Re: [PATCH v2] evm: Correct inode_init_security hooks behaviors

[Casey Schaufler <casey@schaufler-ca.com>]

On 10/21/2022 8:14 AM, Nicolas Bouchinet wrote:
> From: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
>
> Fixes a NULL pointer dereference occurring in the
> `evm_protected_xattr_common` function of the EVM LSM. The bug is
> triggered if a `inode_init_security` hook returns 0 without initializing
> the given `struct xattr` fields (which is the case of BPF) and if no
> other LSM overrides thoses fields after. This also leads to memory
> leaks.
>
> Adds a `call_int_hook_xattr` macro that fetches and feed the
> `new_xattrs` array with every called hook xattr values.
>
> Adds a `evm_init_hmacs` function which init the EVM hmac using every
> entry of the array contrary to `evm_init_hmac`.
>
> Fixes the `evm_inode_init_security` function to use `evm_init_hmacs`.
>
> The `MAX_LSM_EVM_XATTR` value has been raised to 5 which gives room for
> SMACK, SELinux, Apparmor, BPF and IMA/EVM security attributes.

This does not match the code, which sets the value based on the
security modules compiled in rather than the constant 5.

>
> Changes the default return value of the `inode_init_security` hook
> definition to `-EOPNOTSUPP`.
>
> Changes the hook documentation to match the behavior of the LSMs using
> it (only xattr->value is initialised with kmalloc and thus is the only
> one that should be kfreed by the caller).
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
> ---
>
> Changes since v1:
> https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
> * The `call_int_hook_xattr` macro has been inlined into the
>   `security_inode_init_security` hook (as suggested by Paul Moor,
>   Mickaël Salaün and Casey Schaufler).
> * The MAX_LSM_EVM_XATTR value is processed based on compiled LSMs (as
>   proposed by Casey Schaufler).
> * Various typos and cosmetic changes has been fixed (as suggested by
>   Mickaël Salaün)
> * A MAX_LSM_EVM_XATTR test has been wrapped in a WAN_ON_ONCE macro (as
>   suggested by Mickaël Salaün).
> ---
>  include/linux/lsm_hook_defs.h       |  2 +-
>  include/linux/lsm_hooks.h           |  4 ++--
>  security/integrity/evm/evm.h        |  2 ++
>  security/integrity/evm/evm_crypto.c | 22 ++++++++++++++++++++++
>  security/integrity/evm/evm_main.c   |  9 +++++----
>  security/security.c                 | 28 +++++++++++++++++++++-------
>  6 files changed, 53 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 806448173033..e5dd0c0f6345 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -111,7 +111,7 @@ LSM_HOOK(int, 0, path_notify, const struct path *path, u64 mask,
>  	 unsigned int obj_type)
>  LSM_HOOK(int, 0, inode_alloc_security, struct inode *inode)
>  LSM_HOOK(void, LSM_RET_VOID, inode_free_security, struct inode *inode)
> -LSM_HOOK(int, 0, inode_init_security, struct inode *inode,
> +LSM_HOOK(int, -EOPNOTSUPP, inode_init_security, struct inode *inode,
>  	 struct inode *dir, const struct qstr *qstr, const char **name,
>  	 void **value, size_t *len)
>  LSM_HOOK(int, 0, inode_init_security_anon, struct inode *inode,
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 84a0d7e02176..95aff9383de1 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -229,8 +229,8 @@
>   *	This hook is called by the fs code as part of the inode creation
>   *	transaction and provides for atomic labeling of the inode, unlike
>   *	the post_create/mkdir/... hooks called by the VFS.  The hook function
> - *	is expected to allocate the name and value via kmalloc, with the caller
> - *	being responsible for calling kfree after using them.
> + *	is expected to allocate the value via kmalloc, with the caller
> + *	being responsible for calling kfree after using it.
>   *	If the security module does not use security attributes or does
>   *	not wish to put a security attribute on this particular inode,
>   *	then it should return -EOPNOTSUPP to skip this processing.
> diff --git a/security/integrity/evm/evm.h b/security/integrity/evm/evm.h
> index f8b8c5004fc7..a2f9886e924d 100644
> --- a/security/integrity/evm/evm.h
> +++ b/security/integrity/evm/evm.h
> @@ -60,6 +60,8 @@ int evm_calc_hash(struct dentry *dentry, const char *req_xattr_name,
>  		  struct evm_digest *data);
>  int evm_init_hmac(struct inode *inode, const struct xattr *xattr,
>  		  char *hmac_val);
