[PATCH 1/2] signal: Don't always set SA_IMMUTABLE for forced signals

[Eric W. Biederman]

Recently to prevent issues with SECCOMP_RET_KILL and similar signals
being changed before they are delivered SA_IMMUTABLE was added.

Unfortunately this broke debuggers[1][2] which reasonably expect to be
able to trap synchronous SIGTRAP and SIGSEGV even when the target
process is not configured to handle those signals.

Update force_sig_to_task to support both the case when we can
allow the debugger to intercept and possibly ignore the
signal and the case when it is not safe to let userspace
known about the signal until the process has exited.

Reported-by: Kyle Huey <me@xxxxxxxxxxxx>
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
[1] https://lkml.kernel.org/r/CAP045AoMY4xf8aC_4QU_-j7obuEPYgTcnQQP3Yxk=2X90jtpjw@xxxxxxxxxxxxxx
[2] https://lkml.kernel.org/r/20211117150258.GB5403@xsang-OptiPlex-902
Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 kernel/signal.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 7c4b7ae714d4..02058c983bd6 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 proces 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 +1668,7 @@ 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);
 }
 
 /*
@@ -1693,7 +1701,7 @@ 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 +1821,8 @@ 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
-- 
2.20.1