo $PWD/test/0-file_list-compact-
> 079be6d88638e58141ee24bba89813917c44faa55ada4bf5d80335efe1547803 >
> /sys/kernel/security/integrity/diglim/digest_list_add
> >
> > IPE allows the loading of fat.ko:
> > # insmod /fsverity/fat.ko
> > #
> >
> > Regarding authenticity, not shown in this demo, IPE will also
> > ensure that the root digest is signed (diglim_digest_get_info()
> > reports this information).
>
> I apologize for the delay in responses, but it looks like
> you've figured it out.

No problem.

> This kind of thing is exactly what IPE's design is supposed to
> solve, you have some other system which provides the
> integrity mechanism and (optionally) determine if it is trusted or
> not, and IPE can provide the policy aspect very easily to
> make a set of system-wide requirements around your mechanism.
>
> I'm very supportive of adding the functionality, but I wonder
> if it makes more sense to have digilm's extension be a separate
> key instead of tied to the fsverity_digest key - something like
>
> op=EXECUTE diglim_fsverity=TRUE action=DENY
>
> that way the condition that enables this property can depend
> on digilm in the build, and it separates the two systems'
> integrations in a slightly more clean way.

Yes, I agree. It is much more clean.

> As an aside, did you find it difficult to extend?

No, it was very straightforward.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Zhong Ronghua

> > Roberto
> >
> > HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
> > Managing Director: Li Peng, Zhong Ronghua
> >
> >> DIGLIM is a component I'm working on that generically
> >> stores digests. The current use case is to store file digests
> >> from RPMTAG_FILEDIGESTS and use them with IMA, but
> >> the fsverity use case could be easily supported (if the root
> >> digest is stored in the RPM header).
> >>
> >> DIGLIM also tells whether or not the signature of the source
> >> containing file digests (or fsverity digests) is valid (the signature
> >> of the RPM header is taken from RPMTAG_RSAHEADER).
> >>
> >> The memory occupation is relatively small for executables
> >> and shared libraries. I published a demo for Fedora and
> >> openSUSE some time ago:
> >>
> >> https://lore.kernel.org/linux-
> >> integrity/48cd737c504d45208377daa27d625531@huawei.com/
> >>
> >> Thanks
> >>
> >> Roberto
> >>
> >> HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
> >> Managing Director: Li Peng, Zhong Ronghua