| Message ID | 20221201104125.919483-7-roberto.sassu@huaweicloud.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Paul Moore |
| Headers | show |
| Series | evm: Do HMAC of multiple per LSM xattrs for new inodes | expand |
On Thu, 2022-12-01 at 11:41 +0100, Roberto Sassu wrote: > From: Roberto Sassu <roberto.sassu@huawei.com> > > Currently, evm_inode_init_security() processes a single LSM xattr from > the array passed by security_inode_init_security(), and calculates the > HMAC on it and other inode metadata. > > Given that initxattrs() callbacks, called by > security_inode_init_security(), expect that this array is terminated when > the xattr name is set to NULL, reuse the same assumption to scan all xattrs > and to calculate the HMAC on all of them. > > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com> > Reviewed-by: Casey Schaufler <casey@schaufler-ca.com> Normally changing the contents of the EVM HMAC calculation would break existing systems. Assuming for the time being this is safe, at what point will it affect backwards compatability? Should it be documented now or then?
On Sun, 2023-02-19 at 14:42 -0500, Mimi Zohar wrote: > On Thu, 2022-12-01 at 11:41 +0100, Roberto Sassu wrote: > > From: Roberto Sassu <roberto.sassu@huawei.com> > > > > Currently, evm_inode_init_security() processes a single LSM xattr from > > the array passed by security_inode_init_security(), and calculates the > > HMAC on it and other inode metadata. > > > > Given that initxattrs() callbacks, called by > > security_inode_init_security(), expect that this array is terminated when > > the xattr name is set to NULL, reuse the same assumption to scan all xattrs > > and to calculate the HMAC on all of them. > > > > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com> > > Reviewed-by: Casey Schaufler <casey@schaufler-ca.com> > > Normally changing the contents of the EVM HMAC calculation would break > existing systems. Assuming for the time being this is safe, at what > point will it affect backwards compatability? Should it be documented > now or then? Actually, the current patch set continues to fullfill user space expectation on the EVM behavior. If the LSM infrastructure created more xattrs and EVM calculated the HMAC on just one, there would be a problem on subsequent xattr operations and on IMA verification. By updating both the LSM infrastructure and EVM to support multiple xattrs, everything will continue to work. Thanks Roberto
On Mon, 2023-02-20 at 10:49 +0100, Roberto Sassu wrote: > On Sun, 2023-02-19 at 14:42 -0500, Mimi Zohar wrote: > > On Thu, 2022-12-01 at 11:41 +0100, Roberto Sassu wrote: > > > From: Roberto Sassu <roberto.sassu@huawei.com> > > > > > > Currently, evm_inode_init_security() processes a single LSM xattr from > > > the array passed by security_inode_init_security(), and calculates the > > > HMAC on it and other inode metadata. > > > > > > Given that initxattrs() callbacks, called by > > > security_inode_init_security(), expect that this array is terminated when > > > the xattr name is set to NULL, reuse the same assumption to scan all xattrs > > > and to calculate the HMAC on all of them. > > > > > > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com> > > > Reviewed-by: Casey Schaufler <casey@schaufler-ca.com> > > > > Normally changing the contents of the EVM HMAC calculation would break > > existing systems. Assuming for the time being this is safe, at what > > point will it affect backwards compatability? Should it be documented > > now or then? > > Actually, the current patch set continues to fullfill user space > expectation on the EVM behavior. If the LSM infrastructure created more > xattrs and EVM calculated the HMAC on just one, there would be a > problem on subsequent xattr operations and on IMA verification. > > By updating both the LSM infrastructure and EVM to support multiple > xattrs, everything will continue to work. Agreed. Thank you for the reminder of the bug report being addressed by this patch set. Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
diff --git a/security/integrity/evm/evm.h b/security/integrity/evm/evm.h index f8b8c5004fc7..f799d72a59fa 100644 --- a/security/integrity/evm/evm.h +++ b/security/integrity/evm/evm.h @@ -46,6 +46,8 @@ struct evm_digest { char digest[IMA_MAX_DIGEST_SIZE]; } __packed; +int evm_protected_xattr(const char *req_xattr_name); + int evm_init_key(void); int evm_update_evmxattr(struct dentry *dentry, const char *req_xattr_name, diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c index 708de9656bbd..68f99faac316 100644 --- a/security/integrity/evm/evm_crypto.c +++ b/security/integrity/evm/evm_crypto.c @@ -389,6 +389,7 @@ int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattr, char *hmac_val) { struct shash_desc *desc; + const struct xattr *xattr; desc = init_desc(EVM_XATTR_HMAC, HASH_ALGO_SHA1); if (IS_ERR(desc)) { @@ -396,7 +397,13 @@ int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattr, return PTR_ERR(desc); } - crypto_shash_update(desc, lsm_xattr->value, lsm_xattr->value_len); + for (xattr = lsm_xattr; xattr->name != NULL; xattr++) { + if (!evm_protected_xattr(xattr->name)) + continue; + + crypto_shash_update(desc, xattr->value, xattr->value_len); + } + hmac_add_misc(desc, inode, EVM_XATTR_HMAC, hmac_val); kfree(desc); return 0; diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c index 0a312cafb7de..1cf6871a0019 100644 --- a/security/integrity/evm/evm_main.c +++ b/security/integrity/evm/evm_main.c @@ -305,7 +305,7 @@ static int evm_protected_xattr_common(const char *req_xattr_name, return found; } -static int evm_protected_xattr(const char *req_xattr_name) +int evm_protected_xattr(const char *req_xattr_name) { return evm_protected_xattr_common(req_xattr_name, false); } @@ -851,14 +851,20 @@ int evm_inode_init_security(struct inode *inode, struct inode *dir, { struct evm_xattr *xattr_data; struct xattr *xattr, *evm_xattr; + bool evm_protected_xattrs = false; int rc; - if (!(evm_initialized & EVM_INIT_HMAC) || !xattrs || - !evm_protected_xattr(xattrs->name)) + if (!(evm_initialized & EVM_INIT_HMAC) || !xattrs) return -EOPNOTSUPP; - for (xattr = xattrs; xattr->value != NULL; xattr++) - ; + for (xattr = xattrs; xattr->value != NULL; xattr++) { + if (evm_protected_xattr(xattr->name)) + evm_protected_xattrs = true; + } + + /* EVM xattr not needed. */ + if (!evm_protected_xattrs) + return -EOPNOTSUPP; evm_xattr = xattr;