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

[Christian Brauner]

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:

Why does it matter that the ioctl() has the size of the struct embedded
within? Afaik, the kernel itself doesn't do anything with that size. It
merely checks that the size is not pathological and it does so at
compile time.

#ifdef __CHECKER__
#define _IOC_TYPECHECK(t) (sizeof(t))
#else
/* provoke compile error for invalid uses of size argument */
extern unsigned int __invalid_size_argument_for_IOC;
#define _IOC_TYPECHECK(t) \
	((sizeof(t) == sizeof(t[1]) && \
	  sizeof(t) < (1 << _IOC_SIZEBITS)) ? \
	  sizeof(t) : __invalid_size_argument_for_IOC)
#endif

The size itself is not verified at runtime. copy_struct_from_user()
still makes sense at least if we're going to allow expanding the struct
in the future.

Leaving that aside, the proposed direction here seems to mean that any
change to the struct itself will immediately mean a new ioctl() but
afaict, that also means a new struct. Since when you simply extend the
struct for the sake of the new ioctl you also change the size for the
ioctl.

Sure, you can simply treat the struct coming through the old ioctl as
being "capped" by e.g. hiding the size as suggested but then the gain
by having two separate ioctls is 0 compared to simply versioning the
struct with an explicit size member since the size encoded in the ioctl
and the actual size of the struct don't line up anymore which is the
only plus I can see for relying on _IOC_SIZE(). All this manages to do
then is to make it more annoying for userspace since they now need to
maintain multiple ioctls(). And if you have - however unlikely - say
three different ioctls all to be used with a different struct size of
the same struct I now need to know which ioctl() goes with which size of
the struct (I guess you could append the size to the ioctl name?
*shudder*). If you have the size in the struct itself you don't need to
care about any of that.
Maybe I'm not making sense or I misunderstand what's going on though.

Christian

> 
> 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?)
> 
> Diff for just addfd's change:
> 
> 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
> @@ -146,7 +144,7 @@ struct seccomp_notif_addfd {
>  						struct seccomp_notif_resp)
>  #define SECCOMP_IOCTL_NOTIF_ID_VALID	SECCOMP_IOR(2, __u64)
>  /* On success, the return value is the remote process's added fd number */
> -#define SECCOMP_IOCTL_NOTIF_ADDFD	SECCOMP_IOR(3,	\
> +#define SECCOMP_IOCTL_NOTIF_ADDFD	SECCOMP_IOW(3,	\
>  						struct seccomp_notif_addfd)
>  
>  #endif /* _UAPI_LINUX_SECCOMP_H */
> 
> -- 
> Kees Cook