[PATCH v2 6/9] evm: Enforce signatures on unsupported filesystem for EVM_INIT_X509

Stefan Berger
2024-02-05T18:25:37Z

Unsupported filesystems currently do not enforce any signatures. Add
support for signature enforcement of the "original" and "portable &
immutable" signatures when EVM_INIT_X509 is enabled.

The "original" signature type contains filesystem specific metadata.
Thus it cannot be copied up and verified. However with EVM_INIT_X509
and EVM_ALLOW_METADATA_WRITES enabled, the "original" file signature
may be written.

When EVM_ALLOW_METADATA_WRITES is not set or once it is removed from
/sys/kernel/security/evm by setting EVM_INIT_HMAC for example, it is not
possible to write or remove xattrs on the overlay filesystem.

This change still prevents EVM from writing HMAC signatures on
unsupported filesystem when EVM_INIT_HMAC is enabled.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
---
security/integrity/evm/evm_main.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 81c94967f136..c3bd88aba78c 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -192,7 +192,11 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
iint->evm_status == INTEGRITY_PASS_IMMUTABLE))
return iint->evm_status;

- if (is_unsupported_fs(dentry))
+ /*
+ * On unsupported filesystems with EVM_INIT_X509 not enabled, skip
+ * signature verification.
+ */
+ if (!(evm_initialized & EVM_INIT_X509) && is_unsupported_fs(dentry))
return INTEGRITY_UNKNOWN;

/* if status is not PASS, try to check again - against -ENOMEM */
@@ -262,7 +266,8 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
evm_status = INTEGRITY_PASS_IMMUTABLE;
} else if (!IS_RDONLY(inode) &&
!(inode->i_sb->s_readonly_remount) &&
- !IS_IMMUTABLE(inode)) {
+ !IS_IMMUTABLE(inode) &&
+ !is_unsupported_fs(dentry)) {
evm_update_evmxattr(dentry, xattr_name,
xattr_value,
xattr_value_len);
@@ -422,9 +427,6 @@ enum integrity_status evm_verifyxattr(struct dentry *dentry,
if (!evm_key_loaded() || !evm_protected_xattr(xattr_name))
return INTEGRITY_UNKNOWN;

- if (is_unsupported_fs(dentry))
- return INTEGRITY_UNKNOWN;
-
if (!iint) {
iint = integrity_iint_find(d_backing_inode(dentry));
if (!iint)
--
2.43.0