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Subject[PATCH v5 07/12] evm: Allow xattr/attr operations for portable signatures
Date
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 | 31 +++++++++++++++++++++------
security/integrity/ima/ima_appraise.c | 2 ++
3 files changed, 28 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 6556e8c22da9..eab536fa260f 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 +241,12 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
break;
}

- if (rc)
- evm_status = (rc == -ENODATA) ?
- INTEGRITY_NOXATTRS : INTEGRITY_FAIL;
+ if (rc) {
+ evm_status = INTEGRITY_NOXATTRS;
+ if (rc != -ENODATA)
+ evm_status = evm_immutable ?
+ INTEGRITY_FAIL_IMMUTABLE : INTEGRITY_FAIL;
+ }
out:
if (iint)
iint->evm_status = evm_status;
@@ -374,6 +380,14 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
out:
if (evm_ignore_error_safe(evm_status))
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",
@@ -534,8 +548,13 @@ 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_ignore_error_safe(evm_status)))
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 d4b8db1acadd..24d59893aab0 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;
--
2.26.2
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