Re: [PATCH v1 2/4] pid: add pidctl()

[Joel Fernandes]

On Tue, Mar 26, 2019 at 04:55:11PM +0100, Christian Brauner wrote:
> The pidctl() syscalls builds on, extends, and improves translate_pid() [4].
> I quote Konstantins original patchset first that has already been acked and
> picked up by Eric before and whose functionality is preserved in this
> syscall:
> 
> "Each process have different pids, one for each pid namespace it belongs.
>  When interaction happens within single pid-ns translation isn't required.
>  More complicated scenarios needs special handling.
> 
>  For example:
>  - reading pid-files or logs written inside container with pid namespace
>  - writing logs with internal pids outside container for pushing them into
>  - attaching with ptrace to tasks from different pid namespace
> 
>  Generally speaking, any cross pid-ns API with pids needs translation.
> 
>  Currently there are several interfaces that could be used here:
> 
>  Pid namespaces are identified by device and inode of /proc/[pid]/ns/pid.
> 
>  Pids for nested pid namespaces are shown in file /proc/[pid]/status.
>  In some cases pid translation could be easily done using this information.
>  Backward translation requires scanning all tasks and becomes really
>  complicated for deeper namespace nesting.
> 
>  Unix socket automatically translates pid attached to SCM_CREDENTIALS.
>  This requires CAP_SYS_ADMIN for sending arbitrary pids and entering
>  into pid namespace, this expose process and could be insecure."
> 
> The original patchset allowed two distinct operations implicitly:
> - discovering whether pid namespaces (pidns) have a parent-child
>   relationship
> - translating a pid from a source pidns into a target pidns
> 
> Both tasks are accomplished in the original patchset by passing a pid
> along. If the pid argument is passed as 1 the relationship between two pid
> namespaces can be discovered.
> The syscall will gain a lot clearer syntax and will be easier to use for
> userspace if the task it is asked to perform is passed through a
> command argument. Additionally, it allows us to remove an intrinsic race
> caused by using the pid argument as a way to discover the relationship
> between pid namespaces.
> This patch introduces three commands:
> 
> /* PIDCMD_QUERY_PID */
> PIDCMD_QUERY_PID allows to translate a pid between pid namespaces.
> Given a source pid namespace fd return the pid of the process in the target
> namespace:

Could we call this PIDCMD_TRANSLATE_PID please? QUERY is confusing/ambigious
IMO (see below).

> 1. pidctl(PIDCMD_QUERY_PID, pid, source_fd, -1, 0)
>   - retrieve pidns identified by source_fd
>   - retrieve struct pid identifed by pid in pidns identified by source_fd
>   - retrieve callers pidns
>   - return pid in callers pidns
> 
> 2. pidctl(PIDCMD_QUERY_PID, pid, -1, target_fd, 0)
>   - retrieve callers pidns
>   - retrieve struct pid identifed by pid in callers pidns
>   - retrieve pidns identified by target_fd
>   - return pid in pidns identified by target_fd
> 
> 3. pidctl(PIDCMD_QUERY_PID, 1, source_fd, -1, 0)
>   - retrieve pidns identified by source_fd
>   - retrieve struct pid identifed by init task in pidns identified by source_fd
>   - retrieve callers pidns
>   - return pid of init task of pidns identified by source_fd in callers pidns
> 
> 4. pidctl(PIDCMD_QUERY_PID, pid, source_fd, target_fd, 0)
>   - retrieve pidns identified by source_fd
>   - retrieve struct pid identifed by pid in pidns identified by source_fd
>   - retrieve pidns identified by target_fd
>   - check whether struct pid can be found in pidns identified by target_fd
>   - return pid in pidns identified by target_fd
> 
> /* PIDCMD_QUERY_PIDNS */
> PIDCMD_QUERY_PIDNS allows to determine the relationship between pid
> namespaces.
> In the original version of the pachset passing pid as 1 would allow to
> deterimine the relationship between the pid namespaces. This is inherhently
> racy. If pid 1 inside a pid namespace has died it would report false
> negatives. For example, if pid 1 inside of the target pid namespace already
> died, it would report that the target pid namespace cannot be reached from
> the source pid namespace because it couldn't find the pid inside of the
> target pid namespace and thus falsely report to the user that the two pid
> namespaces are not related. This problem is simple to avoid. In the new
> version we simply walk the list of ancestors and check whether the
> namespace are related to each other. By doing it this way we can reliably
> report what the relationship between two pid namespace file descriptors
> looks like.
> 
> 1. pidctl(PIDCMD_QUERY_PIDNS, 0, ns_fd1, ns_fd1, 0) == 0
>    - pidns_of(ns_fd1) and pidns_of(ns_fd2) are unrelated to each other
> 
> 2. pidctl(PIDCMD_QUERY_PIDNS, 0, ns_fd1, ns_fd2, 0) == 1
>    - pidns_of(ns_fd1) == pidns_of(ns_fd2)
> 
> 3. pidctl(PIDCMD_QUERY_PIDNS, 0, ns_fd1, ns_fd2, 0) == 2
>    - pidns_of(ns_fd1) is ancestor of pidns_of(ns_fd2)
> 
> 4. pidctl(PIDCMD_QUERY_PIDNS, 0, ns_fd1, ns_fd2, 0) == 3
>    - pidns_of(ns_fd2) is ancestor of pidns_of(ns_fd1)

