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

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