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?)
Christof
--
http://cmeerw.org sip:cmeerw at cmeerw.org
mailto:cmeerw at cmeerw.org xmpp:cmeerw at cmeerw.org
Christof Meerwald <[email protected]> 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: [email protected]
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/[email protected]/
Signed-off-by: "Eric W. Biederman" <[email protected]>
---
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.
+ */
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);
--
2.20.1
On Mon, Apr 20, 2020 at 12:05:38PM -0500, Eric W. Biederman wrote:
>
> Christof Meerwald <[email protected]> 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: [email protected]
> 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/[email protected]/
> Signed-off-by: "Eric W. Biederman" <[email protected]>
> ---
>
> 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.
> if (valid_signal(sig) && sig)
(Not related to you patch but odd ordering of the if-branch here. I
would've expected this to read
if (sig && valid_signal(sig))
especially since "sig" can be set to 0 right before.
> - __group_send_sig_info(sig, &info, tsk->parent);
> + __send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false);
So below you switch to __send_signal() but set the "force" argument to
to "false". Before that, if the signal was generated from another pid
namespace and we fixed up si_pid and si_uid the "force" argument was set
to "true", i.e. __send_signal(..., ..., ..., ..., true) Even though
from reading through the code I think that change is safe to make let me
verify that this was an intentional change?
Thanks!
Christian
On 04/20, Eric W. Biederman wrote:
>
> The immediate problem is as Christof noticed that "pid_alive(current) == false".
this is slightly offtopic, but we can probably remove this "pid_alive" check,
pid_nr_ns() checks pid != NULL anyway.
> Inspecting the code it appears this problem has existed since the pid
> namespace support started handling this case
Agreed...
> @@ -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.
> + */
> if (valid_signal(sig) && sig)
> - __group_send_sig_info(sig, &info, tsk->parent);
> + __send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false);
Acked-by: Oleg Nesterov <[email protected]>
On 04/21, Christian Brauner wrote:
>
> > - __group_send_sig_info(sig, &info, tsk->parent);
> > + __send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false);
>
> So below you switch to __send_signal() but set the "force" argument to
> to "false".
it must be false, the signal is generated from the parent's namespace or
its descendant
> Before that, if the signal was generated from another pid
> namespace and we fixed up si_pid and si_uid the "force" argument was set
> to "true",
before that the "force" argument could be falsely true by the same reason,
task_pid_nr_ns(tsk, tsk->parent) can return 0 because "tsk" no longer have
pids after __unhash_process().
Oleg.
Starting from 2c4704756cab ("pids: Move the pgrp and session pid pointers
from task_struct to signal_struct") __task_pid_nr_ns() doesn't dereference
task->group_leader, we can remove the pid_alive() check.
pid_nr_ns() has to check pid != NULL anyway, pid_alive() just adds the
unnecessary confusion.
Signed-off-by: Oleg Nesterov <[email protected]>
---
kernel/pid.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/kernel/pid.c b/kernel/pid.c
index bc21c0fb26d8..47221db038e2 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -475,8 +475,7 @@ pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
rcu_read_lock();
if (!ns)
ns = task_active_pid_ns(current);
- if (likely(pid_alive(task)))
- nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
+ nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
rcu_read_unlock();
return nr;
--
2.25.1.362.g51ebf55
On Tue, Apr 21, 2020 at 11:28:47AM +0200, Oleg Nesterov wrote:
> On 04/21, Christian Brauner wrote:
> >
> > > - __group_send_sig_info(sig, &info, tsk->parent);
> > > + __send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false);
> >
> > So below you switch to __send_signal() but set the "force" argument to
> > to "false".
>
> it must be false, the signal is generated from the parent's namespace or
> its descendant
>
> > Before that, if the signal was generated from another pid
> > namespace and we fixed up si_pid and si_uid the "force" argument was set
> > to "true",
>
> before that the "force" argument could be falsely true by the same reason,
> task_pid_nr_ns(tsk, tsk->parent) can return 0 because "tsk" no longer have
> pids after __unhash_process().
As I said in my mail, looking at the codepath it seems safe. My question
was whether it is _always_ incorrectly false which I do think it is
because child subreapers can't come from outside the pid namespace. If
they could you could create a scenario where the signal is generated
from a sibling pid namespace in which case it would be correctly set to
true.
Christian
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 <[email protected]> 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: [email protected]
> > 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/[email protected]/
> > Signed-off-by: "Eric W. Biederman" <[email protected]>
> > ---
> >
> > 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 <[email protected]>
Thanks!
Christian
On Tue, Apr 21, 2020 at 12:19:04PM +0200, Oleg Nesterov wrote:
> Starting from 2c4704756cab ("pids: Move the pgrp and session pid pointers
> from task_struct to signal_struct") __task_pid_nr_ns() doesn't dereference
> task->group_leader, we can remove the pid_alive() check.
