[PATCH] namespaces: introduce sys_hijack (v8)

Serge E. Hallyn serue at us.ibm.com
Fri Nov 2 13:32:26 PDT 2007


Apparently my hijack test program was just too mean-n-lean to show just
how fragile sys_hijack was.  Once it tried to do any kind of actual work
before doing an exec(), it would segfault.  Apparently just letting
dup_task_struct() do "*tsk = *hijacked_task" was bogus.

Mark, I'm sorry, this is without your powerpc/x86_64 patches.  But note
that I took out the 'copy_a_thread' stuff which was bogus anyway.

It's incomplete, I need to walk through the task_struct and copy more of
the relevant pieces from the hijacked task_struct.  But this appears to
be robust.

Tested with entering unshared uts namespace and with cloned
pidns+mountns+utsns container.

thanks,
-serge

PS: also a typo fix in the hijack.c code.

>From 0efe7d1f607438be986d7000571fda406a866b12 Mon Sep 17 00:00:00 2001
From: sergeh at us.ibm.com <sergeh at us.ibm.com>
Date: Tue, 16 Oct 2007 09:36:49 -0700
Subject: [PATCH] namespaces: introduce sys_hijack (v8)

Move most of do_fork() into a new do_fork_task() which acts on
a new argument, task, rather than on current.  do_fork() becomes
a call to do_fork_task(current, ...).

Introduce sys_hijack (for x86 only so far).  It is like clone, but
 in place of a stack pointer (which is assumed null) it accepts a
pid.  The process identified by that pid is the one which is
actually cloned.  Some state - include the file table, the signals
and sighand (and hence tty), and the ->parent are taken from the
calling process.

A process to be hijacked may be identified by process id.
Alternatively, an open fd for a cgroup 'tasks' file may be
specified.  The first available task in that cgroup will then
be hijacked.

In order to hijack a process, the calling process must be
allowed to ptrace the target.

The effect is a sort of namespace enter.  The following program
uses sys_hijack to 'enter' all namespaces of the specified task.
For instance in one terminal, do

	mount -t cgroup -ons /cgroup
	hostname
	  qemu
	ns_exec -u /bin/sh
	  hostname serge
          echo $$
            1073
	  cat /proc/$$/cgroup
	    ns:/node_1073

In another terminal then do

	hostname
	  qemu
	cat /proc/$$/cgroup
	  ns:/
	hijack pid 1073
	  hostname
	    serge
	  cat /proc/$$/cgroup
	    ns:/node_1073
	hijack cgroup /cgroup/node_1073/tasks

Changelog:
	Aug 23: send a stop signal to the hijacked process
		(like ptrace does).
	Oct 09: Update for 2.6.23-rc8-mm2 (mainly pidns)
		Don't take task_lock under rcu_read_lock
		Send hijacked process to cgroup_fork() as
		the first argument.
		Removed some unneeded task_locks.
	Oct 16: Fix bug introduced into alloc_pid.
	Oct 16: Add 'int which' argument to sys_hijack to
		allow later expansion to use cgroup in place
		of pid to specify what to hijack.
	Oct 24: Implement hijack by open cgroup file.
	Nov 02: Switch copying of task info: do full copy
		from current, then copy relevant pieces from
		hijacked task.

==============================================================
hijack.c
==============================================================
 #define _BSD_SOURCE
 #include <unistd.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <sched.h>

void usage(char *me)
{
	printf("Usage: %s pid <pid> | %s cgroup <cgroup_tasks_file>\n", me, me);
}

int exec_shell(void)
{
	execl("/bin/sh", "/bin/sh", NULL);
}

int main(int argc, char *argv[])
{
	int id;
	int ret;
	int status;
	int use_pid = 0;

	if (argc < 3 || !strcmp(argv[1], "-h")) {
		usage(argv[0]);
		return 1;
	}
	if (strcmp(argv[1], "pid") == 0)
		use_pid = 1;

	if (use_pid)
		id = atoi(argv[2]);
	else {
		id = open(argv[2], O_RDONLY);
		if (id == -1) {
			perror("cgroup open");
			return 1;
		}
	}

	ret = syscall(327, SIGCHLD, use_pid ? 1 : 2, (unsigned long)id);

	if (!use_pid)
		close(id);
	if  (ret == 0) {
		return exec_shell();
	} else if (ret < 0) {
		perror("sys_hijack");
	} else {
		printf("waiting on cloned process %d\n", ret);
		while(waitpid(-1, &status, __WALL) != -1)
				;
		printf("cloned process exited with %d (waitpid ret %d)\n",
				status, ret);
	}

