[PATCH v6 06/11] evm: Allow xattr/attr operations for portable signatures

Roberto Sassu
2021-05-05T11:33:59Z

If files with portable signatures are copied from one location to another
or are extracted from an archive, verification can temporarily fail until
all xattrs/attrs are set in the destination. Only portable signatures may
be moved or copied from one file to another, as they don't depend on
system-specific information such as the inode generation. Instead portable
signatures must include security.ima.

Unlike other security.evm types, EVM portable signatures are also
immutable. Thus, it wouldn't be a problem to allow xattr/attr operations
when verification fails, as portable signatures will never be replaced with
the HMAC on possibly corrupted xattrs/attrs.

This patch first introduces a new integrity status called
INTEGRITY_FAIL_IMMUTABLE, that allows callers of
evm_verify_current_integrity() to detect that a portable signature didn't
pass verification and then adds an exception in evm_protect_xattr() and
evm_inode_setattr() for this status and returns 0 instead of -EPERM.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
---
include/linux/integrity.h | 1 +
security/integrity/evm/evm_main.c | 33 ++++++++++++++++++++++-----
security/integrity/ima/ima_appraise.c | 2 ++
3 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/include/linux/integrity.h b/include/linux/integrity.h
index 2271939c5c31..2ea0f2f65ab6 100644
--- a/include/linux/integrity.h
+++ b/include/linux/integrity.h
@@ -13,6 +13,7 @@ enum integrity_status {
INTEGRITY_PASS = 0,
INTEGRITY_PASS_IMMUTABLE,
INTEGRITY_FAIL,
+ INTEGRITY_FAIL_IMMUTABLE,
INTEGRITY_NOLABEL,
INTEGRITY_NOXATTRS,
INTEGRITY_UNKNOWN,
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index f625196eee8e..9faebff029e6 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -27,7 +27,8 @@
int evm_initialized;

static const char * const integrity_status_msg[] = {
- "pass", "pass_immutable", "fail", "no_label", "no_xattrs", "unknown"
+ "pass", "pass_immutable", "fail", "fail_immutable", "no_label",
+ "no_xattrs", "unknown"
};
int evm_hmac_attrs;

@@ -155,7 +156,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
enum integrity_status evm_status = INTEGRITY_PASS;
struct evm_digest digest;
struct inode *inode;
- int rc, xattr_len;
+ int rc, xattr_len, evm_immutable = 0;

if (iint && (iint->evm_status == INTEGRITY_PASS ||
iint->evm_status == INTEGRITY_PASS_IMMUTABLE))
@@ -200,8 +201,10 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
if (rc)
rc = -EINVAL;
break;
- case EVM_IMA_XATTR_DIGSIG:
case EVM_XATTR_PORTABLE_DIGSIG:
+ evm_immutable = 1;
+ fallthrough;
+ case EVM_IMA_XATTR_DIGSIG:
/* accept xattr with non-empty signature field */
if (xattr_len <= sizeof(struct signature_v2_hdr)) {
evm_status = INTEGRITY_FAIL;
@@ -238,9 +241,14 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
break;
}

- if (rc)
- evm_status = (rc == -ENODATA) ?
- INTEGRITY_NOXATTRS : INTEGRITY_FAIL;
+ if (rc) {
+ if (rc == -ENODATA)
+ evm_status = INTEGRITY_NOXATTRS;
+ else if (evm_immutable)
+ evm_status = INTEGRITY_FAIL_IMMUTABLE;
+ else
+ evm_status = INTEGRITY_FAIL;
+ }
out:
if (iint)
iint->evm_status = evm_status;
@@ -379,6 +387,14 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
/* Exception if the HMAC is not going to be calculated. */
if (evm_hmac_disabled() && evm_status == INTEGRITY_NOLABEL)
return 0;
+
+ /*
+ * Writing other xattrs is safe for portable signatures, as portable
+ * signatures are immutable and can never be updated.
+ */
+ if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
+ return 0;
+
if (evm_status != INTEGRITY_PASS)
integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
dentry->d_name.name, "appraise_metadata",
@@ -539,8 +555,13 @@ int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
if (!(ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)))
return 0;
evm_status = evm_verify_current_integrity(dentry);
+ /*
+ * Writing attrs is safe for portable signatures, as portable signatures
+ * are immutable and can never be updated.
+ */
if ((evm_status == INTEGRITY_PASS) ||
(evm_status == INTEGRITY_NOXATTRS) ||
+ (evm_status == INTEGRITY_FAIL_IMMUTABLE) ||
(evm_hmac_disabled() && evm_status == INTEGRITY_NOLABEL))
return 0;
integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index ddac9e727d3a..6d0983a97711 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -416,6 +416,8 @@ int ima_appraise_measurement(enum ima_hooks func,
case INTEGRITY_NOLABEL: /* No security.evm xattr. */
cause = "missing-HMAC";
goto out;
+ case INTEGRITY_FAIL_IMMUTABLE:
+ fallthrough;
case INTEGRITY_FAIL: /* Invalid HMAC/signature. */
cause = "invalid-HMAC";
goto out;
--
2.25.1