Re: [PATCH] fs: split receive_fd_replace from __receive_fd

[Christian Brauner]

On Fri, Apr 16, 2021 at 04:15:43AM +0000, Al Viro wrote:
> On Fri, Apr 02, 2021 at 12:01:05PM -0700, Kees Cook wrote:
> > On Thu, Mar 25, 2021 at 09:22:09AM +0100, Christoph Hellwig wrote:
> > > receive_fd_replace shares almost no code with the general case, so split
> > > it out.  Also remove the "Bump the sock usage counts" comment from
> > > both copies, as that is now what __receive_sock actually does.
> > > 
> > > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > 
> > I'm okay with repeating code in fs/file.c. What I wanted to avoid was
> > open coded combinations in various callers.
> 
> ... and that got you a lovely userland ABI, where you have
> 
> 	(1) newfd >= 0, SECCOMP_ADDFD_FLAG_SETFD is present => replace
> 	(2) newfd < 0, SECCOMP_ADDFD_FLAG_SETFD is present => insert
> 	(3) newfd == 0, SECCOMP_ADDFD_FLAG_SETFD not present => insert
> 	(4) newfd != 0, SECCOMP_ADDFD_FLAG_SETFD not present => -EINVAL
> 
> IMO (2) is a bug.  Whether we still can fix it or not... no idea, depends
> on whether the actual userland has come to depend upon it.

The number of users actively making use of this is rn more or less only
projects I maintain. There's a proposal to make that API part of another
project but they can just adapt to the new behavior too since there's no
released version. So we could just risk that change.

> 
> I suggest turning (2) into an error (-EBADF is what you'd get from
> attempt to set something at such descriptor) and seeing if anything
> breaks.  And having SECCOMP_ADDFD_FLAG_SETFD status passed into kaddfd
> explicitly, with explicit check in seccomp_handle_addfd().  As in
> 
> commit 42eb0d54c08a0331d6d295420f602237968d792b
> Author: Christoph Hellwig <hch@xxxxxx>
> Date:   Thu Mar 25 09:22:09 2021 +0100
> 
>     fs: split receive_fd_replace from __receive_fd
>     
>     receive_fd_replace shares almost no code with the general case, so split
>     it out.  Also remove the "Bump the sock usage counts" comment from
>     both copies, as that is now what __receive_sock actually does.
>     
>     [AV: ... and make the only user of receive_fd_replace() choose between
>     it and receive_fd() according to what userland had passed to it in
>     flags]
>     
>     Signed-off-by: Christoph Hellwig <hch@xxxxxx>
>     Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> 
> diff --git a/fs/file.c b/fs/file.c
> index f3a4bac2cbe9..d8ccb95a7f41 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -1068,8 +1068,6 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
>  
>  /**
>   * __receive_fd() - Install received file into file descriptor table
> - *
> - * @fd: fd to install into (if negative, a new fd will be allocated)
>   * @file: struct file that was received from another process
>   * @ufd: __user pointer to write new fd number to
>   * @o_flags: the O_* flags to apply to the new fd entry
> @@ -1083,7 +1081,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
>   *
>   * Returns newly install fd or -ve on error.
>   */
> -int __receive_fd(int fd, struct file *file, int __user *ufd, unsigned int o_flags)
> +int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
>  {
>  	int new_fd;
>  	int error;
> @@ -1092,32 +1090,33 @@ int __receive_fd(int fd, struct file *file, int __user *ufd, unsigned int o_flag
>  	if (error)
>  		return error;
>  
> -	if (fd < 0) {
> -		new_fd = get_unused_fd_flags(o_flags);
> -		if (new_fd < 0)
> -			return new_fd;
> -	} else {
> -		new_fd = fd;
> -	}
> +	new_fd = get_unused_fd_flags(o_flags);
> +	if (new_fd < 0)
> +		return new_fd;
>  
>  	if (ufd) {
>  		error = put_user(new_fd, ufd);
>  		if (error) {
> -			if (fd < 0)
> -				put_unused_fd(new_fd);
> +			put_unused_fd(new_fd);
>  			return error;
>  		}
>  	}
>  
> -	if (fd < 0) {
> -		fd_install(new_fd, get_file(file));
> -	} else {
> -		error = replace_fd(new_fd, file, o_flags);
> -		if (error)
> -			return error;
> -	}
> +	fd_install(new_fd, get_file(file));
> +	__receive_sock(file);
> +	return new_fd;
> +}
>  
> -	/* Bump the sock usage counts, if any. */
> +int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags)
> +{
> +	int error;
> +
> +	error = security_file_receive(file);
> +	if (error)
> +		return error;
> +	error = replace_fd(new_fd, file, o_flags);
> +	if (error)
> +		return error;
>  	__receive_sock(file);
>  	return new_fd;
>  }
> diff --git a/include/linux/file.h b/include/linux/file.h
> index 225982792fa2..2de2e4613d7b 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -92,23 +92,20 @@ extern void put_unused_fd(unsigned int fd);
>  
>  extern void fd_install(unsigned int fd, struct file *file);
>  
> -extern int __receive_fd(int fd, struct file *file, int __user *ufd,
> +extern int __receive_fd(struct file *file, int __user *ufd,
>  			unsigned int o_flags);
>  static inline int receive_fd_user(struct file *file, int __user *ufd,
>  				  unsigned int o_flags)
>  {
>  	if (ufd == NULL)
>  		return -EFAULT;
> -	return __receive_fd(-1, file, ufd, o_flags);
> +	return __receive_fd(file, ufd, o_flags);
>  }
>  static inline int receive_fd(struct file *file, unsigned int o_flags)
>  {
> -	return __receive_fd(-1, file, NULL, o_flags);
> -}
> -static inline int receive_fd_replace(int fd, struct file *file, unsigned int o_flags)
> -{
> -	return __receive_fd(fd, file, NULL, o_flags);
> +	return __receive_fd(file, NULL, o_flags);
>  }
> +int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags);
>  
>  extern void flush_delayed_fput(void);
>  extern void __fput_sync(struct file *);
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 1d60fc2c9987..4fe19cecaa94 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -119,8 +119,11 @@ struct seccomp_kaddfd {
>  	int fd;
>  	unsigned int flags;
>  
> -	/* To only be set on reply */
> -	int ret;
> +	union {
> +		bool setfd;
> +		/* To only be set on reply */
> +		int ret;
> +	};
>  	struct completion completion;
>  	struct list_head list;
>  };
> @@ -1069,7 +1072,11 @@ static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
>  	 * that it has been handled.
>  	 */
>  	list_del_init(&addfd->list);
> -	addfd->ret = receive_fd_replace(addfd->fd, addfd->file, addfd->flags);
> +	if (!addfd->setfd)
> +		addfd->ret = receive_fd(addfd->file, addfd->flags);
> +	else
> +		addfd->ret = receive_fd_replace(addfd->fd, addfd->file,
> +						addfd->flags);
>  	complete(&addfd->completion);
>  }
>  
> @@ -1583,8 +1590,8 @@ static long seccomp_notify_addfd(struct seccomp_filter *filter,
>  		return -EBADF;
>  
>  	kaddfd.flags = addfd.newfd_flags;
> -	kaddfd.fd = (addfd.flags & SECCOMP_ADDFD_FLAG_SETFD) ?
> -		    addfd.newfd : -1;
> +	kaddfd.setfd = addfd.flags & SECCOMP_ADDFD_FLAG_SETFD;
> +	kaddfd.fd = addfd.newfd;
>  	init_completion(&kaddfd.completion);
>  
>  	ret = mutex_lock_interruptible(&filter->notify_lock);