	return ret;
}
==============================================================

Signed-off-by: Serge Hallyn <serue at us.ibm.com>
---
 arch/i386/kernel/process.c       |   77 +++++++++++++++++++++++++++++++
 arch/i386/kernel/syscall_table.S |    1 +
 include/asm-i386/unistd.h        |    3 +-
 include/linux/cgroup.h           |   10 +++-
 include/linux/ptrace.h           |    1 +
 include/linux/sched.h            |    7 +++
 include/linux/syscalls.h         |    1 +
 kernel/cgroup.c                  |   69 +++++++++++++++++++++++++++--
 kernel/fork.c                    |   92 +++++++++++++++++++++++++++++++-------
 kernel/ptrace.c                  |    7 +++
 10 files changed, 244 insertions(+), 24 deletions(-)

diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index eab6c62..8add6e4 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -39,6 +39,7 @@
 #include <linux/personality.h>
 #include <linux/tick.h>
 #include <linux/percpu.h>
+#include <linux/cgroup.h>
 
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
@@ -784,6 +785,82 @@ asmlinkage int sys_clone(struct pt_regs regs)
 }
 
 /*
+ * Called with task count bumped, drops task count before returning
+ */
+static int hijack_task(struct task_struct *task, struct pt_regs regs)
+{
+	unsigned long clone_flags = regs.ebx;
+	int ret = -EINVAL;
+
+	task_lock(task);
+	put_task_struct(task);
+	if (!ptrace_may_attach_locked(task)) {
+		ret = -EPERM;
+		goto out_put_task;
+	}
+	if (task->ptrace) {
+		ret = -EBUSY;
+		goto out_put_task;
+	}
+	force_sig_specific(SIGSTOP, task);
+
+	task_unlock(task);
+	ret = do_fork_task(task, clone_flags, regs.esp, &regs, 0,
+		NULL, NULL);
+	wake_up_process(task);
+	return ret;
+
+out_put_task:
+	task_unlock(task);
+	return ret;
+}
+
+static int hijack_pid(struct pt_regs regs)
+{
+	pid_t pid = regs.edx;
+	struct task_struct *task;
+
+	rcu_read_lock();
+	task = find_task_by_vpid(pid);
+	if (task)
+		get_task_struct(task);
+	rcu_read_unlock();
+	
+	if (!task)
+		return -EINVAL;
+
+	return hijack_task(task, regs);
+}
+
+static int hijack_cgroup(struct pt_regs regs)
+{
+	unsigned int fd;
+	struct task_struct *task;
+
+	fd = (unsigned int) regs.edx;
+	task = task_from_cgroup_fd(fd);
+	if (IS_ERR(task))
+		return PTR_ERR(task);
+
+	return hijack_task(task, regs);
+}
+
+asmlinkage int sys_hijack(struct pt_regs regs)
+{
+	int which = regs.ecx;
+
+	switch (which) {
+	case HIJACK_PID:
+		return hijack_pid(regs);
+	case HIJACK_CGROUP:
+		return hijack_cgroup(regs);
+	default:
+		return -EINVAL;
+	}
+
+}
+
+/*
  * This is trivial, and on the face of it looks like it
  * could equally well be done in user mode.
  *
diff --git a/arch/i386/kernel/syscall_table.S b/arch/i386/kernel/syscall_table.S
index df6e41e..495930c 100644
--- a/arch/i386/kernel/syscall_table.S
+++ b/arch/i386/kernel/syscall_table.S
@@ -326,3 +326,4 @@ ENTRY(sys_call_table)
 	.long sys_fallocate
 	.long sys_revokeat		/* 325 */
 	.long sys_frevoke
+	.long sys_hijack
diff --git a/include/asm-i386/unistd.h b/include/asm-i386/unistd.h
index 006c1b3..fe6eeb4 100644
--- a/include/asm-i386/unistd.h
+++ b/include/asm-i386/unistd.h
@@ -332,10 +332,11 @@
 #define __NR_fallocate		324
 #define __NR_revokeat		325
 #define __NR_frevoke		326
+#define __NR_hijack		327
 
 #ifdef __KERNEL__
 
-#define NR_syscalls 327
+#define NR_syscalls 328
 
 #define __ARCH_WANT_IPC_PARSE_VERSION
 #define __ARCH_WANT_OLD_READDIR
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 8747932..3edb820 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -26,7 +26,7 @@ extern int cgroup_init(void);
 extern void cgroup_init_smp(void);
 extern void cgroup_lock(void);
 extern void cgroup_unlock(void);
-extern void cgroup_fork(struct task_struct *p);
+extern void cgroup_fork(struct task_struct *parent, struct task_struct *p);
 extern void cgroup_fork_callbacks(struct task_struct *p);
 extern void cgroup_post_fork(struct task_struct *p);
 extern void cgroup_exit(struct task_struct *p, int run_callbacks);
@@ -304,12 +304,14 @@ struct task_struct *cgroup_iter_next(struct cgroup *cont,
 					struct cgroup_iter *it);
 void cgroup_iter_end(struct cgroup *cont, struct cgroup_iter *it);
 
