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. Portable signatures are the only ones that can be moved to different files, as they don't depend on system-specific information such as the inode generation.
Unlike other security.evm types, portable signatures can never be replaced even if an xattr/attr operation is granted, as once evm_update_evmxattr() detects this type, it returns without updating the HMAC. Thus, it wouldn't be a problem to allow those operations so that verification passes on the destination after all xattrs/attrs are copied. 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 <[email protected]> --- include/linux/integrity.h | 1 + security/integrity/evm/evm_main.c | 25 ++++++++++++++++++++----- security/integrity/ima/ima_appraise.c | 1 + 3 files changed, 22 insertions(+), 5 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 4e9f5e8b21d5..30072030f05d 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; @@ -134,7 +135,7 @@ 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)) @@ -179,8 +180,10 @@ 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; @@ -219,7 +222,8 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry, if (rc) evm_status = (rc == -ENODATA) ? - INTEGRITY_NOXATTRS : INTEGRITY_FAIL; + INTEGRITY_NOXATTRS : evm_immutable ? + INTEGRITY_FAIL_IMMUTABLE : INTEGRITY_FAIL; out: if (iint) iint->evm_status = evm_status; @@ -351,6 +355,12 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name, -EPERM, 0); } out: + /* It is safe to allow fail_immutable, portable signatures 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", @@ -488,9 +498,14 @@ 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); + /* It is safe to allow fail_immutable, portable signatures can never + * be updated + */ if ((evm_status == INTEGRITY_PASS) || - (evm_status == INTEGRITY_NOXATTRS)) + (evm_status == INTEGRITY_NOXATTRS) || + (evm_status == INTEGRITY_FAIL_IMMUTABLE)) return 0; + integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry), dentry->d_name.name, "appraise_metadata", integrity_status_msg[evm_status], -EPERM, 0); diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c index a9649b04b9f1..21bda264fc30 100644 --- a/security/integrity/ima/ima_appraise.c +++ b/security/integrity/ima/ima_appraise.c @@ -393,6 +393,7 @@ int ima_appraise_measurement(enum ima_hooks func, case INTEGRITY_NOLABEL: /* No security.evm xattr. */ cause = "missing-HMAC"; goto out; + case INTEGRITY_FAIL_IMMUTABLE: case INTEGRITY_FAIL: /* Invalid HMAC/signature. */ cause = "invalid-HMAC"; goto out; -- 2.17.1
