[PATCH v4 4/20] lsm: Refactor return value of LSM hook inode_listsecurity

[Paul Moore]

On Jul 11, 2024 Xu Kuohai <xukuohai at huaweicloud.com> wrote:
> 
> To be consistent with most LSM hooks, convert the return value of
> hook inode_listsecurity to 0 or a negative error code.
> 
> Before:
> - Hook inode_listsecurity returns number of bytes used/required on
>   success or a negative error code on failure.
> 
> After:
> - Hook inode_listsecurity returns 0 on success or a negative error
>   code on failure. An output parameter @bytes is introduced to hold
>   the number of bytes used/required on success.
> 
> Signed-off-by: Xu Kuohai <xukuohai at huawei.com>
> ---
>  fs/nfs/nfs4proc.c             |  5 ++++-
>  fs/xattr.c                    |  5 ++++-
>  include/linux/lsm_hook_defs.h |  2 +-
>  include/linux/security.h      |  7 ++++---
>  net/socket.c                  |  9 +++++----
>  security/security.c           | 29 +++++++++++++++++++++++++----
>  security/selinux/hooks.c      |  8 +++++---
>  security/smack/smack_lsm.c    |  6 ++++--
>  8 files changed, 52 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> index a691fa10b3e9..6d75758ba3d5 100644
> --- a/fs/nfs/nfs4proc.c
> +++ b/fs/nfs/nfs4proc.c
> @@ -7848,10 +7848,13 @@ static int nfs4_xattr_get_nfs4_label(const struct xattr_handler *handler,
>  static ssize_t
>  nfs4_listxattr_nfs4_label(struct inode *inode, char *list, size_t list_len)
>  {
> +	size_t bytes;
>  	int len = 0;
>  
>  	if (nfs_server_capable(inode, NFS_CAP_SECURITY_LABEL)) {
> -		len = security_inode_listsecurity(inode, list, list_len);
> +		len = security_inode_listsecurity(inode, list, list_len, &bytes);
> +		if (!len)
> +			len = bytes;
>  		if (len >= 0 && list_len && len > list_len)
>  			return -ERANGE;
>  	}

See my comments below.

> diff --git a/fs/xattr.c b/fs/xattr.c
> index f4e3bedf7272..ab7d7123a016 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -485,6 +485,7 @@ vfs_listxattr(struct dentry *dentry, char *list, size_t size)
>  {
>  	struct inode *inode = d_inode(dentry);
>  	ssize_t error;
> +	size_t bytes;
>  
>  	error = security_inode_listxattr(dentry);
>  	if (error)
> @@ -493,7 +494,9 @@ vfs_listxattr(struct dentry *dentry, char *list, size_t size)
>  	if (inode->i_op->listxattr) {
>  		error = inode->i_op->listxattr(dentry, list, size);
>  	} else {
> -		error = security_inode_listsecurity(inode, list, size);
> +		error = security_inode_listsecurity(inode, list, size, &bytes);
> +		if (!error)
> +			error = bytes;
>  		if (size && error > size)
>  			error = -ERANGE;

More on this below, but since the buffer length is fixed we are
already going to have to do a length comparison in the LSMs, why not
do the check and return -ERANGE there?

> diff --git a/net/socket.c b/net/socket.c
> index e416920e9399..43f0e3c9a6e0 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -571,12 +571,13 @@ static struct socket *sockfd_lookup_light(int fd, int *err, int *fput_needed)
>  static ssize_t sockfs_listxattr(struct dentry *dentry, char *buffer,
>  				size_t size)
>  {
> -	ssize_t len;
> +	int err;
> +	size_t len;
>  	ssize_t used = 0;
>  
> -	len = security_inode_listsecurity(d_inode(dentry), buffer, size);
> -	if (len < 0)
> -		return len;
> +	err = security_inode_listsecurity(d_inode(dentry), buffer, size, &len);
> +	if (err < 0)
> +		return err;
>  	used += len;
>  	if (buffer) {
>  		if (size < used)

It doesn't show in the patch/diff, but if the LSM hook handles the length
comparison we can simplify the -ERANGE code in sockfs_listxattr().