+struct task_struct *task_from_cgroup_fd(unsigned int fd);
 #else /* !CONFIG_CGROUPS */
 
 static inline int cgroup_init_early(void) { return 0; }
 static inline int cgroup_init(void) { return 0; }
 static inline void cgroup_init_smp(void) {}
-static inline void cgroup_fork(struct task_struct *p) {}
+static inline void cgroup_fork(struct task_struct *parent,
+					 struct task_struct *p) {}
 static inline void cgroup_fork_callbacks(struct task_struct *p) {}
 static inline void cgroup_post_fork(struct task_struct *p) {}
 static inline void cgroup_exit(struct task_struct *p, int callbacks) {}
@@ -322,6 +324,10 @@ static inline int cgroupstats_build(struct cgroupstats *stats,
 	return -EINVAL;
 }
 
+static inline struct task_struct *task_from_cgroup_fd(unsigned int fd)
+{
+	return ERR_PTR(-EINVAL);
+}
 #endif /* !CONFIG_CGROUPS */
 
 #endif /* _LINUX_CGROUP_H */
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index ae8146a..727a4a9 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -97,6 +97,7 @@ extern void __ptrace_link(struct task_struct *child,
 extern void __ptrace_unlink(struct task_struct *child);
 extern void ptrace_untrace(struct task_struct *child);
 extern int ptrace_may_attach(struct task_struct *task);
+extern int ptrace_may_attach_locked(struct task_struct *task);
 
 static inline void ptrace_link(struct task_struct *child,
 			       struct task_struct *new_parent)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3436b98..18cde38 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -29,6 +29,12 @@
 #define CLONE_NEWNET		0x40000000	/* New network namespace */
 
 /*
+ * Hijack flags
+ */
+#define HIJACK_PID	1	/* 'id' is a pid */
+#define HIJACK_CGROUP	2	/* 'id' is an open fd for a cgroup dir */
+
+/*
  * Scheduling policies
  */
 #define SCHED_NORMAL		0
@@ -1658,6 +1664,7 @@ extern int allow_signal(int);
 extern int disallow_signal(int);
 
 extern int do_execve(char *, char __user * __user *, char __user * __user *, struct pt_regs *);
+extern long do_fork_task(struct task_struct *task, unsigned long, unsigned long, struct pt_regs *, unsigned long, int __user *, int __user *);
 extern long do_fork(unsigned long, unsigned long, struct pt_regs *, unsigned long, int __user *, int __user *);
 struct task_struct *fork_idle(int);
 
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index f696874..31f0c79 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -616,5 +616,6 @@ int kernel_execve(const char *filename, char *const argv[], char *const envp[]);
 
 asmlinkage long sys_revokeat(int dfd, const char __user *filename);
 asmlinkage long sys_frevoke(unsigned int fd);
+asmlinkage long sys_hijack(unsigned long flags, int which, unsigned long id);
 
 #endif
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8720881..2d1d67e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -45,6 +45,7 @@
 #include <linux/sort.h>
 #include <linux/delayacct.h>
 #include <linux/cgroupstats.h>
+#include <linux/file.h>
 
 #include <asm/atomic.h>
 
@@ -2460,12 +2461,12 @@ static struct file_operations proc_cgroupstats_operations = {
  * At the point that cgroup_fork() is called, 'current' is the parent
  * task, and the passed argument 'child' points to the child task.
  */
-void cgroup_fork(struct task_struct *child)
+void cgroup_fork(struct task_struct *parent, struct task_struct *child)
 {
-	task_lock(current);
-	child->cgroups = current->cgroups;
+	task_lock(parent);
+	child->cgroups = parent->cgroups;
 	get_css_set(child->cgroups);
-	task_unlock(current);
+	task_unlock(parent);
 	INIT_LIST_HEAD(&child->cg_list);
 }
 
