Re: [PATCH] signal: Avoid corrupting si_pid and si_uid in do_notify_parent

[Christian Brauner <christian.brauner@ubuntu.com>]

On Tue, Apr 21, 2020 at 10:30:31AM +0200, Christian Brauner wrote:
> On Mon, Apr 20, 2020 at 12:05:38PM -0500, Eric W. Biederman wrote:
> > 
> > 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/
> > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> > ---
> > 
> > Unless someone has an objection I will apply this one and send it to
> > Linus.
> > 
> >  kernel/signal.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/signal.c b/kernel/signal.c
> > index 9899c5f91ee1..a88a89422227 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;
> >  	}
> > +	/*
> > +	 * Bypass send_signal as the si_pid and si_uid values have
> > +	 * been generated in the parent's namespaces.
> > +	 */
> 
> At first I misread that comment as saying that we're skipping sending a
> signal not that it relates to a specific function (and I won't admit that
> I wrote a whole long paragraph on why I'm confused we're skipping
> sending signals on invalid si_pid and si_uid...).
> 
> I think it would be worth to say something that simply states the facts
> such as:
> "If we're not autoreaping, send a signal with si_pid and si_uid set
>  as generated in the parent's namespaces."
> which imho is way clearer then pointing to out that we're skipping
> send_signal(). The logic here and the comment in its current form are
> hard to correlate, especially since send_signal() was never called
> directly here but is rather called by __group_send_sig_info().
> The details of why we switched from __group_send_sign_info() to
> __send_signal() could just go into the commit message. It's more
> confusing in the code.

Forgot before, otherwise:
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

Thanks!
Christian