Re: [PATCH AUTOSEL 5.2 039/249] signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force_sig

[Eric W. Biederman]

Steve French <smfrench@xxxxxxxxx> writes:

> I did some additional testing and it looks like the "allow_signal"
> change may be safe enough
>
> # git diff -a
> diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
> index a4830ced0f98..a15a6e738eb5 100644
> --- a/fs/cifs/connect.c
> +++ b/fs/cifs/connect.c
> @@ -1113,6 +1113,7 @@ cifs_demultiplex_thread(void *p)
>                 mempool_resize(cifs_req_poolp, length + cifs_min_rcv);
>
>         set_freezable();
> +       allow_signal(SIGKILL);
>         while (server->tcpStatus != CifsExiting) {
>                 if (try_to_freeze())
>                         continue;
>
> See below:
> root@smf-Thinkpad-P51:~/cifs-2.6/fs/cifs# insmod ./cifs.ko
> root@smf-Thinkpad-P51:~/cifs-2.6/fs/cifs# mount -t cifs
> //localhost/scratch /mnt -o username=sfrench
> Password for sfrench@//localhost/scratch:  ************
> root@smf-Thinkpad-P51:~/cifs-2.6/fs/cifs# ps -A | grep cifsd
>  5176 ?        00:00:00 cifsd
> root@smf-Thinkpad-P51:~/cifs-2.6/fs/cifs# kill -9 5176
> root@smf-Thinkpad-P51:~/cifs-2.6/fs/cifs# ls /mnt
> 0444  dir0750  dir0754  newfile
> root@smf-Thinkpad-P51:~/cifs-2.6/fs/cifs# umount /mnt
> root@smf-Thinkpad-P51:~/cifs-2.6/fs/cifs# ps -A | grep cifsd
> root@smf-Thinkpad-P51:~/cifs-2.6/fs/cifs# rmmod cifs

Yes.  I just discovered that kthreadd calls a function named
ignore_signals that set all signals to SIG_IGN.  Which becomes
the default for all kernel threads.  So something like allow_signal
to change the signal handler is necessary.  The blocking of SIGKILL is
also concerning but apparently that is not the issue here.

Ideally I think cifs should use kthread_stop, instead of signals for
this purpose.  The logic is convoluted enough that reading through the
cifs code quickly I don't see how sending SIGKILL to the daemon causes
it to stop.

