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Date:   Mon, 12 Aug 2019 23:50:53 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Adrian Reber <areber@redhat.com>
Cc:     Andrei Vagin <avagin@gmail.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Pavel Emelianov <xemul@virtuozzo.com>,
        Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@gmail.com>
Subject: Re: [PATCH v6 1/2] fork: extend clone3() to support setting a PID
Message-ID: <20190812215053.ddqcntgf4p37gzfx@wittgenstein>
References: <20190812200939.23784-1-areber@redhat.com>
 <CANaxB-z+DktJ2Kaqdpyp60oPwAtiKyctnoJA2gFZ2r=044DfjQ@mail.gmail.com>
 <20190812210241.GA23563@dcbz.redhat.com>
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On Mon, Aug 12, 2019 at 11:02:41PM +0200, Adrian Reber wrote:
> On Mon, Aug 12, 2019 at 01:43:53PM -0700, Andrei Vagin wrote:
> > On Mon, Aug 12, 2019 at 1:10 PM Adrian Reber <areber@redhat.com> wrote:
> > >
> > > The main motivation to add 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.
> > >
> > > Extending clone3() to support 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.
> > >
> > > Signed-off-by: Adrian Reber <areber@redhat.com>
> > > ---
> > > 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)
> > >
> > > v3:
> > >  - Return EEXIST if PID is already in use (Christian)
> > >  - Drop CLONE_SET_TID (Christian and Oleg)
> > >  - Use idr_is_empty() instead of idr_get_cursor() (Oleg)
> > >  - Handle different `struct clone_args` sizes (Dmitry)
> > >
> > > v4:
> > >  - Rework struct size check with defines (Christian)
> > >  - Reduce number of set_tid checks (Oleg)
> > >  - Less parentheses and more robust code (Oleg)
> > >  - Do ns_capable() on correct user_ns (Oleg, Christian)
> > >
> > > v5:
> > >  - make set_tid checks earlier in alloc_pid() (Christian)
> > >  - remove unnecessary comment and struct size check (Christian)
> > >
> > > v6:
> > >  - remove CLONE_SET_TID from description (Christian)
> > >  - add clone3() tests for different clone_args sizes (Christian)
> > >  - move more set_tid checks to alloc_pid() (Oleg)
> > >  - make all set_tid checks lockless (Oleg)
> > > ---
> > >  include/linux/pid.h        |  2 +-
> > >  include/linux/sched/task.h |  1 +
> > >  include/uapi/linux/sched.h |  1 +
> > >  kernel/fork.c              | 14 ++++++++++++--
> > >  kernel/pid.c               | 37 ++++++++++++++++++++++++++++++-------
> > >  5 files changed, 45 insertions(+), 10 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..e1ce103a2c47 100644
> > > --- a/include/uapi/linux/sched.h
> > > +++ b/include/uapi/linux/sched.h
> > > @@ -45,6 +45,7 @@ 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..8317d408a8d6 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -117,6 +117,11 @@
> > >   */
> > >  #define MAX_THREADS FUTEX_TID_MASK
> > >
> > > +/*
> > > + * For different sizes of struct clone_args
> > > + */
> > > +#define CLONE3_ARGS_SIZE_V0 64
> > > +
> > >  /*
> > >   * Protected counters by write_lock_irq(&tasklist_lock)
> > >   */
> > > @@ -2031,7 +2036,7 @@ 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;
> > > @@ -2535,9 +2540,13 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > >         if (unlikely(size > PAGE_SIZE))
> > >                 return -E2BIG;
> > >
> > > -       if (unlikely(size < sizeof(struct clone_args)))
> > > +       if (unlikely(size < CLONE3_ARGS_SIZE_V0))
> > >                 return -EINVAL;
> > >
> > > +       if (size < sizeof(struct clone_args))
> > > +               memset((void *)&args + size, 0,
> > > +                               sizeof(struct clone_args) - size);
> > > +
> > >         if (unlikely(!access_ok(uargs, size)))
> > >                 return -EFAULT;
> > >
> > > @@ -2571,6 +2580,7 @@ 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;
> > > diff --git a/kernel/pid.c b/kernel/pid.c
> > > index 0a9f2e437217..5cdab73b9094 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;
> > > @@ -166,6 +166,16 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> > >         struct upid *upid;
> > >         int retval = -ENOMEM;
> > >
> > > +       if (set_tid) {
> > > +               if (set_tid < 0 || set_tid >= pid_max)
> > > +                       return ERR_PTR(-EINVAL);
> > > +               /* Also fail if a PID != 1 is requested and no PID 1 exists */
> > > +               if (set_tid != 1 && !ns->child_reaper)
> > > +                       return ERR_PTR(-EINVAL);
> > 
> > What will happen if I will create a new pid namespace
> > (unsahre(CLONE_NEWPID)) and then call clone3 with tid which isn't
> > equal to 1?
> > 
> > Will I get the init process with the 5 pid?...
> 
> No, you will get -EINVAL. In the selftest exactly this is tested. A PID
> 1 is still required even with this patch.

Correct. unshare(CLONE_PIDNS) leaves ->child_reaper unset.
->child_reaper is set when pid 1 is created in the pid namespace. So you
that check catches the scenarios afaict. Adrians test should also have
caught that if it were possible. I hope he's running them. :)