Why not call this PIDCMD_COMPARE_PIDNS, since a comparison is what you're doing.

Again QUERY is ambigious here. Above you called QUERY to translate something,
now you're calling QUERY to mean compare something. I suggest to be explicit
about the operation PIDCMD_<OPERATION>_<OPERAND-TYPE>.

Arguably, 2 syscalls for this is cleaner:
pid_compare_namespaces(ns_fd1, ns_fd2);
pid_translate(pid, ns_fd1, nds_fd2);


> These two commands - PIDCMD_QUERY_PID and PIDCMD_QUERY_PIDNS - cover and
> improve the functionality expressed implicitly in translate_pid() before.
> 
> /* PIDCMD_GET_PIDFD */

And this can be a third syscall:
pidfd_translate(pid, ns_fd1, ns_fd2).

I am actually supportive of Daniel's view that by combining too many
arguments into a single syscall, becomes confusing and sometimes some
arguments have to be forced to 0 in the single shoe-horned syscall. Like you
don't need a pid to compare to pid-ns, so user has to set that to 0.

More comments below...

> This command allows to retrieve file descriptors for processes and removes
> the dependency of pidfds and thereby the pidfd_send_signal() syscall on
> procfs. First, multiple people have expressed a desire to do this even when
> pidfd_send_signal() was merged. It is even recorded in the commit message
> for pidfd_send_signal() itself
> (cf. commit 3eb39f47934f9d5a3027fe00d906a45fe3a15fad):
> Q-06: (Andrew Morton [1])
>       Is there a cleaner way of obtaining the fd? Another syscall perhaps.
> A-06: Userspace can already trivially retrieve file descriptors from procfs
>       so this is something that we will need to support anyway. Hence,
>       there's no immediate need to add another syscalls just to make
>       pidfd_send_signal() not dependent on the presence of procfs. However,
>       adding a syscalls to get such file descriptors is planned for a
>       future patchset (cf. [1]).
> Alexey made a similar request (cf. [2]).
> Additionally, Andy made an additional, idependent argument that we should
> go forward with non-proc-dirfd file descriptors for the sake of security
> and extensibility (cf. [3]).
> 
> The pidfds are not associated with a specific pid namespaces but rather
> only with struct pid. What the pidctl() syscall enforces is that when a
> caller wants to retrieve a pidfd file descriptor for a pid in a given
> target pid namespace the caller 
> - must have been given access to two file descriptors referring
>   to target and source pid namespace
> - the source pid namespace must be an ancestor of the target pid
>   namespace
> - the pid must be translatable from the source pid namespace into the
>   target pid namespace
> 
> 1. pidctl(PIDCMD_GET_PIDFD, pid, source_fd, -1, 0)
>   - retrieve pidns identified by source_fd
>   - retrieve struct pid identifed by pid in pidns identified by source_fd
>   - retrieve callers pidns
>   - return pidfd
> 
> 2. pidctl(PIDCMD_GET_PIDFD, pid, -1, target_fd, 0)
>   - retrieve callers pidns
>   - retrieve struct pid identifed by pid in callers pidns
>   - retrieve pidns identified by target_fd
>   - return pidfd
> 
> 3. pidctl(PIDCMD_GET_PIDFD, 1, source_fd, -1, 0)
