[PATCH v12 16/25] LSM: Use lsmcontext in security_dentry_init_security

[Stephen Smalley]

On 12/16/19 5:36 PM, Casey Schaufler wrote:
> Change the security_dentry_init_security() interface to
> fill an lsmcontext structure instead of a void * data area
> and a length. The lone caller of this interface is NFS4,
> which may make copies of the data using its own mechanisms.
> A rework of the nfs4 code to use the lsmcontext properly
> is a significant project, so the coward's way out is taken,
> and the lsmcontext data from security_dentry_init_security()
> is copied, then released directly.
> 
> This interface does not use the "display". There is currently
> not case where that is useful or reasonable.
> 
> Reviewed-by: Kees Cook <keescook at chromium.org>
> Reviewed-by: John Johansen <john.johansen at canonical.com>
> Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
> ---
>   fs/nfs/nfs4proc.c        | 26 ++++++++++++++++----------
>   include/linux/security.h |  7 +++----
>   security/security.c      | 29 +++++++++++++++++++++++++----
>   3 files changed, 44 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> index a30e36654c57..78d63f7f0088 100644
> --- a/fs/nfs/nfs4proc.c
> +++ b/fs/nfs/nfs4proc.c
> @@ -112,6 +112,7 @@ static inline struct nfs4_label *
>   nfs4_label_init_security(struct inode *dir, struct dentry *dentry,
>   	struct iattr *sattr, struct nfs4_label *label)
>   {
> +	struct lsmcontext context;
>   	int err;
>   
>   	if (label == NULL)
> @@ -121,21 +122,26 @@ nfs4_label_init_security(struct inode *dir, struct dentry *dentry,
>   		return NULL;
>   
>   	err = security_dentry_init_security(dentry, sattr->ia_mode,
> -				&dentry->d_name, (void **)&label->label, &label->len);
> -	if (err == 0)
> -		return label;
> +					    &dentry->d_name, &context);
> +
> +	if (err)
> +		return NULL;
> +
> +	label->label = kmemdup(context.context, context.len, GFP_KERNEL);

This seems unfortunate; it introduces an extra allocation/copy of the 
context.  I'd prefer to avoid it.  Also wondering if GFP_KERNEL is 
always safe here.

> +	if (label->label == NULL)
> +		label = NULL;
> +	else
> +		label->len = context.len;
> +
> +	security_release_secctx(&context);
> +
> +	return label;
>   
> -	return NULL;
>   }
>   static inline void
>   nfs4_label_release_security(struct nfs4_label *label)
>   {
> -	struct lsmcontext scaff; /* scaffolding */
> -
> -	if (label) {
> -		lsmcontext_init(&scaff, label->label, label->len, 0);
> -		security_release_secctx(&scaff);
> -	}
> +	kfree(label->label);
>   }
>   static inline u32 *nfs4_bitmask(struct nfs_server *server, struct nfs4_label *label)
>   {
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 00421941f683..a5eba06a9382 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -398,8 +398,8 @@ int security_add_mnt_opt(const char *option, const char *val,
>   				int len, void **mnt_opts);
>   int security_move_mount(const struct path *from_path, const struct path *to_path);
>   int security_dentry_init_security(struct dentry *dentry, int mode,
> -					const struct qstr *name, void **ctx,
> -					u32 *ctxlen);
> +					const struct qstr *name,
> +					struct lsmcontext *ctx);
>   int security_dentry_create_files_as(struct dentry *dentry, int mode,
>   					struct qstr *name,
>   					const struct cred *old,
> @@ -790,8 +790,7 @@ static inline void security_inode_free(struct inode *inode)
>   static inline int security_dentry_init_security(struct dentry *dentry,
>   						 int mode,
>   						 const struct qstr *name,
> -						 void **ctx,
> -						 u32 *ctxlen)
> +						 struct lsmcontext *ctx)
>   {
>   	return -EOPNOTSUPP;
>   }
> diff --git a/security/security.c b/security/security.c
> index 4ba1a6ed36e0..8aa107b57af9 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1011,12 +1011,33 @@ void security_inode_free(struct inode *inode)
>   				inode_free_by_rcu);
>   }
>   
> +/*
> + * security_dentry_init_security - initial context for a dentry
> + * @dentry: directory entry
> + * @mode: access mode
> + * @name: path name
> + * @context: resulting security context
> + *
> + * Use at most one security module to get the initial
> + * security context. Do not use the "display".
> + *
> + * Returns -EOPNOTSUPP if not supplied by any module or the module result.
> + */
>   int security_dentry_init_security(struct dentry *dentry, int mode,
> -					const struct qstr *name, void **ctx,
> -					u32 *ctxlen)
> +				  const struct qstr *name,
> +				  struct lsmcontext *cp)
>   {
> -	return call_int_hook(dentry_init_security, -EOPNOTSUPP, dentry, mode,
> -				name, ctx, ctxlen);
> +	struct security_hook_list *hp;
> +
> +	hlist_for_each_entry(hp, &security_hook_heads.dentry_init_security,
> +			     list) {
> +		cp->slot = hp->lsmid->slot;
> +		return hp->hook.dentry_init_security(dentry, mode, name,
> +						     (void **)&cp->context,
> +						     &cp->len);
> +	}
> +
> +	return -EOPNOTSUPP;
>   }
>   EXPORT_SYMBOL(security_dentry_init_security);
>   
>