> +int evm_init_hmacs(struct inode *inode, const struct xattr *xattrs,
> +		  char *hmac_val);
>  int evm_init_secfs(void);
>  
>  #endif
> diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> index 708de9656bbd..db6868875e09 100644
> --- a/security/integrity/evm/evm_crypto.c
> +++ b/security/integrity/evm/evm_crypto.c
> @@ -385,6 +385,28 @@ int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
>  	return rc;
>  }
>  
> +int evm_protected_xattr(const char *req_xattr_name);
> +
> +int evm_init_hmacs(struct inode *inode, const struct xattr *lsm_xattrs,
> +		  char *hmac_val)
> +{
> +	struct shash_desc *desc;
> +
> +	desc = init_desc(EVM_XATTR_HMAC, HASH_ALGO_SHA1);
> +	if (IS_ERR(desc)) {
> +		pr_info("init_desc failed\n");
> +		return PTR_ERR(desc);
> +	}
> +
> +	for (int i = 0; lsm_xattrs[i].value != NULL; i++) {
> +		if (evm_protected_xattr(lsm_xattrs[i].name))
> +			crypto_shash_update(desc, lsm_xattrs[i].value, lsm_xattrs[i].value_len);
> +	}
> +	hmac_add_misc(desc, inode, EVM_XATTR_HMAC, hmac_val);
> +	kfree(desc);
> +	return 0;
> +}
> +
>  int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattr,
>  		  char *hmac_val)
>  {
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 2e6fb6e2ffd2..4f89b9eaf028 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -284,6 +284,8 @@ static int evm_protected_xattr_common(const char *req_xattr_name,
>  	int found = 0;
>  	struct xattr_list *xattr;
>  
> +	if (!req_xattr_name)
> +		return found;
>  	namelen = strlen(req_xattr_name);
>  	list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
>  		if (!all_xattrs && !xattr->enabled)
> @@ -305,7 +307,7 @@ 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);
>  }
> @@ -841,8 +843,7 @@ int evm_inode_init_security(struct inode *inode,
>  	struct evm_xattr *xattr_data;
>  	int rc;
>  
> -	if (!(evm_initialized & EVM_INIT_HMAC) ||
> -	    !evm_protected_xattr(lsm_xattr->name))
> +	if (!(evm_initialized & EVM_INIT_HMAC))
>  		return 0;
>  
>  	xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);
> @@ -850,7 +851,7 @@ int evm_inode_init_security(struct inode *inode,
>  		return -ENOMEM;
>  
>  	xattr_data->data.type = EVM_XATTR_HMAC;
> -	rc = evm_init_hmac(inode, lsm_xattr, xattr_data->digest);
> +	rc = evm_init_hmacs(inode, lsm_xattr, xattr_data->digest);
>  	if (rc < 0)
>  		goto out;
>  
> diff --git a/security/security.c b/security/security.c
> index 14d30fec8a00..cf11bd7d6a1b 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -30,7 +30,11 @@
>  #include <linux/msg.h>
>  #include <net/flow.h>
>  
> -#define MAX_LSM_EVM_XATTR	2
> +#define MAX_LSM_EVM_XATTR                                \
> +	((IS_ENABLED(CONFIG_EVM) ? 1 : 0) +              \
> +	 (IS_ENABLED(CONFIG_SECURITY_SELINUX) ? 1 : 0) + \
> +	 (IS_ENABLED(CONFIG_SECURITY_SMACK) ? 1 : 0) +   \
> +	 (IS_ENABLED(CONFIG_BPF_LSM) ? 1 : 0))
>  
>  /* How many LSMs were built into the kernel? */
>  #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
> @@ -1091,9 +1095,11 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
>  				 const struct qstr *qstr,
>  				 const initxattrs initxattrs, void *fs_data)
>  {
> +	int i = 0;
> +	int ret = -EOPNOTSUPP;
>  	struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
>  	struct xattr *lsm_xattr, *evm_xattr, *xattr;
> -	int ret;
> +	struct security_hook_list *hook_ptr;
>  
>  	if (unlikely(IS_PRIVATE(inode)))
>  		return 0;
> @@ -1103,15 +1109,23 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
>  				     dir, qstr, NULL, NULL, NULL);
>  	memset(new_xattrs, 0, sizeof(new_xattrs));
>  	lsm_xattr = new_xattrs;
> -	ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir, qstr,
> -						&lsm_xattr->name,
> -						&lsm_xattr->value,
> -						&lsm_xattr->value_len);
> +	hlist_for_each_entry(hook_ptr, &security_hook_heads.inode_init_security, list) {
> +		ret = hook_ptr->hook.inode_init_security(inode, dir, qstr, &lsm_xattr->name,
> +				&lsm_xattr->value, &lsm_xattr->value_len);
> +		if (ret == -EOPNOTSUPP)
> +			continue;
> +		if (WARN_ON_ONCE(i >= MAX_LSM_EVM_XATTR))
> +			ret = -ENOMEM;
> +		if (ret != 0)
> +			break;
> +		lsm_xattr++;
> +		i++;
> +	}
>  	if (ret)
>  		goto out;
>  
>  	evm_xattr = lsm_xattr + 1;
> -	ret = evm_inode_init_security(inode, lsm_xattr, evm_xattr);
> +	ret = evm_inode_init_security(inode, new_xattrs, evm_xattr);
>  	if (ret)
>  		goto out;
>  	ret = initxattrs(inode, new_xattrs, fs_data);