On Mon, Apr 27, 2020 at 06:00:21PM +0200, gregkh@xxxxxxxxxxxxxxxxxxx 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@xxxxxxxxxxxxxxx>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 61e713bdca3678e84815f2427f7a063fc353a1fc Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
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@xxxxxxxxxxxxxxx/
Reported-by: Christof Meerwald <cmeerw@xxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
The code just moved around a bit. I've fixed it and queued for all
branches.
--
Thanks,
Sasha