>
> pid_nr_ns() has to check pid != NULL anyway, pid_alive() just adds the
> unnecessary confusion.
>
> Signed-off-by: Oleg Nesterov <[email protected]>
> ---
Acked-by: Christian Brauner <[email protected]>
Sorry Christian, I don't understand...
On 04/21, Christian Brauner wrote:
>
> On Tue, Apr 21, 2020 at 11:28:47AM +0200, Oleg Nesterov wrote:
> > On 04/21, Christian Brauner wrote:
> > >
> > > > - __group_send_sig_info(sig, &info, tsk->parent);
> > > > + __send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false);
> > >
> > > So below you switch to __send_signal() but set the "force" argument to
> > > to "false".
> >
> > it must be false, the signal is generated from the parent's namespace or
> > its descendant
> >
> > > Before that, if the signal was generated from another pid
> > > namespace and we fixed up si_pid and si_uid the "force" argument was set
> > > to "true",
> >
> > before that the "force" argument could be falsely true by the same reason,
> > task_pid_nr_ns(tsk, tsk->parent) can return 0 because "tsk" no longer have
> > pids after __unhash_process().
>
> As I said in my mail, looking at the codepath it seems safe. My question
> was whether it is _always_ incorrectly false which I do think it is
Again, it must be always "false", it can be incorrectly "true" and this
too is fixed by Eric's patch.
> because child subreapers can't come from outside the pid namespace. If
> they could you could create a scenario where the signal is generated
> from a sibling pid namespace in which case it would be correctly set to
> true.
not sure I understand, but probably the answer is "yes"...
task and task->parent either live in the same namespace or the child's
namespace is the descendant of task->parent's namespace. In both cases
task_pid_nr_ns(tsk, tsk->parent) should return the valid pid_nr and
"force" must be false.
The corner case is release_task() when the last exiting sub-thread sends
a signal on behalf of its ->group_leader, and at this point all the tsk's
pid pointers are NULL, that is why "force" can be falsely "true".
Oleg.
On Tue, Apr 21, 2020 at 01:11:40PM +0200, Oleg Nesterov wrote:
> Sorry Christian, I don't understand...
In my original mail, it was really just a clarification question. I
said the patch is correct from looking at the codepaths. :) I was just
trying to see whether there was a potential corner-case we're missing
where "force" could be _validly_ true.
>
> > because child subreapers can't come from outside the pid namespace. If
> > they could you could create a scenario where the signal is generated
> > from a sibling pid namespace in which case it would be correctly set to
> > true.
>
> not sure I understand, but probably the answer is "yes"...
(This is really purely academic now since it isn't possible, but for
pure amusement assume that a child subreaper could cross namespace
boundaries (which they don't). A marks itself as a subreaper and creates
a new process B in a new pid namespace <pidnsB>, process B setnses into
<pidnsC> which is a sibling pid namespace, B clones a new proces in
<pidnsC> which is now a full member of <pidnsC>, B dies and C is
reparented to A, B exits and then you'd be getting a sigchld from a pid
in a pid namespace in which you have no pid nr.)
On 04/21, Oleg Nesterov wrote:
>
> The corner case is release_task() when the last exiting sub-thread sends
> a signal on behalf of its ->group_leader, and at this point all the tsk's
> pid pointers are NULL, that is why "force" can be falsely "true".
Or do_notify_parent() can be called by debugger from the parent namespace,
in this case "force" can be falsely "true" too.
Oleg.
On Tue, Apr 21, 2020 at 01:28:31PM +0200, Oleg Nesterov wrote:
> On 04/21, Oleg Nesterov wrote:
> >
> > The corner case is release_task() when the last exiting sub-thread sends
> > a signal on behalf of its ->group_leader, and at this point all the tsk's
> > pid pointers are NULL, that is why "force" can be falsely "true".
>
> Or do_notify_parent() can be called by debugger from the parent namespace,
> in this case "force" can be falsely "true" too.
That's an interesting scenario to think about as well. Cross-pid-namespace
interactions are fun... That's why the cross-pid-namespace-signal
sending aspects we discussed a while back on-list though pretty nice to
have at some point are somewhat scary.
Christian
On 04/21, Christian Brauner wrote:
>
> process B setnses into
> <pidnsC> which is a sibling pid namespace,
please see pidns_install(), it verifies that
* Only allow entering the current active pid namespace
* or a child of the current active pid namespace.
Oleg.
On Tue, Apr 21, 2020 at 02:17:22PM +0200, Oleg Nesterov wrote:
> On 04/21, Christian Brauner wrote:
> >
> > process B setnses into
> > <pidnsC> which is a sibling pid namespace,
>
> please see pidns_install(), it verifies that
>
> * Only allow entering the current active pid namespace
> * or a child of the current active pid namespace.
