The patch below does not apply to the 4.9-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 61e713bdca3678e84815f2427f7a063fc353a1fc Mon Sep 17 00:00:00 2001

From: "Eric W. Biederman" ebiederm@xmission.com Date: Mon, 20 Apr 2020 11:41:50 -0500 Subject: [PATCH] signal: Avoid corrupting si_pid and si_uid in do_notify_parent

Christof Meerwald cmeerw@cmeerw.org writes:

Hi,

this is probably related to commit 7a0cf094944e2540758b7f957eb6846d5126f535 (signal: Correct namespace fixups of si_pid and si_uid).

With a 5.6.5 kernel I am seeing SIGCHLD signals that don't include a properly set si_pid field - this seems to happen for multi-threaded child processes.

A simple test program (based on the sample from the signalfd man page):

#include <sys/signalfd.h> #include <signal.h> #include <unistd.h> #include <spawn.h> #include <stdlib.h> #include <stdio.h>

#define handle_error(msg) \ do { perror(msg); exit(EXIT_FAILURE); } while (0)

int main(int argc, char *argv[]) { sigset_t mask; int sfd; struct signalfd_siginfo fdsi; ssize_t s;

sigemptyset(&mask); sigaddset(&mask, SIGCHLD);

if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1) handle_error("sigprocmask");

pid_t chldpid; char *chldargv[] = { "./sfdclient", NULL }; posix_spawn(&chldpid, "./sfdclient", NULL, NULL, chldargv, NULL);

sfd = signalfd(-1, &mask, 0); if (sfd == -1) handle_error("signalfd");

for (;;) { s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo)); if (s != sizeof(struct signalfd_siginfo)) handle_error("read");

if (fdsi.ssi_signo == SIGCHLD) {
  printf("Got SIGCHLD %d %d %d %d\n",
      fdsi.ssi_status, fdsi.ssi_code,
      fdsi.ssi_uid, fdsi.ssi_pid);
  return 0;
} else {
  printf("Read unexpected signal\n");
}

} }

and a multi-threaded client to test with:

#include <unistd.h> #include <pthread.h>

void *f(void *arg) { sleep(100); }

int main() { pthread_t t[8];

for (int i = 0; i != 8; ++i) { pthread_create(&t[i], NULL, f, NULL); } }

I tried to do a bit of debugging and what seems to be happening is that

/* From an ancestor pid namespace? */ if (!task_pid_nr_ns(current, task_active_pid_ns(t))) {

fails inside task_pid_nr_ns because the check for "pid_alive" fails.

This code seems to be called from do_notify_parent and there we actually have "tsk != current" (I am assuming both are threads of the current process?)

I instrumented the code with a warning and received the following backtrace:

WARNING: CPU: 0 PID: 777 at kernel/pid.c:501 __task_pid_nr_ns.cold.6+0xc/0x15 Modules linked in: CPU: 0 PID: 777 Comm: sfdclient Not tainted 5.7.0-rc1userns+ #2924 Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 RIP: 0010:__task_pid_nr_ns.cold.6+0xc/0x15 Code: ff 66 90 48 83 ec 08 89 7c 24 04 48 8d 7e 08 48 8d 74 24 04 e8 9a b6 44 00 48 83 c4 08 c3 48 c7 c7 59 9f ac 82 e8 c2 c4 04 00 <0f> 0b e9 3fd RSP: 0018:ffffc9000042fbf8 EFLAGS: 00010046 RAX: 000000000000000c RBX: 0000000000000000 RCX: ffffc9000042faf4 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81193d29 RBP: ffffc9000042fc18 R08: 0000000000000000 R09: 0000000000000001 R10: 000000100f938416 R11: 0000000000000309 R12: ffff8880b941c140 R13: 0000000000000000 R14: 0000000000000000 R15: ffff8880b941c140 FS: 0000000000000000(0000) GS:ffff8880bca00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f2e8c0a32e0 CR3: 0000000002e10000 CR4: 00000000000006f0 Call Trace: send_signal+0x1c8/0x310 do_notify_parent+0x50f/0x550 release_task.part.21+0x4fd/0x620 do_exit+0x6f6/0xaf0 do_group_exit+0x42/0xb0 get_signal+0x13b/0xbb0 do_signal+0x2b/0x670 ? __audit_syscall_exit+0x24d/0x2b0 ? rcu_read_lock_sched_held+0x4d/0x60 ? kfree+0x24c/0x2b0 do_syscall_64+0x176/0x640 ? trace_hardirqs_off_thunk+0x1a/0x1c entry_SYSCALL_64_after_hwframe+0x49/0xb3

The immediate problem is as Christof noticed that "pid_alive(current) == false". This happens because do_notify_parent is called from the last thread to exit in a process after that thread has been reaped.

The bigger issue is that do_notify_parent can be called from any process that manages to wait on a thread of a multi-threaded process from wait_task_zombie. So any logic based upon current for do_notify_parent is just nonsense, as current can be pretty much anything.

So change do_notify_parent to call __send_signal directly.

Inspecting the code it appears this problem has existed since the pid namespace support started handling this case in 2.6.30. This fix only backports to 7a0cf094944e ("signal: Correct namespace fixups of si_pid and si_uid") where the problem logic was moved out of __send_signal and into send_signal.

Cc: stable@vger.kernel.org Fixes: 6588c1e3ff01 ("signals: SI_USER: Masquerade si_pid when crossing pid ns boundary") Ref: 921cf9f63089 ("signals: protect cinit from unblocked SIG_DFL signals") Link: https://lore.kernel.org/lkml/20200419201336.GI22017@edge.cmeerw.net/ Reported-by: Christof Meerwald cmeerw@cmeerw.org Acked-by: Oleg Nesterov oleg@redhat.com Acked-by: Christian Brauner christian.brauner@ubuntu.com Signed-off-by: "Eric W. Biederman" ebiederm@xmission.com

diff --git a/kernel/signal.c b/kernel/signal.c index e58a6c619824..7938c60e11dd 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1993,8 +1993,12 @@ bool do_notify_parent(struct task_struct *tsk, int sig) if (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN) sig = 0; } + /* + * Send with __send_signal as si_pid and si_uid are in the + * parent's namespaces. + */ if (valid_signal(sig) && sig) - __group_send_sig_info(sig, &info, tsk->parent); + __send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false); __wake_up_parent(tsk, tsk->parent); spin_unlock_irqrestore(&psig->siglock, flags);