Re: [PATCH v4] evm: Correct inode_init_security hooks behaviors

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

On Wed, 2022-10-26 at 16:30 +0200, 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.
> 
> The `call_int_hook_xattr` macro has been inlined into the
> `security_inode_init_security` hook in order to check hooks return
> values and skip ones who doesn't init `xattrs`.
> 
> Modify `evm_init_hmac` function to init the EVM hmac using every
> entry of the given xattr array.
> 
> The `MAX_LSM_EVM_XATTR` value is now based on the security modules
> compiled in, which gives room for SMACK, SELinux, Apparmor, BPF and
> IMA/EVM security attributes.
> 
> 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: roberto.sassu@huaweicloud.com
> Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
> ---
> Changes since v3:
> https://lore.kernel.org/linux-integrity/Y1fu4jofqLHVDprT@archlinux/
> 
> * Fixes compilation error reported by the kernel test robot.
> ---
>  include/linux/lsm_hook_defs.h       |  2 +-
>  include/linux/lsm_hooks.h           |  4 ++--
>  security/integrity/evm/evm.h        |  1 +
>  security/integrity/evm/evm_crypto.c |  9 +++++++--
>  security/integrity/evm/evm_main.c   |  7 ++++---
>  security/security.c                 | 31 ++++++++++++++++++++++-------
>  6 files changed, 39 insertions(+), 15 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.

Please also update the description of @name as well (remove allocated).

>   *	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..6d9628ca7c24 100644
> --- a/security/integrity/evm/evm.h
> +++ b/security/integrity/evm/evm.h
> @@ -61,5 +61,6 @@ int evm_calc_hash(struct dentry *dentry, const char *req_xattr_name,
>  int evm_init_hmac(struct inode *inode, const struct xattr *xattr,
>  		  char *hmac_val);
>  int evm_init_secfs(void);
> +int evm_protected_xattr(const char *req_xattr_name);
>  
>  #endif
> diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> index 708de9656bbd..06639f3cfb38 100644
> --- a/security/integrity/evm/evm_crypto.c
> +++ b/security/integrity/evm/evm_crypto.c
> @@ -385,7 +385,7 @@ int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
>  	return rc;
>  }
>  
> -int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattr,
> +int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattrs,
>  		  char *hmac_val)
>  {
>  	struct shash_desc *desc;
> @@ -396,7 +396,12 @@ 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 (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;
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 2e6fb6e2ffd2..0420453a80e8 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;

Remove, and use the check below.

>  	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);
> diff --git a/security/security.c b/security/security.c
> index 14d30fec8a00..79524f8734f1 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 +1109,26 @@ 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;

This does not work properly. Suppose that you have an LSM with xattr
and another without. The final ret will be -EOPNOTSUPP. Instead declare
new_xattrs_set boolean, and set to true if ret = 0. After the loop,
check the boolean instead of ret. If ret != 0 goto out.

> +		if (WARN_ON_ONCE(i >= MAX_LSM_EVM_XATTR))
> +			ret = -ENOMEM;
> +		if (ret != 0)
> +			break;

We can check here if the LSM behaved properly, i.e. it set the xattr
name and value:

if (WARN_ON_ONCE(!lsm_xattr->name || !lsm_xattr->value)) {
	ret = -ENOMEM;
	goto out;
}

> +		lsm_xattr++;
> +		i++;
> +	}
>  	if (ret)
>  		goto out;
>  
>  	evm_xattr = lsm_xattr + 1;

It should be:

	evm_xattr = lsm_xattr;

You incremented lsm_xattr already, after the LSMs set their xattr.

Once you complete the changes, I will send a patch set including your
patch with some more patches.

Roberto

> -	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);