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evm: Allow xattr/attr operations for portable signatures

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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>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
This commit is contained in:
Roberto Sassu 2021-05-14 17:27:47 +02:00 committed by Mimi Zohar
parent 4a804b8a45
commit cdef685be5
3 changed files with 30 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/integrity.h
View File

@ -13,6 +13,7 @@ enum integrity_status {
INTEGRITY_PASS = 0,
INTEGRITY_PASS_IMMUTABLE,
INTEGRITY_FAIL,
INTEGRITY_FAIL_IMMUTABLE,
INTEGRITY_NOLABEL,
INTEGRITY_NOXATTRS,
INTEGRITY_UNKNOWN,

33
security/integrity/evm/evm_main.c
View File

@ -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;

2
security/integrity/ima/ima_appraise.c
View File

@ -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;