From: Christian Brauner christian@brauner.io
[ Upstream commit b3e5838252665ee4cfa76b82bdf1198dca81e5be ]
This patchset makes it possible to retrieve pid file descriptors at process creation time by introducing the new flag CLONE_PIDFD to the clone() system call. Linus originally suggested to implement this as a new flag to clone() instead of making it a separate system call. As spotted by Linus, there is exactly one bit for clone() left.
CLONE_PIDFD creates file descriptors based on the anonymous inode implementation in the kernel that will also be used to implement the new mount api. They serve as a simple opaque handle on pids. Logically, this makes it possible to interpret a pidfd differently, narrowing or widening the scope of various operations (e.g. signal sending). Thus, a pidfd cannot just refer to a tgid, but also a tid, or in theory - given appropriate flag arguments in relevant syscalls - a process group or session. A pidfd does not represent a privilege. This does not imply it cannot ever be that way but for now this is not the case.
A pidfd comes with additional information in fdinfo if the kernel supports procfs. The fdinfo file contains the pid of the process in the callers pid namespace in the same format as the procfs status file, i.e. "Pid:\t%d".
As suggested by Oleg, with CLONE_PIDFD the pidfd is returned in the parent_tidptr argument of clone. This has the advantage that we can give back the associated pid and the pidfd at the same time.
To remove worries about missing metadata access this patchset comes with a sample program that illustrates how a combination of CLONE_PIDFD, and pidfd_send_signal() can be used to gain race-free access to process metadata through /proc/<pid>. The sample program can easily be translated into a helper that would be suitable for inclusion in libc so that users don't have to worry about writing it themselves.
Suggested-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Christian Brauner christian@brauner.io Co-developed-by: Jann Horn jannh@google.com Signed-off-by: Jann Horn jannh@google.com Reviewed-by: Oleg Nesterov oleg@redhat.com Cc: Arnd Bergmann arnd@arndb.de Cc: "Eric W. Biederman" ebiederm@xmission.com Cc: Kees Cook keescook@chromium.org Cc: Thomas Gleixner tglx@linutronix.de Cc: David Howells dhowells@redhat.com Cc: "Michael Kerrisk (man-pages)" mtk.manpages@gmail.com Cc: Andy Lutomirsky luto@kernel.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Aleksa Sarai cyphar@cyphar.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Al Viro viro@zeniv.linux.org.uk Cc: stable@vger.kernel.org # 4.19.x (clone: fix up cherry-pick conflicts for b3e583825266) Signed-off-by: Wen Yang wenyang@linux.alibaba.com --- include/linux/pid.h | 2 + include/uapi/linux/sched.h | 1 + kernel/fork.c | 107 +++++++++++++++++++++++++++++++++++++++++++-- 3 files changed, 106 insertions(+), 4 deletions(-)
diff --git a/include/linux/pid.h b/include/linux/pid.h index 14a9a39..29c0a99 100644 --- a/include/linux/pid.h +++ b/include/linux/pid.h @@ -66,6 +66,8 @@ struct pid
extern struct pid init_struct_pid;
+extern const struct file_operations pidfd_fops; + static inline struct pid *get_pid(struct pid *pid) { if (pid) diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h index 22627f8..ed4ee17 100644 --- a/include/uapi/linux/sched.h +++ b/include/uapi/linux/sched.h @@ -10,6 +10,7 @@ #define CLONE_FS 0x00000200 /* set if fs info shared between processes */ #define CLONE_FILES 0x00000400 /* set if open files shared between processes */ #define CLONE_SIGHAND 0x00000800 /* set if signal handlers and blocked signals shared */ +#define CLONE_PIDFD 0x00001000 /* set if a pidfd should be placed in parent */ #define CLONE_PTRACE 0x00002000 /* set if we want to let tracing continue on the child too */ #define CLONE_VFORK 0x00004000 /* set if the parent wants the child to wake it up on mm_release */ #define CLONE_PARENT 0x00008000 /* set if we want to have the same parent as the cloner */ diff --git a/kernel/fork.c b/kernel/fork.c index f2c92c1..e419891 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -11,6 +11,7 @@ * management can be a bitch. See 'mm/memory.c': 'copy_page_range()' */
+#include <linux/anon_inodes.h> #include <linux/slab.h> #include <linux/sched/autogroup.h> #include <linux/sched/mm.h> @@ -21,6 +22,7 @@ #include <linux/sched/task.h> #include <linux/sched/task_stack.h> #include <linux/sched/cputime.h> +#include <linux/seq_file.h> #include <linux/rtmutex.h> #include <linux/init.h> #include <linux/unistd.h> @@ -1666,6 +1668,58 @@ static void copy_oom_score_adj(u64 clone_flags, struct task_struct *tsk) mutex_unlock(&oom_adj_mutex); }
