Merge branch 'SA_IMMUTABLE-fixes-for-v5.16-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace

Pull exit-vs-signal handling fixes from Eric Biederman:
 "This is a small set of changes where debuggers were no longer able to
  intercept synchronous SIGTRAP and SIGSEGV, introduced by the exit
  cleanups.

  This is essentially the change you suggested with all of i's dotted
  and the t's crossed so that ptrace can intercept all of the cases it
  has been able to intercept the past, and all of the cases that made it
  to exit without giving ptrace a chance still don't give ptrace a
  chance"

* 'SA_IMMUTABLE-fixes-for-v5.16-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
  signal: Replace force_fatal_sig with force_exit_sig when in doubt
  signal: Don't always set SA_IMMUTABLE for forced signals

2 files changed, 31 insertions, 9 deletions

diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
index 4508201847d2..0b6379adff6b 100644
--- a/kernel/entry/syscall_user_dispatch.c
+++ b/kernel/entry/syscall_user_dispatch.c
@@ -48,7 +48,7 @@ bool syscall_user_dispatch(struct pt_regs *regs)
 		 * the selector is loaded by userspace.
 		 */
 		if (unlikely(__get_user(state, sd->selector))) {
-			force_fatal_sig(SIGSEGV);
+			force_exit_sig(SIGSEGV);
 			return true;
 		}
 
@@ -56,7 +56,7 @@ bool syscall_user_dispatch(struct pt_regs *regs)
 			return false;
 
 		if (state != SYSCALL_DISPATCH_FILTER_BLOCK) {
-			force_fatal_sig(SIGSYS);
+			force_exit_sig(SIGSYS);
 			return true;
 		}
 	}
diff --git a/kernel/signal.c b/kernel/signal.c
index 7c4b7ae714d4..a629b11bf3e0 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1298,6 +1298,12 @@ int do_send_sig_info(int sig, struct kernel_siginfo *info, struct task_struct *p
 	return ret;
 }
 
+enum sig_handler {
+	HANDLER_CURRENT, /* If reachable use the current handler */
+	HANDLER_SIG_DFL, /* Always use SIG_DFL handler semantics */
+	HANDLER_EXIT,	 /* Only visible as the process exit code */
+};
+
 /*
  * Force a signal that the process can't ignore: if necessary
  * we unblock the signal and change any SIG_IGN to SIG_DFL.
@@ -1310,7 +1316,8 @@ int do_send_sig_info(int sig, struct kernel_siginfo *info, struct task_struct *p
  * that is why we also clear SIGNAL_UNKILLABLE.
  */
 static int
-force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool sigdfl)
+force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
+	enum sig_handler handler)
 {
 	unsigned long int flags;
 	int ret, blocked, ignored;
@@ -1321,9 +1328,10 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
 	action = &t->sighand->action[sig-1];
 	ignored = action->sa.sa_handler == SIG_IGN;
 	blocked = sigismember(&t->blocked, sig);
-	if (blocked || ignored || sigdfl) {
+	if (blocked || ignored || (handler != HANDLER_CURRENT)) {
 		action->sa.sa_handler = SIG_DFL;
-		action->sa.sa_flags |= SA_IMMUTABLE;
+		if (handler == HANDLER_EXIT)
+			action->sa.sa_flags |= SA_IMMUTABLE;
 		if (blocked) {
 			sigdelset(&t->blocked, sig);
 			recalc_sigpending_and_wake(t);
@@ -1343,7 +1351,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
 
 int force_sig_info(struct kernel_siginfo *info)
 {
-	return force_sig_info_to_task(info, current, false);
+	return force_sig_info_to_task(info, current, HANDLER_CURRENT);
 }
 
 /*
@@ -1660,7 +1668,20 @@ void force_fatal_sig(int sig)
 	info.si_code = SI_KERNEL;
 	info.si_pid = 0;
 	info.si_uid = 0;
-	force_sig_info_to_task(&info, current, true);
+	force_sig_info_to_task(&info, current, HANDLER_SIG_DFL);
+}
+
+void force_exit_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, HANDLER_EXIT);
 }
 
 /*
@@ -1693,7 +1714,7 @@ int force_sig_fault_to_task(int sig, int code, void __user *addr
 	info.si_flags = flags;
 	info.si_isr = isr;
 #endif
-	return force_sig_info_to_task(&info, t, false);
+	return force_sig_info_to_task(&info, t, HANDLER_CURRENT);
 }
 
 int force_sig_fault(int sig, int code, void __user *addr
@@ -1813,7 +1834,8 @@ int force_sig_seccomp(int syscall, int reason, bool force_coredump)
 	info.si_errno = reason;
 	info.si_arch = syscall_get_arch(current);
 	info.si_syscall = syscall;
-	return force_sig_info_to_task(&info, current, force_coredump);
+	return force_sig_info_to_task(&info, current,
+		force_coredump ? HANDLER_EXIT : HANDLER_CURRENT);
 }
 
 /* For the crazy architectures that include trap information in