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Subject[PATCH RFC v5] pidns: introduce syscall translate_pid
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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
- attaching with ptrace to tasks from different pid namespace
- passing pids across pid namespaces in any kind of API

Currently there are several interfaces that could be used here:

Pid namespaces are identified by inode number of /proc/[pid]/ns/pid.

Pids for nested Pid namespaces are shown in file /proc/[pid]/status.
In some cases conversion pid -> vpid could be easily done using this
information, but backward translation requires scanning all tasks.

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.

This patch adds new syscall for converting pids between pid namespaces:

pid_t translate_pid(pid_t pid, int source_type, int source,
int target_type, int target);

@source_type and @target_type defines type of following arguments:

TRANSLATE_PID_CURRENT_PIDNS - current pid namespace, argument is unused
TRANSLATE_PID_TASK_PIDNS - task pid-ns, argument is task pid
TRANSLATE_PID_FD_PIDNS - pidns fd, argument is file descriptor

Syscall returns pid in target pid-ns or zero if task have no pid there.

Error codes:
-EINVAL - @source or @target couldn't be resolved into pid namespace
-ESRCH - task with @pid is not found in @source pid-namespace

Other pid namespaces are referenced either by pid of any process who
lives inside it or by file descriptor pointing to /proc/[pid]/ns/pid.
Latter method provides better protection against races but in some
cases requires CAP_SYS_PTRACE.

Translate_pid could breach pid isolation and return pids from outer pid
namespaces iff process already has file descriptor pointing to them.


Examples:

- get pid in current pid namespace

translate_pid(pid, TRANSLATE_PID_FD_PIDNS, ns_fd,
TRANSLATE_PID_CURRENT_PIDNS, 0)
or
translate_pid(pid, TRANSLATE_PID_TASK_PIDNS, ns_pid,
TRANSLATE_PID_CURRENT_PIDNS, 0)

- get pid in other pid namespace

translate_pid(pid, TRANSLATE_PID_CURRENT_PIDNS, 0,
TRANSLATE_PID_FD_PIDNS, ns_fd)
or
translate_pid(pid, TRANSLATE_PID_CURRENT_PIDNS, 0,
TRANSLATE_PID_TASK_PIDNS, ns_pid)

- get deepest pid

translate_pid(pid, TRANSLATE_PID_CURRENT_PIDNS, 0,
TRANSLATE_PID_TASK_PIDNS, pid)

- get pid of init task for namespace

translate_pid(1, TRANSLATE_PID_FD_PIDNS, ns_fd,
TRANSLATE_PID_CURRENT_PIDNS, 0)


This syscall also could be used for checking topology of pid namespaces:

- ns1 nests inside ns2

translate_pid(1, TRANSLATE_PID_FD_PIDNS, ns1_fd,
TRANSLATE_PID_FD_PIDNS, ns2_fd) > 1

- task1 lives in same pid-namespace as task2

translate_pid(1, TRANSLATE_PID_TASK_PIDNS, task1_pid,
TRANSLATE_PID_TASK_PIDNS, task2_pid) == 1

- task1 is isolated from task2

translate_pid(task1_pid, TRANSLATE_PID_CURRENT_PIDNS, 0,
TRANSLATE_PID_TASK_PIDNS, task2_pid) == 0

- pid is reachable from ns

translate_pid(pid, TRANSLATE_PID_CURRENT_PIDNS, 0,
TRANSLATE_PID_FD_PIDNS, ns_fd) > 0

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

---

v1: https://lkml.org/lkml/2015/9/15/411
v2: https://lkml.org/lkml/2015/9/24/278
* use namespace-fd as second/third argument
* add -pid for getting parent pid
* move code into kernel/sys.c next to getppid
* drop ifdef CONFIG_PID_NS
* add generic syscall
v3: https://lkml.org/lkml/2015/9/28/3
* use proc_ns_fdget()
* update description
* rebase to next-20150925
* fix conflict with mlock2
v4: https://lkml.org/lkml/2017/10/16/852
* rename into translate_pid()
* remove syscall if CONFIG_PID_NS=n
* drop -pid for parent task
* drop fget-fdget optimizations
* add helper get_pid_ns_by_fd()
* wire only into x86
v5:
* rewrite commit message
* resolve pidns by task pid or by pidns fd
* add arguments source_type and target_type