>   - retrieve pidns identified by source_fd
>   - retrieve struct pid identifed by init task in pidns identified by
>     source_fd
>   - retrieve callers pidns
>   - return pidfd
> 
> 4. pidctl(PIDCMD_GET_PIDFD, pid, source_fd, target_fd, 0)
>   - retrieve pidns identified by source_fd
>   - retrieve struct pid identifed by pid in pidns identified by source_fd
>   - retrieve pidns identified by target_fd
>   - check whether struct pid can be found in pidns identified by target_fd
>   - return pidfd
> 
> These pidfds are allocated using anon_inode_getfd(), are O_CLOEXEC by
> default and can be used with the pidfd_send_signal() syscall. They are not
> dirfds and as such have the advantage that we can make them pollable or
> readable in the future if we see a need to do so (which I'm pretty sure we
> will eventually). Currently they do not support any advanced operations.
> 
> /* References */
> [1]: https://lore.kernel.org/lkml/20181228233725.722tdfgijxcssg76@xxxxxxxxxx/
> [2]: https://lore.kernel.org/lkml/20190320203910.GA2842@avx2/
> [3]: https://lore.kernel.org/lkml/CALCETrXO=V=+qEdLDVPf8eCgLZiB9bOTrUfe0V-U-tUZoeoRDA@xxxxxxxxxxxxxx/
> [4]: https://lore.kernel.org/lkml/20181109034919.GA21681@xxxxxxxxxxxx/
[snip]
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index e446806a561f..a4c8c59f7c8f 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -929,6 +929,8 @@ asmlinkage long sys_clock_adjtime32(clockid_t which_clock,
>  				struct old_timex32 __user *tx);
>  asmlinkage long sys_syncfs(int fd);
>  asmlinkage long sys_setns(int fd, int nstype);
> +asmlinkage long sys_pidctl(unsigned int cmd, pid_t pid, int source, int target,
> +			   unsigned int flags);
>  asmlinkage long sys_sendmmsg(int fd, struct mmsghdr __user *msg,
>  			     unsigned int vlen, unsigned flags);
>  asmlinkage long sys_process_vm_readv(pid_t pid,
> diff --git a/include/uapi/linux/wait.h b/include/uapi/linux/wait.h
> index ac49a220cf2a..e9564ec06b07 100644
> --- a/include/uapi/linux/wait.h
> +++ b/include/uapi/linux/wait.h
> @@ -18,5 +18,19 @@
>  #define P_PID		1
>  #define P_PGID		2
>  
> +/* Commands to pass to pidctl() */
> +enum pidcmd {
> +	PIDCMD_QUERY_PID   = 0, /* Get pid in target pid namespace */
> +	PIDCMD_QUERY_PIDNS = 1, /* Determine relationship between pid namespaces */
> +	PIDCMD_GET_PIDFD   = 2, /* Get pidfd for a process */
> +};
> +
> +/* Return values of PIDCMD_QUERY_PIDNS */
> +enum pidcmd_query_pidns {
> +	PIDNS_UNRELATED          = 0, /* The pid namespaces are unrelated */
> +	PIDNS_EQUAL              = 1, /* The pid namespaces are equal */
> +	PIDNS_SOURCE_IS_ANCESTOR = 2, /* Source pid namespace is ancestor of target pid namespace */
> +	PIDNS_TARGET_IS_ANCESTOR = 3, /* Target pid namespace is ancestor of source pid namespace */
> +};
>  
>  #endif /* _UAPI_LINUX_WAIT_H */
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 20881598bdfa..3213a137a63e 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -26,6 +26,7 @@
>   *
>   */
>  
> +#include <linux/anon_inodes.h>
>  #include <linux/mm.h>
>  #include <linux/export.h>
>  #include <linux/slab.h>
> @@ -40,6 +41,7 @@
>  #include <linux/proc_fs.h>
