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Subject: Re: [PATCH v6] pidns: introduce syscall translate_pid
To:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Jann Horn <jannh@google.com>,
        Serge Hallyn <serge.hallyn@ubuntu.com>,
        Prakash Sangappa <prakash.sangappa@oracle.com>,
        Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
References: <152788068212.768348.15192457501079586650.stgit@buzz>
From:   Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>
Message-ID: <6dcb9f67-03dc-fa58-4690-fe69b23ada23@oracle.com>
Date:   Tue, 12 Jun 2018 10:08:28 -0700
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On 06/01/2018 12:18 PM, Konstantin Khlebnikov wrote:
> 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.
>
> This patch adds new syscall for converting pids between pid namespaces:
>
> pid_t translate_pid(pid_t pid, int source, int target);
>
> Pid-namespaces are referred file descriptors opened to proc files
> /proc/[pid]/ns/pid or /proc/[pid]/ns/pid_for_children.
> Negative argument points to current pid namespace.
>
> Syscall returns pid in target pid-ns or zero if task have no pid there.
>
> Error codes:
> EBADF    - file descriptor is closed
> EINVAL   - file descriptor isn't pid namespace
> ESRCH    - task not found in @source namespace
>
> Translation could breach pid-ns isolation and return pids from outer pid
> namespaces iff process already has file descriptor for these namespaces.
>
> Examples:
> translate_pid(pid, ns, -1)      - get pid in our pid namespace
> translate_pid(pid, -1, ns)      - get pid in other pid namespace
> translate_pid(1, ns, -1)        - get pid of init task for namespace
> translate_pid(pid, -1, ns) > 0  - is pid is reachable from ns?
> translate_pid(1, ns1, ns2) > 0  - is ns1 inside ns2?
> translate_pid(1, ns1, ns2) == 0 - is ns1 outside ns2?
> translate_pid(1, ns1, ns2) == 1 - is ns1 equal ns2?
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Reanimated-by: Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>
>
> ---
>
> 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/2017/10/13/177
>   * rename from getvpid() 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: https://lkml.org/lkml/2018/4/4/677
>   * rewrite commit message
>   * resolve pidns by task pid or by pidns fd
>   * add arguments source_type and target_type
> v6:
>   * revert back minimized v4 design
>   * rebase to next-20180601
>   * fix COND_SYSCALL stub
>   * use next syscall number, old used for io_pgetevents
>
> --- sample tool ---
>
> #define _GNU_SOURCE
> #include <sys/syscall.h>
> #include <sys/types.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <err.h>
>
> #ifndef SYS_translate_pid
> #ifdef __x86_64__
> #define SYS_translate_pid 334
> #elif defined __i386__
> #define SYS_translate_pid 386
> #endif
> #endif
>
> pid_t translate_pid(pid_t pid, int source, int target) {
> 	return syscall(SYS_translate_pid, pid, source, target);
> }
>
> int main(int argc, char **argv) {
> 	int pid, source, target;
> 	char buf[64];
>
> 	if (argc != 4)
> 		errx(1, "usage: %s <pid> <source> <target>", argv[0]);
>
> 	pid = atoi(argv[1]);
> 	source = atoi(argv[2]);
> 	target = atoi(argv[3]);
>
> 	if (source > 0) {
> 		snprintf(buf, sizeof(buf), "/proc/%d/ns/pid", source);
> 		source = open(buf, O_RDONLY);
> 		if (source < 0)
> 			err(2, "open source %s", buf);
> 	}
>
> 	if (target > 0) {
> 		snprintf(buf, sizeof(buf), "/proc/%d/ns/pid", target);
> 		target = open(buf, O_RDONLY);
> 		if (target < 0)
> 			err(2, "open target %s", buf);
> 	}
>
> 	pid = translate_pid(pid, source, target);
> 	if (pid < 0)
> 		err(2, "translate_pid");
>
> 	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               |    1
>   kernel/pid_namespace.c                 |   66 ++++++++++++++++++++++++++++++++
>   kernel/sys_ni.c                        |    3 +
>   5 files changed, 72 insertions(+)
>
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 14a2f996e543..e70685750d43 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -397,3 +397,4 @@
>   383	i386	statx			sys_statx			__ia32_sys_statx
>   384	i386	arch_prctl		sys_arch_prctl			__ia32_compat_sys_arch_prctl
>   385	i386	io_pgetevents		sys_io_pgetevents		__ia32_compat_sys_io_pgetevents
> +386	i386	translate_pid		sys_translate_pid		__ia32_sys_translate_pid
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index cd36232ab62f..ebfd89055424 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -342,6 +342,7 @@
>   331	common	pkey_free		__x64_sys_pkey_free
>   332	common	statx			__x64_sys_statx
>   333	common	io_pgetevents		__x64_sys_io_pgetevents
> +334	common	translate_pid		__x64_sys_translate_pid

Looks like the syscall numbers used here were claimed by "resq" in the 
linux-next tree.