> diff --git a/security/security.c b/security/security.c
> index 614f14cbfff7..26eea8f4cd74 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2597,20 +2597,41 @@ int security_inode_setsecurity(struct inode *inode, const char *name,
>   * @inode: inode
>   * @buffer: buffer
>   * @buffer_size: size of buffer
> + * @bytes: number of bytes used/required
>   *
>   * Copy the extended attribute names for the security labels associated with
>   * @inode into @buffer.  The maximum size of @buffer is specified by
>   * @buffer_size.  @buffer may be NULL to request the size of the buffer
>   * required.
>   *
> - * Return: Returns number of bytes used/required on success.
> + * Return: Returns 0 on success or a negative error code on failure.
>   */
>  int security_inode_listsecurity(struct inode *inode,
> -				char *buffer, size_t buffer_size)
> +				char *buffer, size_t buffer_size,
> +				size_t *bytes)
>  {
> +	int rc;
> +	size_t used;
> +	struct security_hook_list *hp;
> +
>  	if (unlikely(IS_PRIVATE(inode)))
> -		return 0;
> -	return call_int_hook(inode_listsecurity, inode, buffer, buffer_size);
> +		return *bytes = 0;
> +
> +	used = 0;
> +	hlist_for_each_entry(hp, &security_hook_heads.inode_listsecurity,
> +			     list) {
> +		rc = hp->hook.inode_listsecurity(inode, buffer, buffer_size,
> +						 &used);
> +		if (rc < 0)
> +			return rc;
> +		if (used != 0)
> +			break;
> +	}
> +
> +	*bytes = used;
> +
> +	return 0;
> +
>  }
>  EXPORT_SYMBOL(security_inode_listsecurity);

For reasons associated with the static_call work, we really need to
limit ourselves to the call_{int,void}_hook() macros on any new code.
The good news is that I think we can do that here as the existing
code isn't multi-LSM friendly.

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 70792bba24d9..5dedd3917d57 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3481,16 +3481,18 @@ static int selinux_inode_setsecurity(struct inode *inode, const char *name,
>  	return 0;
>  }
>  
> -static int selinux_inode_listsecurity(struct inode *inode, char *buffer, size_t buffer_size)
> +static int selinux_inode_listsecurity(struct inode *inode, char *buffer,
> +				      size_t buffer_size, size_t *bytes)
>  {
>  	const int len = sizeof(XATTR_NAME_SELINUX);
>  
>  	if (!selinux_initialized())
> -		return 0;
> +		return *bytes = 0;
>  
>  	if (buffer && len <= buffer_size)
>  		memcpy(buffer, XATTR_NAME_SELINUX, len);
> -	return len;
> +	*bytes = len;
> +	return 0;
>  }

Let's do something like below so we can catch -ERANGE in the LSMs
themselves.

  if (!selinux_initialized())
    return *bytes = 0;

  *bytes = sizeof(XATTR_NAME_SELINUX);
  if (len > buffer_size);
    return -ERANGE;
  if (buffer)
    memcpy(buffer, XATTR_NAME_SELINUX, *bytes);
  
  return 0;

>  static void selinux_inode_getsecid(struct inode *inode, u32 *secid)
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index e7a5f6fd9a2d..6f73906bf7ea 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -1611,16 +1611,18 @@ static int smack_inode_getsecurity(struct mnt_idmap *idmap,
>   * @inode: the object
>   * @buffer: where they go
>   * @buffer_size: size of buffer
> + * @bytes: number of data bytes in buffer
>   */
>  static int smack_inode_listsecurity(struct inode *inode, char *buffer,
> -				    size_t buffer_size)
> +				    size_t buffer_size, size_t *bytes)
>  {
>  	int len = sizeof(XATTR_NAME_SMACK);
>  
>  	if (buffer != NULL && len <= buffer_size)
>  		memcpy(buffer, XATTR_NAME_SMACK, len);
>  
> -	return len;
> +	*bytes = len;
> +	return 0;
>  }

A similar approach could be used here.

--
paul-moore.com