>  #include <linux/sched/task.h>
>  #include <linux/idr.h>
> +#include <linux/wait.h>
>  
>  struct pid init_struct_pid = {
>  	.count 		= ATOMIC_INIT(1),
> @@ -451,6 +453,165 @@ struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
>  	return idr_get_next(&ns->idr, &nr);
>  }
>  
> +static int pidfd_release(struct inode *inode, struct file *file)
> +{
> +	struct pid *pid = file->private_data;
> +
> +	if (pid) {
> +		file->private_data = NULL;
> +		put_pid(pid);
> +	}
> +
> +	return 0;
> +}
> +
> +const struct file_operations pidfd_fops = {
> +	.release = pidfd_release,
> +};
> +
> +static int pidfd_create_fd(struct pid *pid, unsigned int o_flags)
> +{
> +	int fd;
> +
> +	fd = anon_inode_getfd("pidfd", &pidfd_fops, get_pid(pid), O_RDWR | o_flags);
> +	if (fd < 0)
> +		put_pid(pid);
> +
> +	return fd;
> +}
> +
> +static struct pid_namespace *get_pid_ns_by_fd(int fd)
> +{
> +	struct pid_namespace *pidns = ERR_PTR(-EINVAL);
> +
> +	if (fd >= 0) {
> +#ifdef CONFIG_PID_NS
> +		struct ns_common *ns;
> +		struct file *file = proc_ns_fget(fd);
> +		if (IS_ERR(file))
> +			return ERR_CAST(file);
> +
> +		ns = get_proc_ns(file_inode(file));
> +		if (ns->ops->type == CLONE_NEWPID) {
> +			pidns = container_of(ns, struct pid_namespace, ns);
> +			get_pid_ns(pidns);
> +		}
> +
> +		fput(file);
> +#endif
> +	} else {
> +		pidns = task_active_pid_ns(current);
> +		get_pid_ns(pidns);
> +	}
> +
> +	return pidns;
> +}
> +
> +static int pidns_related(struct pid_namespace *source,
> +			 struct pid_namespace *target)
> +{
> +	int query;
> +
> +	query = pidnscmp(source, target);
> +	switch (query) {
> +	case 0:
> +		return PIDNS_EQUAL;
> +	case 1:
> +		return PIDNS_SOURCE_IS_ANCESTOR;
> +	}
> +
> +	query = pidnscmp(target, source);
> +	if (query == 1)
> +		return PIDNS_TARGET_IS_ANCESTOR;
> +
> +	return PIDNS_UNRELATED;
> +}
> +
> +/*
> + * pidctl - perform operations on pids
> + * @cmd:    command to execute
> + * @pid:    pid for translation
> + * @source: pid-ns file descriptor or -1 for active namespace
> + * @target: pid-ns file descriptor or -1 for active namesapce
> + * @flags:  flags to pass
> + *
> + * If cmd is PIDCMD_QUERY_PID translates pid between pid namespaces
> + * identified by @source and @target. Returns pid if process has pid in
> + * @target, -ESRCH if process does not have a pid in @source, -ENOENT if
> + * process has no pid in @target.
> + *
> + * If cmd is PIDCMD_QUERY_PIDNS determines the relations between two pid
> + * namespaces. Returns 2 if @source is an ancestor pid namespace
> + * of @target, 1 if @source and @target refer to the same pid namespace,
> + * 3 if @target is an ancestor pid namespace of @source, 0 if they have
> + * no parent-child relationship in either direction.
> + *
> + * If cmd is PIDCMD_GET_PIDFD returns pidfd for process in @target pid
> + * namespace. Returns pidfd if process has pid in @target, -ESRCH if
> + * process does not have a pid in @source, -ENOENT if process does not
> + * have a pid in @target pid namespace.
> + *
> + */
> +SYSCALL_DEFINE5(pidctl, unsigned int, cmd, pid_t, pid, int, source, int, target,
> +		unsigned int, flags)

