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

[Kees Cook]

On Thu, Jun 11, 2020 at 10:39:23AM +0000, Sargun Dhillon wrote:
> On Thu, Jun 11, 2020 at 11:19:42AM +0200, Christian Brauner wrote:
> > 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
> Why not change the public API in seccomp.h to:
> #define SECCOMP_IOCTL_NOTIF_ID_VALID	SECCOMP_IOW(2, __u64)
> 
> And then in seccomp.c:
> #define SECCOMP_IOCTL_NOTIF_ID_VALID_OLD	SECCOMP_IOR(2, __u64)
> static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
> 				 unsigned long arg)
> {
> 	struct seccomp_filter *filter = file->private_data;
> 	void __user *buf = (void __user *)arg;
> 
> 	switch (cmd) {
> 	case SECCOMP_IOCTL_NOTIF_RECV:
> 		return seccomp_notify_recv(filter, buf);
> 	case SECCOMP_IOCTL_NOTIF_SEND:
> 		return seccomp_notify_send(filter, buf);
> 	case SECCOMP_IOCTL_NOTIF_ID_VALID_OLD:
> 		pr_warn_once("Detected usage of legacy (incorrect) version of seccomp notifier notif_id_valid ioctl\n");
> 	case SECCOMP_IOCTL_NOTIF_ID_VALID:
> 		return seccomp_notify_id_valid(filter, buf);
> 	default:
> 		return -EINVAL;
> 	}
> }
> ---- 
> 
> So, both will work fine, and whenevery anyone recompiles, or picks up new 
> headers, they will start calling the "right" one without a code change, and
> we wont break any userspace.

Yeah, that's what I'd prefer here.

-- 
Kees Cook