I forgot about that.
Though, don't we have the same problem in:
static void do_notify_parent_cldstop(struct task_struct *tsk,
bool for_ptracer, int why)
at least for the for_ptrace is false case?
Christian
Christian Brauner <[email protected]> writes:
> On Tue, Apr 21, 2020 at 02:17:22PM +0200, Oleg Nesterov wrote:
>> On 04/21, Christian Brauner wrote:
>> >
>> > process B setnses into
>> > <pidnsC> which is a sibling pid namespace,
>>
>> please see pidns_install(), it verifies that
>>
>> * Only allow entering the current active pid namespace
>> * or a child of the current active pid namespace.
>
> I forgot about that.
>
> Though, don't we have the same problem in:
>
> static void do_notify_parent_cldstop(struct task_struct *tsk,
> bool for_ptracer, int why)
>
> at least for the for_ptrace is false case?
The same problem does not exist with do_notify_parent_cldstop because
do_notify_parent_cldstop is always called from current (there is
one case of current->group_leader but that is close enough calculations
made against current are true).
However because do_notify_parent_cldstop calculates si_pid and si_uid of
the target parent process I think we can still get the wrong si_uid.
So it probably makes sense to generalize the fixup code in send_signal
and make do_notify_parent_cldstop just generate ids in the current
namespace and let the fixup code do it's job.
Eric
Christian Brauner <[email protected]> writes:
> On Mon, Apr 20, 2020 at 12:05:38PM -0500, Eric W. Biederman wrote:
>> 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 have updated the comment to read:
+ /*
+ * Send with __send_signal as si_pid and si_uid are in the
+ * parent's namespaces.
+ */
That should be enough of a hint for someone to read the code and figure
out what is going on.
Eric
Christof Meerwald <[email protected]> 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.
Christof I want to say very good spotting and reporting of this issue.
Thank you.
Eric
Oleg Nesterov <[email protected]> writes:
> Starting from 2c4704756cab ("pids: Move the pgrp and session pid pointers
> from task_struct to signal_struct") __task_pid_nr_ns() doesn't dereference
> task->group_leader, we can remove the pid_alive() check.
>
> pid_nr_ns() has to check pid != NULL anyway, pid_alive() just adds the
> unnecessary confusion.
>
> Signed-off-by: Oleg Nesterov <[email protected]>
Reviewed-by: "Eric W. Biederman" <[email protected]>
> ---
Good catch that does simplify things.
Eric
> kernel/pid.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index bc21c0fb26d8..47221db038e2 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -475,8 +475,7 @@ pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
> rcu_read_lock();
> if (!ns)
> ns = task_active_pid_ns(current);
> - if (likely(pid_alive(task)))
> - nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
> + nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
> rcu_read_unlock();
>
> return nr;
On Tue, Apr 21, 2020 at 09:57:09AM -0500, Eric W. Biederman wrote:
> Christian Brauner <[email protected]> writes:
>
> > On Mon, Apr 20, 2020 at 12:05:38PM -0500, Eric W. Biederman wrote:
> >> 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 have updated the comment to read:
> + /*
> + * Send with __send_signal as si_pid and si_uid are in the
> + * parent's namespaces.
> + */
>
> That should be enough of a hint for someone to read the code and figure
> out what is going on.
Perfect, thanks!
Acked-by: Christian Brauner <[email protected]>
Christian
On 04/21, Eric W. Biederman wrote:
>
> Oleg Nesterov <[email protected]> writes:
>
> > Starting from 2c4704756cab ("pids: Move the pgrp and session pid pointers
> > from task_struct to signal_struct") __task_pid_nr_ns() doesn't dereference
> > task->group_leader, we can remove the pid_alive() check.
> >
> > pid_nr_ns() has to check pid != NULL anyway, pid_alive() just adds the
> > unnecessary confusion.
> >
> > Signed-off-by: Oleg Nesterov <[email protected]>
> Reviewed-by: "Eric W. Biederman" <[email protected]>
Thanks, can you take this patch?
Oleg.
Oleg Nesterov <[email protected]> writes:
> On 04/21, Eric W. Biederman wrote:
>>
>> Oleg Nesterov <[email protected]> writes:
>>
>> > Starting from 2c4704756cab ("pids: Move the pgrp and session pid pointers
>> > from task_struct to signal_struct") __task_pid_nr_ns() doesn't dereference
>> > task->group_leader, we can remove the pid_alive() check.
>> >
>> > pid_nr_ns() has to check pid != NULL anyway, pid_alive() just adds the
>> > unnecessary confusion.
>> >
>> > Signed-off-by: Oleg Nesterov <[email protected]>
>> Reviewed-by: "Eric W. Biederman" <[email protected]>
>
> Thanks, can you take this patch?
I plan to. Probably on a topic branch for signals. I am sorting
that out now.
Eric