[PATCH] do_exit(): panic() when double fault detected

[Vladimir Kondratiev]

Double fault detected in do_exit() is symptom of integrity
compromised. For safety critical systems, it may be better to
panic() in this case to minimize risk.

Signed-off-by: Vladimir Kondratiev <vladimir.kondratiev@xxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt | 5 +++++
 include/linux/kernel.h                          | 1 +
 kernel/exit.c                                   | 7 +++++++
 kernel/sysctl.c                                 | 9 +++++++++
 4 files changed, 22 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 44fde25bb221..6cb2a63c47f3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3521,6 +3521,11 @@
 			extra details on the taint flags that users can pick
 			to compose the bitmask to assign to panic_on_taint.
 
+	panic_on_double_fault
+			panic() when double fault detected in do_exit().
+			Useful on safety critical systems; double fault is
+			a symptom of kernel integrity compromised.
+
 	panic_on_warn	panic() instead of WARN().  Useful to cause kdump
 			on a WARN().
 
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 2f05e9128201..0d8822259a36 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -539,6 +539,7 @@ extern int sysctl_panic_on_rcu_stall;
 extern int sysctl_panic_on_stackoverflow;
 
 extern bool crash_kexec_post_notifiers;
+extern int panic_on_double_fault;
 
 /*
  * panic_cpu is used for synchronizing panic() and crash_kexec() execution. It
diff --git a/kernel/exit.c b/kernel/exit.c
index 1f236ed375f8..e67ae43644f9 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -68,6 +68,9 @@
 #include <asm/unistd.h>
 #include <asm/mmu_context.h>
 
+int panic_on_double_fault __read_mostly;
+core_param(panic_on_double_fault, panic_on_double_fault, int, 0644);
+
 static void __unhash_process(struct task_struct *p, bool group_dead)
 {
 	nr_threads--;
@@ -757,6 +760,10 @@ void __noreturn do_exit(long code)
 	 */
 	if (unlikely(tsk->flags & PF_EXITING)) {
 		pr_alert("Fixing recursive fault but reboot is needed!\n");
+		if (panic_on_double_fault)
+			panic("Double fault detected in %s[%d]\n",
+			      current->comm, task_pid_nr(current));
+
 		futex_exit_recursive(tsk);
 		set_current_state(TASK_UNINTERRUPTIBLE);
 		schedule();
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index afad085960b8..869a2ca41e8e 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2600,6 +2600,15 @@ static struct ctl_table kern_table[] = {
 		.extra2		= &one_thousand,
 	},
 #endif
+	{
+		.procname	= "panic_on_double_fault",
+		.data		= &panic_on_double_fault,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
 	{
 		.procname	= "panic_on_warn",
 		.data		= &panic_on_warn,
-- 
2.27.0

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