Re: [PATCH v2 1/2] fork: extend clone3() to support CLONE_SET_TID

[Christian Brauner]

On Wed, Jul 31, 2019 at 06:12:22PM +0200, Adrian Reber wrote:
> The main motivation to add CLONE_SET_TID to clone3() is CRIU.
> 
> To restore a process with the same PID/TID CRIU currently uses
> /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> ns_last_pid and then (quickly) does a clone(). This works most of the
> time, but it is racy. It is also slow as it requires multiple syscalls.

Can you elaborate how this is racy, please. Afaict, CRIU will always
usually restore in a new pid namespace that it controls, right? What is
the exact race?

> 
> Extending clone3() to support CLONE_SET_TID makes it possible restore a
> process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> race free (as long as the desired PID/TID is available).
> 
> This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> on clone3() with set_tid as they are currently in place for ns_last_pid.
> 
> v2:
>  - Removed (size < sizeof(struct clone_args)) as discussed with
>    Christian and Dmitry
>  - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
>  - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)

Fwiw, the changelog should be placed after the "---" after your SOB, so
rather :):

Signed-off-by: Adrian Reber <areber@xxxxxxxxxx>
---
v2:
 - Removed (size < sizeof(struct clone_args)) as discussed with
   Christian and Dmitry
 - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
 - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)
---

> 
> Signed-off-by: Adrian Reber <areber@xxxxxxxxxx>
> ---
>  include/linux/pid.h        |  2 +-
>  include/linux/sched/task.h |  1 +
>  include/uapi/linux/sched.h |  2 ++
>  kernel/fork.c              | 25 ++++++++++++++++---------
>  kernel/pid.c               | 30 +++++++++++++++++++++++-------
>  5 files changed, 43 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 2a83e434db9d..052000db0ced 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
>  extern struct pid *find_get_pid(int nr);
>  extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
>  
> -extern struct pid *alloc_pid(struct pid_namespace *ns);
> +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
>  extern void free_pid(struct pid *pid);
>  extern void disable_pid_allocation(struct pid_namespace *ns);
>  
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 0497091e40c1..4f2a80564332 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -26,6 +26,7 @@ struct kernel_clone_args {
>  	unsigned long stack;
>  	unsigned long stack_size;
>  	unsigned long tls;
> +	pid_t set_tid;
>  };
>  
>  /*
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index b3105ac1381a..8c4e803e8147 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -32,6 +32,7 @@
>  #define CLONE_NEWPID		0x20000000	/* New pid namespace */
>  #define CLONE_NEWNET		0x40000000	/* New network namespace */
>  #define CLONE_IO		0x80000000	/* Clone io context */
> +#define CLONE_SET_TID		0x100000000ULL	/* set if the desired TID is set in set_tid */
>  
>  /*
>   * Arguments for the clone3 syscall
> @@ -45,6 +46,7 @@ struct clone_args {
>  	__aligned_u64 stack;
>  	__aligned_u64 stack_size;
>  	__aligned_u64 tls;
> +	__aligned_u64 set_tid;
>  };
>  
>  /*
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 2852d0e76ea3..405f98ce4c83 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2031,7 +2031,7 @@ static __latent_entropy struct task_struct *copy_process(
>  	stackleak_task_init(p);
>  
>  	if (pid != &init_struct_pid) {
> -		pid = alloc_pid(p->nsproxy->pid_ns_for_children);
> +		pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
>  		if (IS_ERR(pid)) {
>  			retval = PTR_ERR(pid);
>  			goto bad_fork_cleanup_thread;
> @@ -2530,14 +2530,12 @@ 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)))
> -		return -EINVAL;
> -
>  	if (unlikely(!access_ok(uargs, size)))
>  		return -EFAULT;
>  
> @@ -2562,6 +2560,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  	if (copy_from_user(&args, uargs, size))
>  		return -EFAULT;
>  
> +	if ((args.flags & CLONE_SET_TID) && !ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
> +		return -EPERM;

Have you made sure that this makes sense with all flags, e.g. does this
make sense when specified with CLONE_THREAD?

> +
>  	*kargs = (struct kernel_clone_args){
>  		.flags		= args.flags,
>  		.pidfd		= u64_to_user_ptr(args.pidfd),
> @@ -2571,6 +2572,7 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  		.stack		= args.stack,
>  		.stack_size	= args.stack_size,
>  		.tls		= args.tls,
> +		.set_tid	= args.set_tid,
>  	};
>  
>  	return 0;
> @@ -2578,11 +2580,16 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  
>  static bool clone3_args_valid(const struct kernel_clone_args *kargs)
>  {
> -	/*
> -	 * All lower bits of the flag word are taken.
> -	 * Verify that no other unknown flags are passed along.
> -	 */
> -	if (kargs->flags & ~CLONE_LEGACY_FLAGS)
> +	/* Verify that no other unknown flags are passed along. */
> +	if (kargs->flags & ~(CLONE_LEGACY_FLAGS | CLONE_SET_TID))
> +		return false;
> +
> +	/* Fail if set_tid is set without CLONE_SET_TID */
> +	if (kargs->set_tid && !(kargs->flags & CLONE_SET_TID))
> +		return false;
> +
> +	/* Also fail if set_tid is invalid */
> +	if ((kargs->set_tid <= 0) && (kargs->flags & CLONE_SET_TID))
>  		return false;
>  
>  	/*
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 0a9f2e437217..977f3ac39d7f 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
>  	call_rcu(&pid->rcu, delayed_put_pid);
>  }
>  
> -struct pid *alloc_pid(struct pid_namespace *ns)
> +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
>  {
>  	struct pid *pid;
>  	enum pid_type type;
> @@ -186,12 +186,28 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  		if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
>  			pid_min = RESERVED_PIDS;
>  
> -		/*
> -		 * Store a null pointer so find_pid_ns does not find
> -		 * a partially initialized PID (see below).
> -		 */
> -		nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> -				      pid_max, GFP_ATOMIC);
> +		if (set_tid) {
> +			/*
> +			 * Also fail if a PID != 1 is requested
> +			 * and no PID 1 exists.
> +			 */
> +			if ((set_tid >= pid_max) || ((set_tid != 1) &&
> +				(idr_get_cursor(&tmp->idr) <= 1))) {
> +				spin_unlock_irq(&pidmap_lock);
> +				retval = -EINVAL;
> +				goto out_free;
> +			}
> +			nr = idr_alloc(&tmp->idr, NULL, set_tid,
> +				       set_tid + 1, GFP_ATOMIC);

Hm, feels to me that we should report a different error code than EAGAIN
when the allocation fails for set_tid. Right now you get EAGAIN for both
the non set_tid and the set_tid codepath.
But for set_tid the error that you likely should surface is EEXIST, i.e.
that pid is already taken.

> +			set_tid = 0;
> +		} else {
> +			/*
> +			 * Store a null pointer so find_pid_ns does not find
> +			 * a partially initialized PID (see below).
> +			 */
> +			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> +					      pid_max, GFP_ATOMIC);
> +		}
>  		spin_unlock_irq(&pidmap_lock);
>  		idr_preload_end();
>  
> -- 
> 2.21.0
>