Re: FAILED: patch "[PATCH] signal: Avoid corrupting si_pid and si_uid in" failed to apply to 4.19-stable tree

[Sasha Levin <sashal@kernel.org>]

On Mon, Apr 27, 2020 at 06:00:21PM +0200, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 4.19-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>

The code just moved around a bit. I've fixed it and queued for all
branches.

-- 
Thanks,
Sasha