@@ -2819,3 +2820,63 @@ static void cgroup_release_agent(struct work_struct *work)
 	spin_unlock(&release_list_lock);
 	mutex_unlock(&cgroup_mutex);
 }
+
+static inline int task_available(struct task_struct *task)
+{
+	switch(task->state) {
+	case TASK_RUNNING:
+	case TASK_INTERRUPTIBLE:
+		return 1;
+	default:
+		 return 0;
+	}
+}
+
+/*
+ * Takes an integer which is a open fd in current for a valid
+ * cgroupfs file.  Returns a task in that cgroup, with its
+ * refcount bumped.
+ */
+struct task_struct *task_from_cgroup_fd(unsigned int fd)
+{
+	struct file *file;
+	struct cgroup *cgroup;
+	struct cgroup_iter it;
+	struct task_struct *task = NULL;
+
+	file = fget(fd);
+	if (!file)
+		return ERR_PTR(-EINVAL);
+
+	if (!file->f_dentry || !file->f_dentry->d_sb)
+		goto out_fput;
+	if (file->f_dentry->d_parent->d_sb->s_magic != CGROUP_SUPER_MAGIC)
+		goto out_fput;
+	if (strcmp(file->f_dentry->d_name.name, "tasks"))
+		goto out_fput;
+
+	rcu_read_lock();
+	cgroup = __d_cont(file->f_dentry->d_parent);
+printk(KERN_NOTICE "cgroup is %lu\n", (unsigned long)cgroup);
+	if (!cgroup)
+		goto out_unlock;
+	cgroup_iter_start(cgroup, &it);
+	do {
+		task = cgroup_iter_next(cgroup, &it);
+		if (task)
+			printk(KERN_NOTICE "task %d state %d\n",
+				task->pid, task->state);
+	} while (task && !task_available(task));
+	cgroup_iter_end(cgroup, &it);
+printk(KERN_NOTICE "task is %lu\n", (unsigned long)task);
+	if (task)
+		get_task_struct(task);
+
+out_unlock:
+	rcu_read_unlock();
+out_fput:
+	fput(file);
+	if (!task)
+		return ERR_PTR(-EINVAL);
+	return task;
+}
diff --git a/kernel/fork.c b/kernel/fork.c
index 64860ef..4d3d3f7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -190,7 +190,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
 		return NULL;
 	}
 
-	setup_thread_stack(tsk, orig);
+	setup_thread_stack(tsk, current);
 
 #ifdef CONFIG_CC_STACKPROTECTOR
 	tsk->stack_canary = get_random_int();
@@ -621,13 +621,14 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
 
 EXPORT_SYMBOL_GPL(copy_fs_struct);
 
-static int copy_fs(unsigned long clone_flags, struct task_struct *tsk)
+static inline int copy_fs(unsigned long clone_flags,
+		struct task_struct * src, struct task_struct * tsk)
 {
 	if (clone_flags & CLONE_FS) {
-		atomic_inc(&current->fs->count);
+		atomic_inc(&src->fs->count);
 		return 0;
 	}
-	tsk->fs = __copy_fs_struct(current->fs);
+	tsk->fs = __copy_fs_struct(src->fs);
 	if (!tsk->fs)
 		return -ENOMEM;
 	return 0;
@@ -965,6 +966,30 @@ static void rt_mutex_init_task(struct task_struct *p)
 #endif
 }
 
+void copy_hijackable_taskinfo(struct task_struct *p,
+		struct task_struct *task)
+{
+	p->uid = task->uid;
+	p->euid = task->euid;
+	p->suid = task->suid;
+	p->fsuid = task->fsuid;
+	p->gid = task->gid;
+	p->egid = task->egid;
+	p->sgid = task->sgid;
+	p->fsgid = task->fsgid;
+	p->cap_effective = task->cap_effective;
+	p->cap_inheritable = task->cap_inheritable;
+	p->cap_permitted = task->cap_permitted;
+	p->keep_capabilities = task->keep_capabilities;
+	p->user = task->user;
+	p->nsproxy = task->nsproxy;
+	/*
+	 * TODO: continue taking relevant pieces from
+	 * hijacked task, starting at:
+	 *     key info
+	 */
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -973,7 +998,8 @@ static void rt_mutex_init_task(struct task_struct *p)
  * parts of the process environment (as per the clone
  * flags). The actual kick-off is left to the caller.
  */
-static struct task_struct *copy_process(unsigned long clone_flags,
+static struct task_struct *copy_process(struct task_struct *task,
+					unsigned long clone_flags,
 					unsigned long stack_start,
 					struct pt_regs *regs,
 					unsigned long stack_size,
@@ -1010,12 +1036,17 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	p = dup_task_struct(current);
 	if (!p)
 		goto fork_out;
+	if (current != task) {
+		copy_hijackable_taskinfo(p, task);
+	}
 
 	rt_mutex_init_task(p);
 
