Re: [PATCH v5 3/3] Wire UFFD up to SELinux

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Apr 01, 2020 at 02:39:03PM -0700, Daniel Colascione wrote:
> This change gives userfaultfd file descriptors a real security
> context, allowing policy to act on them.
> 
> Signed-off-by: Daniel Colascione <dancol@google.com>
> ---
>  fs/userfaultfd.c | 30 ++++++++++++++++++++++++++----
>  1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 37df7c9eedb1..78ff5d898733 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -76,6 +76,8 @@ struct userfaultfd_ctx {
>  	bool mmap_changing;
>  	/* mm with one ore more vmas attached to this userfaultfd_ctx */
>  	struct mm_struct *mm;
> +	/* The inode that owns this context --- not a strong reference.  */
> +	const struct inode *owner;
>  };

Adding this field seems wrong.  There's no reference held to it, so it can only
be used if the caller holds a reference to the inode anyway.  The only user of
this field is via userfafultfd_read(), so why not just use the inode of the
struct file passed to userfaultfd_read()?

>  SYSCALL_DEFINE1(userfaultfd, int, flags)
>  {
> +	struct file *file;
>  	struct userfaultfd_ctx *ctx;
>  	int fd;
>  
> @@ -1974,8 +1979,25 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
>  	/* prevent the mm struct to be freed */
>  	mmgrab(ctx->mm);
>  
> -	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
> -			      O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
> +	file = anon_inode_getfile_secure(
> +		"[userfaultfd]", &userfaultfd_fops, ctx,
> +		O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS),
> +		NULL);
> +	if (IS_ERR(file)) {
> +		fd = PTR_ERR(file);
> +		goto out;
> +	}
> +
> +	fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
> +	if (fd < 0) {
> +		fput(file);
> +		goto out;
> +	}
> +
> +	ctx->owner = file_inode(file);
> +	fd_install(fd, file);
> +
> +out:
>  	if (fd < 0) {
>  		mmdrop(ctx->mm);
>  		kmem_cache_free(userfaultfd_ctx_cachep, ctx);

What's the point of anon_inode_getfile_secure()?  anon_inode_getfd_secure()
would work here too.

- Eric