flags seems not needed since it is unused, but I get it that you may want to
have flags in the future? If yes, give example of future flags?

> +{
> +	struct pid_namespace *source_ns = NULL, *target_ns = NULL;

Setting these to NULL is no longer needed.

> +	struct pid *struct_pid;
> +	pid_t result;
> +
> +	if (flags)
> +		return -EINVAL;
> +
> +	switch (cmd) {
> +	case PIDCMD_QUERY_PID:
> +		break;
> +	case PIDCMD_QUERY_PIDNS:
> +		if (pid)
> +			return -EINVAL;
> +		break;
> +	case PIDCMD_GET_PIDFD:
> +		break;
> +	default:
> +		return -EOPNOTSUPP;
> +	}
> +
> +	source_ns = get_pid_ns_by_fd(source);
> +	if (IS_ERR(source_ns))
> +		return PTR_ERR(source_ns);
> +
> +	target_ns = get_pid_ns_by_fd(target);
> +	if (IS_ERR(target_ns)) {
> +		put_pid_ns(source_ns);
> +		return PTR_ERR(target_ns);
> +	}
> +
> +	if (cmd == PIDCMD_QUERY_PIDNS) {
> +		result = pidns_related(source_ns, target_ns);
> +	} else {
> +		rcu_read_lock();
> +		struct_pid = get_pid(find_pid_ns(pid, source_ns));
> +		rcu_read_unlock();
> +
> +		if (struct_pid)
> +			result = pid_nr_ns(struct_pid, target_ns);
> +		else
> +			result = -ESRCH;
> +
> +		if (cmd == PIDCMD_GET_PIDFD && (result > 0))
> +			result = pidfd_create_fd(struct_pid, O_CLOEXEC);

pidfd_create_fd already does put_pid on errors..

> +
> +		if (!result)
> +			result = -ENOENT;
> +
> +		put_pid(struct_pid);

so on error you would put_pid twice which seems odd..  I would suggest, don't
release the pid ref from within pidfd_create_fd, release the ref from the
caller. Speaking of which, I added to my list to convert the pid->count to
refcount_t at some point :)

> +	}
> +
> +	put_pid_ns(target_ns);
> +	put_pid_ns(source_ns);

This part looks more clean than before so good.

thanks,

 - Joel


> +	return result;
> +}
> +
>  void __init pid_idr_init(void)
>  {
>  	/* Verify no one has done anything silly: */
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index aa6e72fb7c08..1c863fb3d55a 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -429,6 +429,31 @@ static struct ns_common *pidns_get_parent(struct ns_common *ns)
>  	return &get_pid_ns(pid_ns)->ns;
>  }
>  
> +/**
> + * pidnscmp - Determine if @ancestor is ancestor of @descendant
> + * @ancestor:   pidns suspected to be the ancestor of @descendant
> + * @descendant: pidns suspected to be the descendant of @ancestor
> + *
> + * Returns -1 if @ancestor is not an ancestor of @descendant,
> + * 0 if @ancestor is the same pidns as @descendant, 1 if @ancestor
> + * is an ancestor of @descendant.
> + */
> +int pidnscmp(struct pid_namespace *ancestor, struct pid_namespace *descendant)
> +{
> +	if (ancestor == descendant)
> +		return 0;
> +
> +	for (;;) {
> +		if (!descendant)
> +			return -1;
> +		if (descendant == ancestor)
> +			break;
> +		descendant = descendant->parent;
> +	}
> +
> +	return 1;
> +}
> +
>  static struct user_namespace *pidns_owner(struct ns_common *ns)
>  {
>  	return to_pid_ns(ns)->user_ns;
> -- 
> 2.21.0
>