Eric


> On Tue, Jul 23, 2019 at 9:19 PM Steve French <smfrench@xxxxxxxxx> wrote:
>>
>> Pavel noticed I missed a line from the attempt to do a similar patch
>> to Eric's suggestion
>> (it still didn't work though - although "allow_signal" does albeit is
>> possibly dangerous as user space can kill cifsd)
>>
>> # git diff -a
>> diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
>> index a4830ced0f98..8758dff18c15 100644
>> --- a/fs/cifs/connect.c
>> +++ b/fs/cifs/connect.c
>> @@ -1104,6 +1104,7 @@ cifs_demultiplex_thread(void *p)
>>         struct task_struct *task_to_wake = NULL;
>>         struct mid_q_entry *mids[MAX_COMPOUND];
>>         char *bufs[MAX_COMPOUND];
>> +       sigset_t mask, oldmask;
>>
>>         current->flags |= PF_MEMALLOC;
>>         cifs_dbg(FYI, "Demultiplex PID: %d\n", task_pid_nr(current));
>> @@ -1113,6 +1114,9 @@ cifs_demultiplex_thread(void *p)
>>                 mempool_resize(cifs_req_poolp, length + cifs_min_rcv);
>>
>>         set_freezable();
>> +       sigfillset(&mask);
>> +       sigdelset(&mask, SIGKILL);
>> +       sigprocmask(SIG_BLOCK, &mask, &oldmask);
>>         while (server->tcpStatus != CifsExiting) {
>>                 if (try_to_freeze())
>>                         continue;
>>
>> On Tue, Jul 23, 2019 at 9:02 PM Steve French <smfrench@xxxxxxxxx> wrote:
>> >
>> > On Tue, Jul 23, 2019 at 8:32 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>> > >
>> > > Steve French <smfrench@xxxxxxxxx> writes:
>> > >
>> > > > Very easy to see what caused the regression with this global change:
>> > > >
>> > > > mount (which launches "cifsd" thread to read the socket)
>> > > > umount (which kills the "cifsd" thread)
>> > > > rmmod   (rmmod now fails since "cifsd" thread is still active)
>> > > >
>> > > > mount launches a thread to read from the socket ("cifsd")
>> > > > umount is supposed to kill that thread (but with the patch
>> > > > "signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of
>> > > > force_sig" that no longer works).  So the regression is that after
>> > > > unmount you still see the "cifsd" thread, and the reason that cifsd
>> > > > thread is still around is that that patch no longer force kills the
>> > > > process (see line 2652 of fs/cifs/connect.c) which regresses module
>> > > > removal.
>> > > >
>> > > > -               force_sig(SIGKILL, task);
>> > > > +               send_sig(SIGKILL, task, 1);
>> > > >
>> > > > The comment in the changeset indicates "The signal SIGKILL can not be
>> > > > ignored" but obviously it can be ignored - at least on 5.3-rc1 it is
>> > > > being ignored.
>> > > >
>> > > > If send_sig(SIGKILL ...) doesn't work and if force_sig(SIGKILL, task)
>> > > > is removed and no longer possible - how do we kill a helper process
>> > > > ...
>> > >
>> > > I think I see what is happening.  It looks like as well as misuinsg
>> > > force_sig, cifs is also violating the invariant that keeps SIGKILL out
>> > > of the blocked signal set.
>> > >
>> > > For that force_sig will act differently.  I did not consider it because
>> > > that is never supposed to happen.
>> > >
>> > > Can someone test this code below and confirm the issue goes away?
>> > >
>> > > diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
>> > > index 5d6d44bfe10a..2a782ebc7b65 100644
>> > > --- a/fs/cifs/transport.c
>> > > +++ b/fs/cifs/transport.c
>> > > @@ -347,6 +347,7 @@ __smb_send_rqst(struct TCP_Server_Info *server, int num_rqst,
>> > >          */
>> > >
>> > >         sigfillset(&mask);
>> > > +       sigdelset(&mask, SIGKILL);
>> > >         sigprocmask(SIG_BLOCK, &mask, &oldmask);
>> > >
>> > >         /* Generate a rfc1002 marker for SMB2+ */
>> > >
>> > >
>> > > Eric
>> >
>> > I just tried your suggestion and it didn't work.   I also tried doing
>> > a similar thing on the thread we are trying to kills ("cifsd" - ie
>> > which is blocked in the function cifs_demultiplex_thread waiting to
>> > read from the socket)
>> > # git diff -a
>> > diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
>> > index a4830ced0f98..b73062520a17 100644
>> > --- a/fs/cifs/connect.c
>> > +++ b/fs/cifs/connect.c
>> > @@ -1104,6 +1104,7 @@ cifs_demultiplex_thread(void *p)
>> >         struct task_struct *task_to_wake = NULL;
>> >         struct mid_q_entry *mids[MAX_COMPOUND];
>> >         char *bufs[MAX_COMPOUND];
>> > +       sigset_t mask;
>> >
>> >         current->flags |= PF_MEMALLOC;
>> >         cifs_dbg(FYI, "Demultiplex PID: %d\n", task_pid_nr(current));
>> > @@ -1113,6 +1114,8 @@ cifs_demultiplex_thread(void *p)
>> >                 mempool_resize(cifs_req_poolp, length + cifs_min_rcv);
>> >
>> >         set_freezable();
>> > +       sigfillset(&mask);
>> > +       sigdelset(&mask, SIGKILL);
>> >         while (server->tcpStatus != CifsExiting) {
>> >                 if (try_to_freeze())
>> >                         continue;
>> >
>> >
>> > That also didn't work.     The only thing I have been able to find
>> > which worked was:
>> >
>> > diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
>> > index a4830ced0f98..e74f04163fc9 100644
>> > --- a/fs/cifs/connect.c
>> > +++ b/fs/cifs/connect.c
>> > @@ -1113,6 +1113,7 @@ cifs_demultiplex_thread(void *p)
>> >                 mempool_resize(cifs_req_poolp, length + cifs_min_rcv);
>> >
>> >         set_freezable();
>> > +      allow_signal(SIGKILL);
>> >         while (server->tcpStatus != CifsExiting) {
>> >                 if (try_to_freeze())
>> >                         continue;
>> >
>> >
>> > That fixes the problem ... but ... as Ronnie and others have noted it
>> > would allow a userspace process to make the mount unusable (all you
>> > would have to do would be to do a kill -9 of the "cifsd" process from
>> > some userspace process like bash and the mount would be unusable - so
>> > this sounds dangerous.
>> >
>> > Is there an alternative that, in the process doing the unmount in
>> > kernel, would allow us to do the equivalent of:
>> >       "allow_signal(SIGKILL, <the id of the cifsd process>"
>> > In otherwords, to minimize the risk of some userspace process killing
>> > cifsd, could we delay enabling allow_signal(SIGKILL) till the unmount
>> > begins by doing it for a different process (have the unmount process
>> > enable signals for the cifsd process).   Otherwise is there a way to
>> > force kill a process from the kernel as we used to do - without
>> > running the risk of a user space process killing cifsd (which is bad).
>> >
>> > --
>> > Thanks,
>> >
>> > Steve
>>
>>
>>
>> --
>> Thanks,
>>
>> Steve