Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET_TID

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

On Wed, Aug 07, 2019 at 05:48:29PM +0200, Oleg Nesterov wrote:
> On 08/06, Adrian Reber wrote:
> >
> > @@ -2530,12 +2530,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> >  					      struct clone_args __user *uargs,
> >  					      size_t size)
> >  {
> > +	struct pid_namespace *pid_ns = task_active_pid_ns(current);
> >  	struct clone_args args;
> >  
> >  	if (unlikely(size > PAGE_SIZE))
> >  		return -E2BIG;
> >  
> > -	if (unlikely(size < sizeof(struct clone_args)))
> > +	/* The struct needs to be at least the size of the original struct. */
> > +	if (size < (sizeof(struct clone_args) - sizeof(__aligned_u64)))
> >  		return -EINVAL;
> 
> slightly off-topic, but with or without this patch I do not understand
> -EINVAL. Can't we replace this check with
> 
> 	if (size < sizeof(struct clone_args))
> 		memset((void*)&args + size, sizeof(struct clone_args) - size, 0);
> 
> ?
> 
> this way we can new members at the end of clone_args and this matches
> the "if (size > sizeof(struct clone_args))" block below which promises
> that whatever we add into clone_args a zero value should work.

Hm, I actually think we should define:

#define CLONE3_ARGS_SIZE_V0 64
#define CLONE3_ARGS_SIZE_V1 ...
and then later on for future extensions
#define CLONE3_ARGS_SIZE_V2 ...

then do
if (size < CLONE3_ARGS_SIZE_V0)
	return -EINVAL;

then do what you suggested:

if (size < sizeof(struct clone_args))
	memset((void*)&args + size, sizeof(struct clone_args) - size, 0);

> 
> 
> And if we do this
> 
> > +	if (size == sizeof(struct clone_args)) {
> > +		/* Only check permissions if set_tid is actually set. */
> > +		if (args.set_tid &&
> > +			!ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
> > +			return -EPERM;
> > +		kargs->set_tid = args.set_tid;
> > +	}
> 
> we can move this check into clone3_args_valid() or even copy_process()
> 
> 	if (kargs->set_tid) {
> 		if (!ns_capable(...))
> 			return -EPERM;
> 	}
> 
> 
> Either way,
> 
> > @@ -2585,6 +2595,10 @@ static bool clone3_args_valid(const struct kernel_clone_args *kargs)
> >  	if (kargs->flags & ~CLONE_LEGACY_FLAGS)
> >  		return false;
> >
> > +	/* Fail if set_tid is invalid */
> > +	if (kargs->set_tid < 0)
> > +		return false;
> 
> I think it would be more clean to do this along with ns_capable() check,
> or along with the "set_tid >= pid_max" check in alloc_pid().
> 
> I won't insist, but I do not really like the fact we check set_tid 3 times
> in copy_clone_args_from_user(), clone3_args_valid(), and alloc_pid().

Agreed on that part.