 #ifdef CONFIG_TRACE_IRQFLAGS
-	DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled);
-	DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
+	if (task == current) {
+		DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled);
+		DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
+	}
 #endif
 	retval = -EAGAIN;
 	if (atomic_read(&p->user->processes) >=
@@ -1084,7 +1115,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 #endif
 	p->io_context = NULL;
 	p->audit_context = NULL;
-	cgroup_fork(p);
+	cgroup_fork(task, p);
 #ifdef CONFIG_NUMA
  	p->mempolicy = mpol_copy(p->mempolicy);
  	if (IS_ERR(p->mempolicy)) {
@@ -1132,7 +1163,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		goto bad_fork_cleanup_audit;
 	if ((retval = copy_files(clone_flags, p)))
 		goto bad_fork_cleanup_semundo;
-	if ((retval = copy_fs(clone_flags, p)))
+	if ((retval = copy_fs(clone_flags, task, p)))
 		goto bad_fork_cleanup_files;
 	if ((retval = copy_sighand(clone_flags, p)))
 		goto bad_fork_cleanup_fs;
@@ -1164,7 +1195,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	p->pid = pid_nr(pid);
 	p->tgid = p->pid;
 	if (clone_flags & CLONE_THREAD)
-		p->tgid = current->tgid;
+		p->tgid = task->tgid;
 
 	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
 	/*
@@ -1379,7 +1410,7 @@ struct task_struct * __cpuinit fork_idle(int cpu)
 	struct task_struct *task;
 	struct pt_regs regs;
 
-	task = copy_process(CLONE_VM, 0, idle_regs(&regs), 0, NULL,
+	task = copy_process(current, CLONE_VM, 0, idle_regs(&regs), 0, NULL,
 				&init_struct_pid);
 	if (!IS_ERR(task))
 		init_idle(task, cpu);
@@ -1404,12 +1435,12 @@ static int fork_traceflag(unsigned clone_flags)
 }
 
 /*
- *  Ok, this is the main fork-routine.
- *
- * It copies the process, and if successful kick-starts
- * it and waits for it to finish using the VM if required.
+ * if called with task!=current, then caller must ensure that
+ *    1. it has a reference to task
+ *    2. current must have ptrace permission to task
  */
-long do_fork(unsigned long clone_flags,
+long do_fork_task(struct task_struct *task,
+		unsigned long clone_flags,
 	      unsigned long stack_start,
 	      struct pt_regs *regs,
 	      unsigned long stack_size,
@@ -1420,13 +1451,23 @@ long do_fork(unsigned long clone_flags,
 	int trace = 0;
 	long nr;
 
+	if (task != current) {
+		/* sanity checks */
+		/* we only want to allow hijacking the simplest cases */
+		if (clone_flags & CLONE_SYSVSEM)
+			return -EINVAL;
+		if (current->ptrace)
+			return -EPERM;
+		if (task->ptrace)
+			return -EINVAL;
+	}
 	if (unlikely(current->ptrace)) {
 		trace = fork_traceflag (clone_flags);
 		if (trace)
 			clone_flags |= CLONE_PTRACE;
 	}
 
-	p = copy_process(clone_flags, stack_start, regs, stack_size,
+	p = copy_process(task, clone_flags, stack_start, regs, stack_size,
 			child_tidptr, NULL);
 	/*
 	 * Do this prior waking up the new thread - the thread pointer
@@ -1481,6 +1522,23 @@ long do_fork(unsigned long clone_flags,
 	return nr;
 }
 
+/*
+ *  Ok, this is the main fork-routine.
+ *
+ * It copies the process, and if successful kick-starts
+ * it and waits for it to finish using the VM if required.
+ */
+long do_fork(unsigned long clone_flags,
+	      unsigned long stack_start,
+	      struct pt_regs *regs,
+	      unsigned long stack_size,
+	      int __user *parent_tidptr,
+	      int __user *child_tidptr)
+{
+	return do_fork_task(current, clone_flags, stack_start,
+		regs, stack_size, parent_tidptr, child_tidptr);
+}
+
 #ifndef ARCH_MIN_MMSTRUCT_ALIGN
 #define ARCH_MIN_MMSTRUCT_ALIGN 0
 #endif
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 7c76f2f..c65c9fe 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -159,6 +159,13 @@ int ptrace_may_attach(struct task_struct *task)
 	return !err;
 }
 
+int ptrace_may_attach_locked(struct task_struct *task)
+{
+	int err;
+	err = may_attach(task);
+	return !err;
+}
+
 int ptrace_attach(struct task_struct *task)
 {
 	int retval;
-- 
1.5.1



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