+static int pidfd_release(struct inode *inode, struct file *file) +{ + struct pid *pid = file->private_data; + + file->private_data = NULL; + put_pid(pid); + return 0; +} + +#ifdef CONFIG_PROC_FS +static void pidfd_show_fdinfo(struct seq_file *m, struct file *f) +{ + struct pid_namespace *ns = proc_pid_ns(file_inode(m->file)); + struct pid *pid = f->private_data; + + seq_put_decimal_ull(m, "Pid:\t", pid_nr_ns(pid, ns)); + seq_putc(m, '\n'); +} +#endif + +const struct file_operations pidfd_fops = { + .release = pidfd_release, +#ifdef CONFIG_PROC_FS + .show_fdinfo = pidfd_show_fdinfo, +#endif +}; + +/** + * pidfd_create() - Create a new pid file descriptor. + * + * @pid: struct pid that the pidfd will reference + * + * This creates a new pid file descriptor with the O_CLOEXEC flag set. + * + * Note, that this function can only be called after the fd table has + * been unshared to avoid leaking the pidfd to the new process. + * + * Return: On success, a cloexec pidfd is returned. + * On error, a negative errno number will be returned. + */ +static int pidfd_create(struct pid *pid) +{ + int fd; + + fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid), + O_RDWR | O_CLOEXEC); + if (fd < 0) + put_pid(pid); + + return fd; +} + /* * This creates a new process as a copy of the old one, * but does not actually start it yet. @@ -1678,13 +1732,14 @@ static __latent_entropy struct task_struct *copy_process( unsigned long clone_flags, unsigned long stack_start, unsigned long stack_size, + int __user *parent_tidptr, int __user *child_tidptr, struct pid *pid, int trace, unsigned long tls, int node) { - int retval; + int pidfd = -1, retval; struct task_struct *p; struct multiprocess_signals delayed;
@@ -1734,6 +1789,31 @@ static __latent_entropy struct task_struct *copy_process( return ERR_PTR(-EINVAL); }
+ if (clone_flags & CLONE_PIDFD) { + int reserved; + + /* + * - CLONE_PARENT_SETTID is useless for pidfds and also + * parent_tidptr is used to return pidfds. + * - CLONE_DETACHED is blocked so that we can potentially + * reuse it later for CLONE_PIDFD. + * - CLONE_THREAD is blocked until someone really needs it. + */ + if (clone_flags & + (CLONE_DETACHED | CLONE_PARENT_SETTID | CLONE_THREAD)) + return ERR_PTR(-EINVAL); + + /* + * Verify that parent_tidptr is sane so we can potentially + * reuse it later. + */ + if (get_user(reserved, parent_tidptr)) + return ERR_PTR(-EFAULT); + + if (reserved != 0) + return ERR_PTR(-EINVAL); + } + /* * Force any signals received before this point to be delivered * before the fork happens. Collect up signals sent to multiple @@ -1934,6 +2014,22 @@ static __latent_entropy struct task_struct *copy_process( } }
+ /* + * This has to happen after we've potentially unshared the file + * descriptor table (so that the pidfd doesn't leak into the child + * if the fd table isn't shared). + */ + if (clone_flags & CLONE_PIDFD) { + retval = pidfd_create(pid); + if (retval < 0) + goto bad_fork_free_pid; + + pidfd = retval; + retval = put_user(pidfd, parent_tidptr); + if (retval) + goto bad_fork_put_pidfd; + } + #ifdef CONFIG_BLOCK p->plug = NULL; #endif @@ -1989,7 +2085,7 @@ static __latent_entropy struct task_struct *copy_process( */ retval = cgroup_can_fork(p); if (retval) - goto bad_fork_free_pid; + goto bad_fork_put_pidfd;
/* * From this point on we must avoid any synchronous user-space @@ -2111,6 +2207,9 @@ static __latent_entropy struct task_struct *copy_process( spin_unlock(¤t->sighand->siglock); write_unlock_irq(&tasklist_lock); cgroup_cancel_fork(p); +bad_fork_put_pidfd: + if (clone_flags & CLONE_PIDFD) + ksys_close(pidfd); bad_fork_free_pid: cgroup_threadgroup_change_end(current); if (pid != &init_struct_pid) @@ -2178,7 +2277,7 @@ static inline void init_idle_pids(struct task_struct *idle) struct task_struct *fork_idle(int cpu) { struct task_struct *task; - task = copy_process(CLONE_VM, 0, 0, NULL, &init_struct_pid, 0, 0, + task = copy_process(CLONE_VM, 0, 0, NULL, NULL, &init_struct_pid, 0, 0, cpu_to_node(cpu)); if (!IS_ERR(task)) { init_idle_pids(task); @@ -2225,7 +2324,7 @@ long _do_fork(unsigned long clone_flags, trace = 0; }
- p = copy_process(clone_flags, stack_start, stack_size, + p = copy_process(clone_flags, stack_start, stack_size, parent_tidptr, child_tidptr, NULL, trace, tls, NUMA_NO_NODE); add_latent_entropy();