signal: Implement force_fatal_sig

Add a simple helper force_fatal_sig that causes a signal to be
delivered to a process as if the signal handler was set to SIG_DFL.

Reimplement force_sigsegv based upon this new helper.  This fixes
force_sigsegv so that when it forces the default signal handler
to be used the code now forces the signal to be unblocked as well.

Reusing the tested logic in force_sig_info_to_task that was built for
force_sig_seccomp this makes the implementation trivial.

This is interesting both because it makes force_sigsegv simpler and
because there are a couple of buggy places in the kernel that call
do_exit(SIGILL) or do_exit(SIGSYS) because there is no straight
forward way today for those places to simply force the exit of a
process with the chosen signal.  Creating force_fatal_sig allows
those places to be implemented with normal signal exits.

Link: https://lkml.kernel.org/r/20211020174406.17889-13-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index e5f4ce6..e2dc9f1 100644 (file)
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -338,6 +338,7 @@ extern int kill_pid(struct pid *pid, int sig, int priv);
 extern __must_check bool do_notify_parent(struct task_struct *, int);
 extern void __wake_up_parent(struct task_struct *p, struct task_struct *parent);
 extern void force_sig(int);
+extern void force_fatal_sig(int);
 extern int send_sig(int, struct task_struct *, int);
 extern int zap_other_threads(struct task_struct *p);
 extern struct sigqueue *sigqueue_alloc(void);

diff --git a/kernel/signal.c b/kernel/signal.c
index 952741f..6a5e180 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1662,6 +1662,19 @@ void force_sig(int sig)
 }
 EXPORT_SYMBOL(force_sig);
 
+void force_fatal_sig(int sig)
+{
+       struct kernel_siginfo info;
+
+       clear_siginfo(&info);
+       info.si_signo = sig;
+       info.si_errno = 0;
+       info.si_code = SI_KERNEL;
+       info.si_pid = 0;
+       info.si_uid = 0;
+       force_sig_info_to_task(&info, current, true);
+}
+
 /*
  * When things go south during signal handling, we
  * will force a SIGSEGV. And if the signal that caused
@@ -1670,15 +1683,10 @@ EXPORT_SYMBOL(force_sig);
  */
 void force_sigsegv(int sig)
 {
-       struct task_struct *p = current;
-
-       if (sig == SIGSEGV) {
-               unsigned long flags;
-               spin_lock_irqsave(&p->sighand->siglock, flags);
-               p->sighand->action[sig - 1].sa.sa_handler = SIG_DFL;
-               spin_unlock_irqrestore(&p->sighand->siglock, flags);
-       }
-       force_sig(SIGSEGV);
+       if (sig == SIGSEGV)
+               force_fatal_sig(SIGSEGV);
+       else
+               force_sig(SIGSEGV);
 }
 
 int force_sig_fault_to_task(int sig, int code, void __user *addr