--- sample tool translate_pid.c ---

#define _GNU_SOURCE
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sched.h>
#include <fcntl.h>
#include <err.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

#ifndef SYS_translate_pid
#ifdef __x86_64__
#define SYS_translate_pid 333
#endif
#endif

#ifndef TRANSLATE_PID_CURRENT_PIDNS
#define TRANSLATE_PID_CURRENT_PIDNS 0
#define TRANSLATE_PID_TASK_PIDNS 1
#define TRANSLATE_PID_FD_PIDNS 2
#endif

pid_t translate_pid(pid_t pid, int source_type, int source,
int target_type, int target) {
return syscall(SYS_translate_pid, pid, source_type, source,
target_type, target);
}

int main(int argc, char **argv) {
int pid, source, target;
char buf[64];

if (argc != 4)
errx(1, "usage: %s <pid> <source> <traget>", argv[0]);

pid = atoi(argv[1]);
int source_type, target_type;
source = atoi(argv[2]);
target = atoi(argv[3]);

if (source < 0) {
source_type = TRANSLATE_PID_TASK_PIDNS;
source = -source;
} else if (source > 0) {
source_type = TRANSLATE_PID_FD_PIDNS;
sprintf(buf, "/proc/%d/ns/pid", source);
source = open(buf, O_RDONLY);
if (source < 0)
err(2, "open source %s", buf);
} else {
source_type = TRANSLATE_PID_CURRENT_PIDNS;
}

if (target < 0) {
target_type = TRANSLATE_PID_TASK_PIDNS;
target = -target;
} else if (target > 0) {
target_type = TRANSLATE_PID_FD_PIDNS;
sprintf(buf, "/proc/%d/ns/pid", target);
target = open(buf, O_RDONLY);
if (target < 0)
err(2, "open target %s", buf);
} else {
target_type = TRANSLATE_PID_CURRENT_PIDNS;
}

pid = translate_pid(pid, source_type, source, target_type, target);
if (pid < 0)
err(2, "translate");

printf("%d\n", pid);
return 0;
}

---
---
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
include/linux/syscalls.h | 4 ++
include/uapi/linux/sched.h | 7 ++++
kernel/pid_namespace.c | 64 ++++++++++++++++++++++++++++++++
kernel/sys_ni.c | 3 ++
6 files changed, 80 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index c58f75b088c5..aef52c709845 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -391,3 +391,4 @@
382 i386 pkey_free sys_pkey_free
383 i386 statx sys_statx
384 i386 arch_prctl sys_arch_prctl compat_sys_arch_prctl
+385 i386 translate_pid sys_translate_pid
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 5aef183e2f85..1ebdab83c6f4 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -339,6 +339,7 @@
330 common pkey_alloc sys_pkey_alloc
331 common pkey_free sys_pkey_free
332 common statx sys_statx
+333 common translate_pid sys_translate_pid

#
# x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index b961184f597a..d189a1f61160 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -553,6 +553,10 @@ asmlinkage long sys_clock_nanosleep(clockid_t which_clock, int flags,
/* kernel/printk.c */
asmlinkage long sys_syslog(int type, char __user *buf, int len);

+/* kernel/pid_namespace.c */
+asmlinkage long sys_translate_pid(pid_t pid, int source_type, int source,
+ int target_type, int target);
+
/* kernel/ptrace.c */
asmlinkage long sys_ptrace(long request, long pid, unsigned long addr,
unsigned long data);
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 22627f80063e..7c45fd8d33d7 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -55,4 +55,11 @@
SCHED_FLAG_RECLAIM | \
SCHED_FLAG_DL_OVERRUN)

