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

[Steve French <smfrench@gmail.com>]

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@gmail.com> wrote:
>
> On Tue, Jul 23, 2019 at 8:32 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> >
> > Steve French <smfrench@gmail.com> 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