[PATCH v3 03/10] 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 type 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-
system since the metadata their signature covers does not include file-
system-specific data such as a file's inode number, generation, and UUID.

Co-developed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
---
 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 b0a862bfd74a..c658d2f1494b 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -948,9 +948,34 @@ static void evm_inode_post_setattr(struct mnt_idmap *idmap,
 
 static 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