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

From: Deven Bowers [mailto:deven.desai@linux.microsoft.com] Sent: Friday, October 15, 2021 9:26 PM On 10/13/2021 12:24 PM, Eric Biggers wrote: > On Wed, Oct 13, 2021 at 12:06:31PM -0700, deven.desai(a)linux.microsoft.com wrote: >> From: Fan Wu <wufan(a)linux.microsoft.com&gt; >> >> 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.

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. 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. IPE does not want to recalculate a digest, that’s expensive and doesn’t provide any value. IPE, in this case, treats this as a buffer to compare the policy-provided one above to make a policy decision about access to the resource. >> Also changes the implementaion inside the hook function to let >> multiple LSMs can add hooks. > Please split fs/verity/ changes and security/ changes into separate patches, if > possible. Sorry, will do, not a problem. >> @@ -177,6 +178,17 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode, >> fsverity_err(inode, "Error %d computing file digest", err); >> goto out; >> } >> + >> + err = security_inode_setsecurity((struct inode *)inode, > If a non-const inode is needed, please propagate that into the callers rather > than randomly casting away the const. > >> + FS_VERITY_DIGEST_SEC_NAME, >> + vi->file_digest, >> + vi->tree_params.hash_alg- >digest_size, >> + 0); >> @@ -84,7 +85,9 @@ int fsverity_verify_signature(const struct fsverity_info *vi, >> >> pr_debug("Valid signature for file digest %s:%*phN\n", >> hash_alg->name, hash_alg->digest_size, vi->file_digest); >> - return 0; >> + return security_inode_setsecurity((struct inode *)inode, >> > Likewise, please don't cast away const. Sorry, I should've caught these myself. I'll change fsverity_create_info to accept the non-const inode, and change fsverity_verify_signature to accept an additional inode struct as the first arg instead of changing the fsverity_info structure to have a non-const inode field. >> + 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. > and what the LSM hook will do with them? At the moment, this will just signal to IPE that these fs-verity files were enabled with a built-in signature as opposed to enabled without a signature. In v7, it copies the signature data into IPE's LSM blob attached to the inode. In v8+, I'm changing this to store “true” in IPE's LSM blob instead, as copying the signature data is an unnecessary waste of space and point of failure. This has a _slightly_ different functionality then fs.verity.require_signatures, because even if someone were to disable the require signatures option, IPE would still know if these files were signed or not and be able to make the access control decision based IPE's policy. Very concretely, this powers this kind of rule in IPE:   op=EXECUTE fsverity_signature=TRUE action=ALLOW if that fsverity_signature value in IPE’s LSM blob attached to the inode is true, then fsverity_signature in IPE’s policy will evaluate to true and match this rule. The inverse is also applicable.