[PATCH 5/7][v7] Protect cinit from blocked fatal signals

From: Sukadev Bhattiprolu
Date: Sat Jan 17 2009 - 15:40:33 EST

From: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
Date: Wed, 24 Dec 2008 14:04:24 -0800
Subject: [PATCH 5/7][v7] Protect cinit from blocked fatal signals

Normally SIG_DFL signals to global and container-init are dropped early.
But if a signal is blocked when it is posted, we cannot drop the signal
since the receiver may install a handler before unblocking the signal.
Once this signal is queued however, the receiver container-init has
no way of knowing if the signal was sent from an ancestor or descendant
namespace. This patch ensures that contianer-init drops all SIG_DFL
signals in get_signal_to_deliver() except SIGKILL/SIGSTOP.

If SIGSTOP/SIGKILL originate from a descendant of container-init they
are never queued (i.e dropped in sig_ignored() in an earler patch).

If SIGSTOP/SIGKILL originate from parent namespace, the signal is queued
and container-init processes the signal.

IOW, if get_signal_to_deliver() sees a sig_kernel_only() signal for global
or container-init, the signal must have been generated internally or must
have come from an ancestor ns and we process the signal.

Further, the signal_group_exit() check was needed to cover the case of
a multi-threaded init sending SIGKILL to other threads when doing an
exit() or exec(). But since the new sig_kernel_only() check covers the
SIGKILL, the signal_group_exit() check is no longer needed and can be

Finally, now that we have all pieces in place, set SIGNAL_UNKILLABLE for

- Add a note regarding the signal_group_exit() in patch description.

- (Oleg Nesterov) Drop signal_unkillable(), simplify check in
get_signal_to_deliver() and drop check for signal_group_exit()
since it is covered by sig_kernel_only().

- Rename sig_unkillable() to unkillable_by_sig()
- Remove SIGNAL_UNKILLABLE_FROM_NS flag and simplify (Oleg Nesterov)
- Set SIGNAL_UNKILLABLE for container-init in this patch.

Signed-off-by: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
kernel/fork.c | 2 ++
kernel/signal.c | 9 ++++++++-
2 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index dba2d3f..d3e93ef 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -812,6 +812,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
atomic_set(&sig->live, 1);
sig->flags = 0;
+ if (clone_flags & CLONE_NEWPID)
+ sig->flags |= SIGNAL_UNKILLABLE;
sig->group_exit_code = 0;
sig->group_exit_task = NULL;
sig->group_stop_count = 0;
diff --git a/kernel/signal.c b/kernel/signal.c
index 41060ae..ccc020f 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1857,9 +1857,16 @@ relock:

* Global init gets no signals it doesn't want.
+ * Container-init gets no signals it doesn't want from same
+ * container.
+ *
+ * Note that if global/container-init sees a sig_kernel_only()
+ * signal here, the signal must have been generated internally
+ * or must have come from an ancestor namespace. In either
+ * case, the signal cannot be dropped.
if (unlikely(signal->flags & SIGNAL_UNKILLABLE) &&
- !signal_group_exit(signal))
+ !sig_kernel_only(signr))

if (sig_kernel_stop(signr)) {

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