fsverity: expose verified fsverity built-in signatures to LSMs

This patch enhances fsverity's capabilities to support both integrity and
authenticity protection by introducing the exposure of built-in
signatures through a new LSM hook. This functionality allows LSMs,
e.g. IPE, to enforce policies based on the authenticity and integrity of
files, specifically focusing on built-in fsverity signatures. It enables
a policy enforcement layer within LSMs for fsverity, offering granular
control over the usage of authenticity claims. For instance, a policy
could be established to only permit the execution of all files with
verified built-in fsverity signatures.

The introduction of a security_inode_setintegrity() hook call within
fsverity's workflow ensures that the verified built-in signature of a file
is exposed to LSMs. This enables LSMs to recognize and label fsverity files
that contain a verified built-in fsverity signature. This hook is invoked
subsequent to the fsverity_verify_signature() process, guaranteeing the
signature's verification against fsverity's keyring. This mechanism is
crucial for maintaining system security, as it operates in kernel space,
effectively thwarting attempts by malicious binaries to bypass user space
stack interactions.

The second to last commit in this patch set will add a link to the IPE
documentation in fsverity.rst.

Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
Acked-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 17 insertions, 1 deletions

diff --git a/fs/verity/signature.c b/fs/verity/signature.c
index 90c07573dd77..0302a4e506ec 100644
--- a/fs/verity/signature.c
+++ b/fs/verity/signature.c
@@ -17,6 +17,7 @@
 
 #include <linux/cred.h>
 #include <linux/key.h>
+#include <linux/security.h>
 #include <linux/slab.h>
 #include <linux/verification.h>
 
@@ -41,7 +42,11 @@ static struct key *fsverity_keyring;
  * @sig_size: size of signature in bytes, or 0 if no signature
  *
  * If the file includes a signature of its fs-verity file digest, verify it
- * against the certificates in the fs-verity keyring.
+ * against the certificates in the fs-verity keyring. Note that signatures
+ * are verified regardless of the state of the 'fsverity_require_signatures'
+ * variable and the LSM subsystem relies on this behavior to help enforce
+ * file integrity policies. Please discuss changes with the LSM list
+ * (thank you!).
  *
  * Return: 0 on success (signature valid or not required); -errno on failure
  */
@@ -106,6 +111,17 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
 		return err;
 	}
 
+	err = security_inode_setintegrity(inode,
+					  LSM_INT_FSVERITY_BUILTINSIG_VALID,
+					  signature,
+					  sig_size);
+
+	if (err) {
+		fsverity_err(inode, "Error %d exposing file signature to LSMs",
+			     err);
+		return err;
+	}
+
 	return 0;
 }