From cdef685be5b4ae55c3959289e72d520402839c29 Mon Sep 17 00:00:00 2001 From: Roberto Sassu Date: Fri, 14 May 2021 17:27:47 +0200 Subject: evm: Allow xattr/attr operations for portable signatures 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 Signed-off-by: Mimi Zohar --- security/integrity/evm/evm_main.c | 33 +++++++++++++++++++++++++++------ security/integrity/ima/ima_appraise.c | 2 ++ 2 files changed, 29 insertions(+), 6 deletions(-) (limited to 'security/integrity') diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c index 4206c7e492ae..333524e879b5 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; @@ -380,6 +388,14 @@ out: if (evm_hmac_disabled() && (evm_status == INTEGRITY_NOLABEL || evm_status == INTEGRITY_UNKNOWN)) 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", @@ -553,8 +569,13 @@ 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 || evm_status == INTEGRITY_UNKNOWN))) return 0; diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c index 03894769dffa..9bb351b933fb 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; -- cgit v1.2.3