pid: make sub-init creation retryable

Patch series "pid: make sub-init creation retryable".


This patch (of 2):

Currently we allow only one attempt to create init in a new namespace.  If
the first fork() fails after alloc_pid() succeeds, free_pid() clears
PIDNS_ADDING and thus disables further PID allocations.

Nowadays this looks like an unnecessary limitation.  The original reason
to handle "case PIDNS_ADDING" in free_pid() is gone, most probably after
commit 69879c01a0c3 ("proc: Remove the now unnecessary internal mount of
proc").

Change free_pid() to keep ns->pid_allocated == PIDNS_ADDING, and change
alloc_pid() to reset the cursor early, right after taking pidmap_lock.

Test-case:

	#define _GNU_SOURCE
	#include <linux/sched.h>
	#include <sys/syscall.h>
	#include <sys/wait.h>
	#include <assert.h>
	#include <sched.h>
	#include <errno.h>

	int main(void)
	{
		struct clone_args args = {
			.exit_signal = SIGCHLD,
			.flags	= CLONE_PIDFD,
			.pidfd	= 0,
		};
		unsigned long pidfd;
		int pid;

		assert(unshare(CLONE_NEWPID) == 0);

		pid = syscall(__NR_clone3, &args, sizeof(args));
		assert(pid == -1 && errno == EFAULT);

		args.pidfd = (unsigned long)&pidfd;
		pid = syscall(__NR_clone3, &args, sizeof(args));
		if (pid)
			assert(pid > 0 && wait(NULL) == pid);
		else
			assert(getpid() == 1);

		return 0;
	}

Link: https://lkml.kernel.org/r/aaGHu3ixbw9Y7kFj@redhat.com
Link: https://lkml.kernel.org/r/aaGIHa7vGdwhEc_D@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Andrei Vagin <avagin@gmail.com>
Cc: Adrian Reber <areber@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Kirill Tkhai <tkhai@ya.ru>
Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 6 insertions, 7 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index 3b96571d0fe6..aff5bf0f638f 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -131,9 +131,8 @@ void free_pid(struct pid *pid)
 			wake_up_process(ns->child_reaper);
 			break;
 		case PIDNS_ADDING:
-			/* Handle a fork failure of the first process */
-			WARN_ON(ns->child_reaper);
-			ns->pid_allocated = 0;
+			/* Only possible if the 1st fork fails */
+			WARN_ON(READ_ONCE(ns->child_reaper));
 			break;
 		}
 
@@ -236,6 +235,10 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 	retried_preload = false;
 	idr_preload(GFP_KERNEL);
 	spin_lock(&pidmap_lock);
+	/* For the case when the previous attempt to create init failed */
+	if (ns->pid_allocated == PIDNS_ADDING)
+		idr_set_cursor(&ns->idr, 0);
+
 	for (tmp = ns, i = ns->level; i >= 0;) {
 		int tid = set_tid[ns->level - i];
 
@@ -338,10 +341,6 @@ out_free:
 		idr_remove(&upid->ns->idr, upid->nr);
 	}
 
-	/* On failure to allocate the first pid, reset the state */
-	if (ns->pid_allocated == PIDNS_ADDING)
-		idr_set_cursor(&ns->idr, 0);
-
 	spin_unlock(&pidmap_lock);
 	idr_preload_end();