Re: [PATCH v3 04/11] lsm: inode_pre_setxattr hook

["Serge E. Hallyn" <serge@hallyn.com>]

On Fri, Jul 24, 2015 at 12:04:38PM +0200, Lukasz Pawelczyk wrote:
> Add a new LSM hook called before inode's setxattr. It is required for
> LSM to be able to reliably replace the xattr's value to be set to
> filesystem in __vfs_setxattr_noperm(). Useful for mapped values, like in
> the upcoming Smack namespace patches.
> 
> Signed-off-by: Lukasz Pawelczyk <l.pawelczyk@samsung.com>

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

Could get confusing if userspace passes in 1 char and gets ENOSPC because
the unmapped label is too long :)

> ---
>  fs/xattr.c                | 10 ++++++++++
>  include/linux/lsm_hooks.h |  9 +++++++++
>  include/linux/security.h  | 10 ++++++++++
>  security/security.c       | 12 ++++++++++++
>  4 files changed, 41 insertions(+)
> 
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 072fee1..cbc8d19 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -100,12 +100,22 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
>  	if (issec)
>  		inode->i_flags &= ~S_NOSEC;
>  	if (inode->i_op->setxattr) {
> +		bool alloc = false;
> +
> +		error = security_inode_pre_setxattr(dentry, name, &value,
> +						    &size, flags, &alloc);
> +		if (error)
> +			return error;
> +
>  		error = inode->i_op->setxattr(dentry, name, value, size, flags);
>  		if (!error) {
>  			fsnotify_xattr(dentry);
>  			security_inode_post_setxattr(dentry, name, value,
>  						     size, flags);
>  		}
> +
> +		if (alloc)
> +			kfree(value);
>  	} else if (issec) {
>  		const char *suffix = name + XATTR_SECURITY_PREFIX_LEN;
>  		error = security_inode_setsecurity(inode, suffix, value,
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 1751864..0aeed91 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -349,6 +349,11 @@
>   *	Check permission before setting the extended attributes
>   *	@value identified by @name for @dentry.
>   *	Return 0 if permission is granted.
> + * @inode_pre_setxattr:
> + *	Be able to do some operation before setting the @value identified
> + *	by @name on the filesystem. Replacing the @value and its @size is
> + *	possible. Useful for mapped values. Set @alloc to true if @value
> + *	needs to be kfreed afterwards.
>   * @inode_post_setxattr:
>   *	Update inode security field after successful setxattr operation.
>   *	@value identified by @name for @dentry.
> @@ -1448,6 +1453,9 @@ union security_list_options {
>  	int (*inode_getattr)(const struct path *path);
>  	int (*inode_setxattr)(struct dentry *dentry, const char *name,
>  				const void *value, size_t size, int flags);
> +	int (*inode_pre_setxattr)(struct dentry *dentry, const char *name,
> +				  const void **value, size_t *size,
> +				  int flags, bool *alloc);
>  	void (*inode_post_setxattr)(struct dentry *dentry, const char *name,
>  					const void *value, size_t size,
>  					int flags);
> @@ -1730,6 +1738,7 @@ struct security_hook_heads {
>  	struct list_head inode_setattr;
>  	struct list_head inode_getattr;
>  	struct list_head inode_setxattr;
> +	struct list_head inode_pre_setxattr;
>  	struct list_head inode_post_setxattr;
>  	struct list_head inode_getxattr;
>  	struct list_head inode_listxattr;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index f0d2914..24f91e0 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -263,6 +263,9 @@ int security_inode_setattr(struct dentry *dentry, struct iattr *attr);
>  int security_inode_getattr(const struct path *path);
>  int security_inode_setxattr(struct dentry *dentry, const char *name,
>  			    const void *value, size_t size, int flags);
> +int security_inode_pre_setxattr(struct dentry *dentry, const char *name,
> +				const void **value, size_t *size, int flags,
> +				bool *alloc);
>  void security_inode_post_setxattr(struct dentry *dentry, const char *name,
>  				  const void *value, size_t size, int flags);
>  int security_inode_getxattr(struct dentry *dentry, const char *name);
> @@ -691,6 +694,13 @@ static inline int security_inode_setxattr(struct dentry *dentry,
>  	return cap_inode_setxattr(dentry, name, value, size, flags);
>  }
>  
> +static inline int security_inode_pre_setxattr(struct dentry *dentry,
> +				const char *name, const void **value,
> +				size_t *size, int flags, bool *alloc)
> +{
> +	return 0;
> +}
> +
>  static inline void security_inode_post_setxattr(struct dentry *dentry,
>  		const char *name, const void *value, size_t size, int flags)
>  { }
> diff --git a/security/security.c b/security/security.c
> index 88a3b78..e1d2c6f 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -649,6 +649,16 @@ int security_inode_setxattr(struct dentry *dentry, const char *name,
>  	return evm_inode_setxattr(dentry, name, value, size);
>  }
>  
> +int security_inode_pre_setxattr(struct dentry *dentry, const char *name,
> +				const void **value, size_t *size, int flags,
> +				bool *alloc)
> +{
> +	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> +		return 0;
> +	return call_int_hook(inode_pre_setxattr, 0, dentry, name, value, size,
> +			     flags, alloc);
> +}
> +
>  void security_inode_post_setxattr(struct dentry *dentry, const char *name,
>  				  const void *value, size_t size, int flags)
>  {
> @@ -1666,6 +1676,8 @@ struct security_hook_heads security_hook_heads = {
>  		LIST_HEAD_INIT(security_hook_heads.inode_getattr),
>  	.inode_setxattr =
>  		LIST_HEAD_INIT(security_hook_heads.inode_setxattr),
> +	.inode_pre_setxattr =
> +		LIST_HEAD_INIT(security_hook_heads.inode_pre_setxattr),
>  	.inode_post_setxattr =
>  		LIST_HEAD_INIT(security_hook_heads.inode_post_setxattr),
>  	.inode_getxattr =
> -- 
> 2.4.3