Thanks,
Nagarathnam.
>   
>   #
>   # 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 390e814fdc8d..3f33971cf1c8 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -843,6 +843,7 @@ asmlinkage long sys_clock_adjtime(clockid_t which_clock,
>   				struct timex __user *tx);
>   asmlinkage long sys_syncfs(int fd);
>   asmlinkage long sys_setns(int fd, int nstype);
> +asmlinkage long sys_translate_pid(pid_t pid, int source, int target);
>   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/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index 2a2ac53d8b8b..3b872cbbe264 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 +381,71 @@ static void pidns_put(struct ns_common *ns)
>   	put_pid_ns(to_pid_ns(ns));
>   }
>   
> +static struct pid_namespace *get_pid_ns_by_fd(int fd)
> +{
> +	struct pid_namespace *pidns;
> +	struct ns_common *ns;
> +	struct file *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 = get_pid_ns(to_pid_ns(ns));
> +	else
> +		pidns = ERR_PTR(-EINVAL);
> +
> +	fput(file);
> +	return pidns;
> +}
> +
> +/*
> + * translate_pid - convert pid in source pid-ns into target pid-ns.
> + * @pid:    pid for translation
> + * @source: pid-ns file descriptor or -1 for active namespace
> + * @target: pid-ns file descriptor or -1 for active namesapce
> + *
> + * 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.
> + */
> +SYSCALL_DEFINE3(translate_pid, pid_t, pid, int, source, int, target)
> +{
> +	struct pid_namespace *source_ns, *target_ns;
> +	struct pid *struct_pid;
> +	pid_t result;
> +
> +	if (source >= 0) {
> +		source_ns = get_pid_ns_by_fd(source);
> +		result = PTR_ERR(source_ns);
> +		if (IS_ERR(source_ns))
> +			goto err_source;
> +	} else
> +		source_ns = task_active_pid_ns(current);
> +
> +	if (target >= 0) {
> +		target_ns = get_pid_ns_by_fd(target);
> +		result = PTR_ERR(target_ns);
> +		if (IS_ERR(target_ns))
> +			goto err_target;
> +	} else
> +		target_ns = task_active_pid_ns(current);
> +
> +	rcu_read_lock();
> +	struct_pid = find_pid_ns(pid, source_ns);
> +	result = struct_pid ? pid_nr_ns(struct_pid, target_ns) : -ESRCH;
> +	rcu_read_unlock();
> +
> +	if (target >= 0)
> +		put_pid_ns(target_ns);
> +err_target:
> +	if (source >= 0)
> +		put_pid_ns(source_ns);
> +err_source:
> +	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 06b4ccee0047..bf276e9ace9a 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -153,6 +153,9 @@ COND_SYSCALL_COMPAT(kexec_load);
>   COND_SYSCALL(init_module);
>   COND_SYSCALL(delete_module);
>   
> +/* kernel/pid_namespace.c */
> +COND_SYSCALL(translate_pid);
> +
>   /* kernel/posix-timers.c */
>   
>   /* kernel/printk.c */
>