[PATCH 2/2] procfs/tasks: add a simple per-task procfs hidepid= field

[Lafcadio Wluiki <wluikil@gmail.com>]

(Second, rebased submission, since first submission yielded no replies.)

This adds a new per-task hidepid= flag that is honored by procfs when
presenting /proc to the user, in addition to the existing hidepid= mount
option. So far, hidepid= was exclusively a per-pidns setting. Locking
down a set of processes so that they cannot see other user's processes
without affecting the rest of the system thus currently requires
creation of a private PID namespace, with all the complexity it brings,
including maintaining a stub init process as PID 1 and losing the
ability to see processes of the same user on the rest of the system.

With this patch all acesss and visibility checks in procfs now
honour two fields:

	a) the existing hide_pid field in the PID namespace
	b) the new hide_pid in struct task_struct

Access/visibility is only granted if both fields permit it; the more
restrictive one wins. By default the new task_struct hide_pid value
defaults to 0, which means behaviour is not changed from the status quo.

Setting the per-process hide_pid value is done via a new PR_SET_HIDEPID
prctl() option which takes the same three supported values as the
hidepid= mount option. The per-process hide_pid may only be increased,
never decreased, thus ensuring that once applied, processes can never
escape such a hide_pid jail.  When a process forks it inherits its
parent's hide_pid value.

Suggested usecase: let's say nginx runs as user "www-data". After
dropping privileges it may now call:

	…
	prctl(PR_SET_HIDEPID, 2);
	…

And from that point on neither nginx itself, nor any of its child
processes may see processes in /proc anymore that belong to a different
user than "www-data". Other services running on the same system remain
unaffected.

This should permit Linux distributions to more comprehensively lock down
their services, as it allows an isolated opt-in for hidepid= for
specific services. Previously hidepid= could only be set system-wide,
and then specific services had to be excluded by group membership,
essentially a more complex concept of opt-out.

A test-tool that validates this functionality is available here:

	https://paste.fedoraproject.org/412975/71967605/

Signed-off-by: Lafcadio Wluiki <wluikil@gmail.com>
---
 fs/proc/array.c            |  3 +++
 fs/proc/base.c             |  6 ++++--
 include/linux/init_task.h  |  1 +
 include/linux/sched.h      |  1 +
 include/uapi/linux/prctl.h |  4 ++++
 kernel/fork.c              |  1 +
 kernel/sys.c               | 10 ++++++++++
 7 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 89600fd..2135616 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -163,6 +163,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 	const struct cred *cred;
 	pid_t ppid, tpid = 0, tgid, ngid;
 	unsigned int max_fds = 0;
+	int hide_pid;
 
 	rcu_read_lock();
 	ppid = pid_alive(p) ?
@@ -183,6 +184,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 	task_lock(p);
 	if (p->files)
 		max_fds = files_fdtable(p->files)->max_fds;
+	hide_pid = p->hide_pid;
 	task_unlock(p);
 	rcu_read_unlock();
 
@@ -201,6 +203,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 	seq_put_decimal_ull(m, "\t", from_kgid_munged(user_ns, cred->egid));
 	seq_put_decimal_ull(m, "\t", from_kgid_munged(user_ns, cred->sgid));
 	seq_put_decimal_ull(m, "\t", from_kgid_munged(user_ns, cred->fsgid));
+	seq_put_decimal_ull(m, "\nHidePID:\t", hide_pid);
 	seq_put_decimal_ull(m, "\nFDSize:\t", max_fds);
 
 	seq_puts(m, "\nGroups:\t");
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 2680794..84524d6 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -726,7 +726,8 @@ static bool has_pid_permissions(struct pid_namespace *pid,
 				 struct task_struct *task,
 				 int hide_pid_min)
 {
-	if (pid->hide_pid < hide_pid_min)
+	if (pid->hide_pid < hide_pid_min &&
+	    current->hide_pid < hide_pid_min)
 		return true;
 	if (in_group_p(pid->pid_gid))
 		return true;
@@ -747,7 +748,8 @@ static int proc_pid_permission(struct inode *inode, int mask)
 	put_task_struct(task);
 
 	if (!has_perms) {
-		if (pid->hide_pid == HIDEPID_INVISIBLE) {
+		if (pid->hide_pid == HIDEPID_INVISIBLE ||
+		    current->hide_pid == HIDEPID_INVISIBLE) {
 			/*
 			 * Let's make getdents(), stat(), and open()
 			 * consistent with each other.  If a process
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 325f649..c87de0e 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -250,6 +250,7 @@ extern struct task_group root_task_group;
 	.cpu_timers	= INIT_CPU_TIMERS(tsk.cpu_timers),		\
 	.pi_lock	= __RAW_SPIN_LOCK_UNLOCKED(tsk.pi_lock),	\
 	.timer_slack_ns = 50000, /* 50 usec default slack */		\
+	.hide_pid	= 0,						\
 	.pids = {							\
 		[PIDTYPE_PID]  = INIT_PID_LINK(PIDTYPE_PID),		\
 		[PIDTYPE_PGID] = INIT_PID_LINK(PIDTYPE_PGID),		\
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 348f51b..3e8ca16 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1572,6 +1572,7 @@ struct task_struct {
 	/* unserialized, strictly 'current' */
 	unsigned in_execve:1; /* bit to tell LSMs we're in execve */
 	unsigned in_iowait:1;
+	unsigned hide_pid:2; /* per-process procfs hidepid= */
 #if !defined(TIF_RESTORE_SIGMASK)
 	unsigned restore_sigmask:1;
 #endif
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index a8d0759..ada62b6 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -197,4 +197,8 @@ struct prctl_mm_map {
 # define PR_CAP_AMBIENT_LOWER		3
 # define PR_CAP_AMBIENT_CLEAR_ALL	4
 
+/* Per process, non-revokable procfs hidepid= option */
+#define PR_SET_HIDEPID 48
+#define PR_GET_HIDEPID 49
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index 6d42242..a781d35 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1560,6 +1560,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 #endif
 
 	p->default_timer_slack_ns = current->timer_slack_ns;
+	p->hide_pid = current->hide_pid;
 
 	task_io_accounting_init(&p->ioac);
 	acct_clear_integrals(p);
diff --git a/kernel/sys.c b/kernel/sys.c
index 89d5be4..c0a1d3e 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2270,6 +2270,16 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case PR_GET_FP_MODE:
 		error = GET_FP_MODE(me);
 		break;
+	case PR_SET_HIDEPID:
+		if (arg2 < HIDEPID_OFF || arg2 > HIDEPID_INVISIBLE)
+			return -EINVAL;
+		if (arg2 < me->hide_pid)
+			return -EPERM;
+		me->hide_pid = arg2;
+		break;
+	case PR_GET_HIDEPID:
+		error = put_user((int) me->hide_pid, (int __user *)arg2);
+		break;
 	default:
 		error = -EINVAL;
 		break;
-- 
2.7.4