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Message-ID: <20190810011033.ns23e7ivlnzkwj6f@wittgenstein>
Date: Sat, 10 Aug 2019 03:10:34 +0200
From: Christian Brauner <christian.brauner@...ntu.com>
To: Adrian Reber <areber@...hat.com>
Cc: Eric Biederman <ebiederm@...ssion.com>,
Pavel Emelianov <xemul@...tuozzo.com>,
Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
Dmitry Safonov <0x7f454c46@...il.com>,
linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
Mike Rapoport <rppt@...ux.ibm.com>,
Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v4 1/2] fork: extend clone3() to support CLONE_SET_TID
On Thu, Aug 08, 2019 at 11:22:21PM +0200, Adrian Reber 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@...hat.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)
> ---
> include/linux/pid.h | 2 +-
> include/linux/sched/task.h | 1 +
> include/uapi/linux/sched.h | 1 +
> kernel/fork.c | 25 +++++++++++++++++++++++--
> kernel/pid.c | 34 +++++++++++++++++++++++++++-------
> 5 files changed, 53 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..2a03f0e201e9 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -117,6 +117,13 @@
> */
> #define MAX_THREADS FUTEX_TID_MASK
>
> +/*
> + * Different sizes of struct clone_args
> + */
> +#define CLONE3_ARGS_SIZE_V0 64
> +/* V1 includes set_tid */
> +#define CLONE3_ARGS_SIZE_V1 72
> +
> /*
> * Protected counters by write_lock_irq(&tasklist_lock)
> */
> @@ -2031,7 +2038,13 @@ 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);
> + if (args->set_tid && !ns_capable(
> + p->nsproxy->pid_ns_for_children->user_ns,
> + CAP_SYS_ADMIN)) {
> + retval = -EPERM;
> + goto bad_fork_cleanup_thread;
> + }
> + 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 +2548,14 @@ 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)))
> + /* The struct needs to be at least the size of the original struct. */
I don't think you need that comment. I think the macro is pretty
self-explanatory. If you want it to be even clearer you could even make
it CLONE3_ARGS_SIZE_MIN but V0 is good enough. :)
> + 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;
>
> @@ -2573,6 +2591,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> .tls = args.tls,
> };
>
> + if (size >= CLONE3_ARGS_SIZE_V1)
> + kargs->set_tid = args.set_tid;
Hm, the if-condition is not needed though, right? At this point we will
have already copied from struct clone_args __user *uargs into struct
clone_args args. If we hit that codepath that means the kernel
definitely has a field for set_tid in its struct clone_args. :) So this
could probably just be:
.tls = args.tls,
.set_tid = args.set_tid,
}
?
> +
> return 0;
> }
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 0a9f2e437217..9ce89c35c5be 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 +186,32 @@ 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.
> + */
> + nr = -EINVAL;
> + if (set_tid < pid_max && set_tid > 0 &&
Hm, you're already in the if-branch hat verified if (set_tid) so the
set_tid > 0 conjunct seems redundant. :)
> + (set_tid == 1 || !idr_is_empty(&tmp->idr)))
> + nr = idr_alloc(&tmp->idr, NULL, set_tid,
> + set_tid + 1, GFP_ATOMIC);
I'm confused, shouldn't this be
if (set_tid < pid_max || (set_tid == 1 && !idr_is_emtpy(&tmp->idf)))
> + /*
> + * If ENOSPC is returned it means that the PID is
> + * alreay in use. Return EEXIST in that case.
> + */
> + if (nr == -ENOSPC)
> + nr = -EEXIST;
> + /* Only use set_tid for one PID namespace. */
> + 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
>
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