[PATCH v2 3/9] evm: Implement per signature type decision in security_inode_copy_up_xattr

[Stefan Berger]

To support portable and immutable signatures on otherwise unsupported
filesystems, determine the EVM signature type by the content of a file's
xattr. If the file has the appropriate signature then allow it to be
copied up. All other signature types are discarded as before.

"Portable and immutable" EVM signatures can be copied up by stacked file-
systems since the metadata their signature covers does not include file-
system-specific data such as a file's inode number, generation, and UUID.

Signed-off-by: Stefan Berger <stefanb at linux.ibm.com>
---
 security/integrity/evm/evm_main.c | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 2555aa4501ae..565c36471408 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -898,9 +898,34 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
 
 int evm_inode_copy_up_xattr(struct dentry *src, const char *name)
 {
-	if (strcmp(name, XATTR_NAME_EVM) == 0)
-		return 1; /* Discard */
-	return -EOPNOTSUPP;
+	struct evm_ima_xattr_data *xattr_data = NULL;
+	int rc;
+
+	if (strcmp(name, XATTR_NAME_EVM) != 0)
+		return -EOPNOTSUPP;
+
+	/* first need to know the sig type */
+	rc = vfs_getxattr_alloc(&nop_mnt_idmap, src, XATTR_NAME_EVM,
+				(char **)&xattr_data, 0, GFP_NOFS);
+	if (rc <= 0)
+		return -EPERM;
+
+	if (rc < offsetof(struct evm_ima_xattr_data, type) +
+		 sizeof(xattr_data->type))
+		return -EPERM;
+
+	switch (xattr_data->type) {
+	case EVM_XATTR_PORTABLE_DIGSIG:
+		rc = 0; /* allow copy-up */
+		break;
+	case EVM_XATTR_HMAC:
+	case EVM_IMA_XATTR_DIGSIG:
+	default:
+		rc = 1; /* discard */
+	}
+
+	kfree(xattr_data);
+	return rc;
 }
 
 /*
-- 
2.43.0