+/*
+ * For translate_pid()
+ */
+#define TRANSLATE_PID_CURRENT_PIDNS 0 /* Current pid namespace */
+#define TRANSLATE_PID_TASK_PIDNS 1 /* Namespace by task pid */
+#define TRANSLATE_PID_FD_PIDNS 2 /* Namespace by pidns fd */
+
#endif /* _UAPI_LINUX_SCHED_H */
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 2a2ac53d8b8b..84c8b47289d5 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -13,6 +13,7 @@
#include <linux/user_namespace.h>
#include <linux/syscalls.h>
#include <linux/cred.h>
+#include <linux/file.h>
#include <linux/err.h>
#include <linux/acct.h>
#include <linux/slab.h>
@@ -380,6 +381,69 @@ static void pidns_put(struct ns_common *ns)
put_pid_ns(to_pid_ns(ns));
}

+/* Under rcu_read_lock(). Returns pointer to pid_namespace or NULL. */
+static struct pid_namespace *resolve_pid_ns(int type, int fd_or_pid)
+{
+ struct pid_namespace *current_ns = task_active_pid_ns(current);
+ struct pid_namespace *pidns = NULL;
+ struct ns_common *ns;
+ struct file *file;
+
+ switch (type) {
+ case TRANSLATE_PID_CURRENT_PIDNS:
+ pidns = current_ns;
+ break;
+ case TRANSLATE_PID_TASK_PIDNS:
+ pidns = ns_of_pid(find_pid_ns(fd_or_pid, current_ns));
+ break;
+ case TRANSLATE_PID_FD_PIDNS:
+ file = proc_ns_fget(fd_or_pid);
+ if (!IS_ERR(file)) {
+ ns = get_proc_ns(file_inode(file));
+ if (ns->ops->type == CLONE_NEWPID)
+ pidns = to_pid_ns(ns);
+ fput(file);
+ }
+ break;
+ }
+
+ return pidns;
+}
+
+/*
+ * translate_pid - convert pid in source pid-ns into target pid-ns.
+ * @pid: pid for translation
+ * @source_type: one of TRANSLATE_PID_*
+ * @source: depending on @source_type pid-ns fd, pid, or nothing
+ * @target_type: one of TRANSLATE_PID_*
+ * @target: depending on @target_type pid-ns fd, pid, or nothing
+ *
+ * Returns pid in @target pid-ns, zero if task have no pid there,
+ * or -ESRCH if task with @pid does not found in @source pid-ns,
+ * or -EINVAL if @source or @target couldn't be resolved into pid-ns.
+ */
+SYSCALL_DEFINE5(translate_pid, pid_t, pid,
+ int, source_type, int, source,
+ int, target_type, int, target)
+{
+ struct pid_namespace *source_ns, *target_ns;
+ struct pid *struct_pid;
+ pid_t result = -EINVAL;
+
+ rcu_read_lock();
+ source_ns = resolve_pid_ns(source_type, source);
+ if (!source_ns)
+ goto out;
+ target_ns = resolve_pid_ns(target_type, target);
+ if (!target_ns)
+ goto out;
+ struct_pid = find_pid_ns(pid, source_ns);
+ result = struct_pid ? pid_nr_ns(struct_pid, target_ns) : -ESRCH;
+out:
+ rcu_read_unlock();
+ return result;
+}
+
static int pidns_install(struct nsproxy *nsproxy, struct ns_common *ns)
{
struct pid_namespace *active = task_active_pid_ns(current);
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 6cafc008f6db..777689bce406 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -146,6 +146,9 @@ COND_SYSCALL(delete_module);
/* kernel/printk.c */
COND_SYSCALL(syslog);

+/* kernel/pid_namespace.c */
+COND_SYSCALL(sys_translate_pid);
+
/* kernel/ptrace.c */

/* kernel/sched/core.c */
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