> From: Roberto Sassu [mailto:roberto.sassu@xxxxxxxxxx] > Sent: Wednesday, October 20, 2021 5:09 PM > > From: Eric Biggers [mailto:ebiggers@xxxxxxxxxx] > > 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@xxxxxxxxxxxxxxxxxxx wrote: > > > > > From: Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx> > > > > > > > > > > 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 sha256:079be6d88638e58141ee24bba89813917c44faa55ada4bf5d80335efe1547803 /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). 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@xxxxxxxxxx/ > > Thanks > > Roberto > > HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 > Managing Director: Li Peng, Zhong Ronghua > > > > This brings me to your next comment: > > > > > > > The digest isn't meaningful without knowing the hash algorithm it uses. > > > It's available here, but you aren't passing it to this function. > > > > > > The digest is meaningful without the algorithm in this case. > > > > No, it's not. > > > > Digests are meaningless without knowing what algorithm they were created > > with. > > > > If your security policy is something like "Trust the file with digest $foo" and > > multiple hash algorithms are possible, then the alorithm intended to be used > > needs to be explicitly specified. Otherwise any algorithm with the same length > > digest will be accepted. That's a fatal flaw if any of these algorithms is > > cryptographically broken or was never intended to be a cryptographic > algorithm > > in the first place (e.g., a non-cryptographic checksum). > > > > Cryptosystems always need to specify the crypto algorithm(s) used; the > > adversary > > must not be allowed to choose the algorithms. > > > > I'm not sure how these patches can be taken seriously when they're getting > this > > sort of thing wrong. > > > > > > > + > FS_VERITY_SIGNATURE_SEC_NAME, > > > > > + signature, sig_size, 0); > > > > This is only for fs-verity built-in signatures which aren't the only way to do > > > > signatures with fs-verity. Are you sure this is what you're looking for? > > > > > > Could you elaborate on the other signature types that can be used > > > with fs-verity? I’m 99% sure this is what I’m looking for as this > > > is a signature validated in the kernel against the fs-verity keyring > > > as part of the “fsverity enable” utility. > > > > > > It's important that the signature is validated in the kernel, as > > > userspace is considered untrusted until the signature is validated > > > for this case. > > > > > > > Can you elaborate on your use case for fs-verity built-in signatures, > > > Sure, signatures, like digests, also provide a way to prove integrity, > > > and the trust component comes from the validation against the keyring, > > > as opposed to a fixed value in IPE’s policy. The use case for fs-verity > > > built-in signatures is that we have a rw ext4 filesystem that has some > > > executable files, and we want to have a execution policy (through IPE) > > > that only _trusted_ executables can run. Perf is important here, hence > > > fs-verity. > > > > Most users of fs-verity built-in signatures have actually been enforcing their > > security policy in userspace, by checking whether specific files have the > > fs-verity bit set or not. Such users could just store and verify signatures in > > userspace instead, without any kernel involvement. So that's what I've been > > recommending (with limited success, unfortunately). > > > > If you really do need in-kernel signature verification, then that may be a > > legitimate use case for the fs-verity built-in signatures, although I do wonder > > why you aren't using IMA and its signature mechanism instead. > > > > - Eric
