Re: Q: selinux_bprm_committed_creds() && signals/do_wait

[David Howells <dhowells@redhat.com>]

Oleg Nesterov <oleg@redhat.com> wrote:

> > I suspect we should pass SIGKILL
> 
> Or we can fliter out SIGKILLs, yes.

How about the attached patch?

David
---
From: David Howells <dhowells@redhat.com>
Subject: [PATCH] SELinux: Don't flush inherited SIGKILL during execve()

Don't flush inherited SIGKILL during execve() in SELinux's post cred commit
hook.  This isn't really a security problem: if the SIGKILL came before the
credentials were changed, then we were right to receive it at the time, and
should honour it; if it came after the creds were changed, then we definitely
should honour it; and in any case, all that will happen is that the process
will be scrapped before it ever returns to userspace.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 include/linux/sched.h    |    1 +
 kernel/signal.c          |   11 ++++++++---
 security/selinux/hooks.c |   11 +++++++----
 3 files changed, 16 insertions(+), 7 deletions(-)


diff --git a/include/linux/sched.h b/include/linux/sched.h
index b4c38bc..3fa82b3 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1885,6 +1885,7 @@ extern void sched_dead(struct task_struct *p);
 
 extern void proc_caches_init(void);
 extern void flush_signals(struct task_struct *);
+extern void __flush_signals(struct task_struct *);
 extern void ignore_signals(struct task_struct *);
 extern void flush_signal_handlers(struct task_struct *, int force_default);
 extern int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info);
diff --git a/kernel/signal.c b/kernel/signal.c
index d803473..d2dd9cf 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -249,14 +249,19 @@ void flush_sigqueue(struct sigpending *queue)
 /*
  * Flush all pending signals for a task.
  */
+void __flush_signals(struct task_struct *t)
+{
+	clear_tsk_thread_flag(t, TIF_SIGPENDING);
+	flush_sigqueue(&t->pending);
+	flush_sigqueue(&t->signal->shared_pending);
+}
+
 void flush_signals(struct task_struct *t)
 {
 	unsigned long flags;
 
 	spin_lock_irqsave(&t->sighand->siglock, flags);
-	clear_tsk_thread_flag(t, TIF_SIGPENDING);
-	flush_sigqueue(&t->pending);
-	flush_sigqueue(&t->signal->shared_pending);
+	__flush_signals(t);
 	spin_unlock_irqrestore(&t->sighand->siglock, flags);
 }
 
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index ba808ef..b3ff7fa 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2398,11 +2398,14 @@ static void selinux_bprm_committed_creds(struct linux_binprm *bprm)
 		memset(&itimer, 0, sizeof itimer);
 		for (i = 0; i < 3; i++)
 			do_setitimer(i, &itimer, NULL);
-		flush_signals(current);
 		spin_lock_irq(&current->sighand->siglock);
-		flush_signal_handlers(current, 1);
-		sigemptyset(&current->blocked);
-		recalc_sigpending();
+		if (!sigismember(&current->pending.signal, SIGKILL) &&
+		    !sigismember(&current->signal->shared_pending.signal,
+				 SIGKILL)) {
+			__flush_signals(current);
+			flush_signal_handlers(current, 1);
+			sigemptyset(&current->blocked);
+		}
 		spin_unlock_irq(&current->sighand->siglock);
 	}