Re: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Jun 10, 2020 at 07:59:55PM -0700, Kees Cook wrote:
> On Wed, Jun 10, 2020 at 08:12:38AM +0000, Sargun Dhillon wrote:
> > As an aside, all of this junk should be dropped:
> > +	ret = get_user(size, &uaddfd->size);
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = copy_struct_from_user(&addfd, sizeof(addfd), uaddfd, size);
> > +	if (ret)
> > +		return ret;
> > 
> > and the size member of the seccomp_notif_addfd struct. I brought this up 
> > off-list with Tycho that ioctls have the size of the struct embedded in them. We 
> > should just use that. The ioctl definition is based on this[2]:
> > #define _IOC(dir,type,nr,size) \
> > 	(((dir)  << _IOC_DIRSHIFT) | \
> > 	 ((type) << _IOC_TYPESHIFT) | \
> > 	 ((nr)   << _IOC_NRSHIFT) | \
> > 	 ((size) << _IOC_SIZESHIFT))
> > 
> > 
> > We should just use copy_from_user for now. In the future, we can either 
> > introduce new ioctl names for new structs, or extract the size dynamically from 
> > the ioctl (and mask it out on the switch statement in seccomp_notify_ioctl.
> 
> Yeah, that seems reasonable. Here's the diff for that part:
> 
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 7b6028b399d8..98bf19b4e086 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -118,7 +118,6 @@ struct seccomp_notif_resp {
>  
>  /**
>   * struct seccomp_notif_addfd
> - * @size: The size of the seccomp_notif_addfd datastructure
>   * @id: The ID of the seccomp notification
>   * @flags: SECCOMP_ADDFD_FLAG_*
>   * @srcfd: The local fd number
> @@ -126,7 +125,6 @@ struct seccomp_notif_resp {
>   * @newfd_flags: The O_* flags the remote FD should have applied
>   */
>  struct seccomp_notif_addfd {
> -	__u64 size;
>  	__u64 id;
>  	__u32 flags;
>  	__u32 srcfd;
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 3c913f3b8451..00cbdad6c480 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -1297,14 +1297,9 @@ static long seccomp_notify_addfd(struct seccomp_filter *filter,
>  	struct seccomp_notif_addfd addfd;
>  	struct seccomp_knotif *knotif;
>  	struct seccomp_kaddfd kaddfd;
> -	u64 size;
>  	int ret;
>  
> -	ret = get_user(size, &uaddfd->size);
> -	if (ret)
> -		return ret;
> -
> -	ret = copy_struct_from_user(&addfd, sizeof(addfd), uaddfd, size);
> +	ret = copy_from_user(&addfd, uaddfd, sizeof(addfd));
>  	if (ret)
>  		return ret;
>  
> 
> > 
> > ----
> > +#define SECCOMP_IOCTL_NOTIF_ADDFD	SECCOMP_IOR(3,	\
> > +						struct seccomp_notif_addfd)
> > 
> > Lastly, what I believe to be a small mistake, it should be SECCOMP_IOW, based on 
> > the documentation in ioctl.h -- "_IOW means userland is writing and kernel is 
> > reading."
> 
> Oooooh. Yeah; good catch. Uhm, that means SECCOMP_IOCTL_NOTIF_ID_VALID
> is wrong too, yes? Tycho, Christian, how disruptive would this be to
> fix? (Perhaps support both and deprecate the IOR version at some point
> in the future?)

We have custom defines in our source code, i.e.
#define SECCOMP_IOCTL_NOTIF_ID_VALID  SECCOMP_IOR(2, __u64)
so ideally we'd have a SECCOMP_IOCTL_NOTIF_ID_VALID_V2

Does that sound ok?

Christian