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Message-ID: <CAHC9VhSDVv30ce2652kridRU7iaQQ19tiGubWpyP0mi7pf+JJw@mail.gmail.com>
Date: Thu, 30 Mar 2023 18:55:23 -0400
From: Paul Moore <paul@...l-moore.com>
To: Roberto Sassu <roberto.sassu@...weicloud.com>
Cc: zohar@...ux.ibm.com, dmitry.kasatkin@...il.com, jmorris@...ei.org,
serge@...lyn.com, stephen.smalley.work@...il.com,
eparis@...isplace.org, casey@...aufler-ca.com,
reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-integrity@...r.kernel.org,
linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
bpf@...r.kernel.org, kpsingh@...nel.org, keescook@...omium.org,
nicolas.bouchinet@...p-os.org,
Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [PATCH v9 3/4] evm: Align evm_inode_init_security() definition
with LSM infrastructure
On Wed, Mar 29, 2023 at 9:05 AM Roberto Sassu
<roberto.sassu@...weicloud.com> wrote:
>
> From: Roberto Sassu <roberto.sassu@...wei.com>
>
> Change the evm_inode_init_security() definition to align with the LSM
> infrastructure. Keep the existing behavior of including in the HMAC
> calculation only the first xattr provided by LSMs.
>
> Changing the evm_inode_init_security() definition requires passing the
> xattr array allocated by security_inode_init_security(), and the number of
> xattrs filled by previously invoked LSMs.
>
> Use the newly introduced lsm_find_xattr_slot() to position EVM correctly in
> the xattrs array, like a regular LSM, and to increment the number of filled
> slots. For now, the LSM infrastructure allocates enough xattrs slots to
> store the EVM xattr, without using the reservation mechanism.
>
> Finally, make evm_inode_init_security() return value compatible with the
> inode_init_security hook conventions, i.e. return -EOPNOTSUPP if it is not
> setting an xattr.
>
> EVM is a bit tricky, because xattrs is both an input and an output. If it
> was just output, EVM should have returned zero if xattrs is NULL. But,
> since xattrs is also input, EVM is unable to do its calculations, so return
> -EOPNOTSUPP and handle this error in security_inode_init_security().
I don't quite understand why EVM would return EOPNOTSUPP if it is
enabled but there are not xattrs to measure. It seems like EVM should
return success/0 in the no-xattr case; there were no xattrs to
measure, so it succeeded in measuring nothing. Am I missing
something?
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> ---
> include/linux/evm.h | 14 ++++++++------
> security/integrity/evm/evm_main.c | 18 +++++++++++-------
> security/security.c | 6 +++---
> 3 files changed, 22 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/evm.h b/include/linux/evm.h
> index 7dc1ee74169..3c0e8591b69 100644
> --- a/include/linux/evm.h
> +++ b/include/linux/evm.h
> @@ -56,9 +56,10 @@ static inline void evm_inode_post_set_acl(struct dentry *dentry,
> {
> return evm_inode_post_setxattr(dentry, acl_name, NULL, 0);
> }
> -extern int evm_inode_init_security(struct inode *inode,
> - const struct xattr *xattr_array,
> - struct xattr *evm);
> +extern int evm_inode_init_security(struct inode *inode, struct inode *dir,
> + const struct qstr *qstr,
> + struct xattr *xattrs,
> + int *num_filled_xattrs);
> extern bool evm_revalidate_status(const char *xattr_name);
> extern int evm_protected_xattr_if_enabled(const char *req_xattr_name);
> extern int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
> @@ -157,9 +158,10 @@ static inline void evm_inode_post_set_acl(struct dentry *dentry,
> return;
> }
>
> -static inline int evm_inode_init_security(struct inode *inode,
> - const struct xattr *xattr_array,
> - struct xattr *evm)
> +static inline int evm_inode_init_security(struct inode *inode, struct inode *dir,
> + const struct qstr *qstr,
> + struct xattr *xattrs,
> + int *num_filled_xattrs)
> {
> return 0;
> }
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index cf24c525558..9e75759150c 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -21,6 +21,7 @@
> #include <linux/evm.h>
> #include <linux/magic.h>
> #include <linux/posix_acl_xattr.h>
> +#include <linux/lsm_hooks.h>
>
> #include <crypto/hash.h>
> #include <crypto/hash_info.h>
> @@ -864,23 +865,26 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
> /*
> * evm_inode_init_security - initializes security.evm HMAC value
> */
> -int evm_inode_init_security(struct inode *inode,
> - const struct xattr *lsm_xattr,
> - struct xattr *evm_xattr)
> +int evm_inode_init_security(struct inode *inode, struct inode *dir,
> + const struct qstr *qstr, struct xattr *xattrs,
> + int *num_filled_xattrs)
> {
> struct evm_xattr *xattr_data;
> + struct xattr *evm_xattr;
> int rc;
>
> - if (!(evm_initialized & EVM_INIT_HMAC) ||
> - !evm_protected_xattr(lsm_xattr->name))
> - return 0;
> + if (!(evm_initialized & EVM_INIT_HMAC) || !xattrs ||
> + !evm_protected_xattr(xattrs->name))
> + return -EOPNOTSUPP;
> +
> + evm_xattr = lsm_find_xattr_slot(xattrs, num_filled_xattrs);
>
> xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);
> if (!xattr_data)
> return -ENOMEM;
>
> xattr_data->data.type = EVM_XATTR_HMAC;
> - rc = evm_init_hmac(inode, lsm_xattr, xattr_data->digest);
> + rc = evm_init_hmac(inode, xattrs, xattr_data->digest);
> if (rc < 0)
> goto out;
>
> diff --git a/security/security.c b/security/security.c
> index be33d643a81..22ab4fb7ebf 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1674,9 +1674,9 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> if (!num_filled_xattrs)
> goto out;
>
> - ret = evm_inode_init_security(inode, new_xattrs,
> - new_xattrs + num_filled_xattrs);
> - if (ret)
> + ret = evm_inode_init_security(inode, dir, qstr, new_xattrs,
> + &num_filled_xattrs);
> + if (ret && ret != -EOPNOTSUPP)
> goto out;
> ret = initxattrs(inode, new_xattrs, fs_data);
> out:
> --
> 2.25.1
>
--
paul-moore.com
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