[PATCH v2 2/2] sysctl: introduce kernel.pkill_on_warn

[Alexander Popov]

Currently, the Linux kernel provides two types of reaction to kernel
warnings:
 1. Do nothing (by default),
 2. Call panic() if panic_on_warn is set. That's a very strong reaction,
    so panic_on_warn is usually disabled on production systems.

>From a safety point of view, the Linux kernel misses a middle way of
handling kernel warnings:
 - The kernel should stop the activity that provokes a warning,
 - But the kernel should avoid complete denial of service.

>From a security point of view, kernel warning messages provide a lot of
useful information for attackers. Many GNU/Linux distributions allow
unprivileged users to read the kernel log, so attackers use kernel
warning infoleak in vulnerability exploits. See the examples:
https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html
https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
https://googleprojectzero.blogspot.com/2018/09/a-cache-invalidation-bug-in-linux.html

Let's introduce the pkill_on_warn sysctl.
If this parameter is set, the kernel kills all threads in a process that
provoked a kernel warning. This behavior is reasonable from a safety point of
view described above. It is also useful for kernel security hardening because
the system kills an exploit process that hits a kernel warning.

Moreover, bugs usually don't come alone, and a kernel warning may be
followed by memory corruption or other bad effects. So pkill_on_warn allows
the kernel to stop the process when the first signs of wrong behavior
are detected.

Signed-off-by: Alexander Popov <alex.popov@xxxxxxxxx>
---
 Documentation/admin-guide/sysctl/kernel.rst | 14 +++++++++++++
 include/asm-generic/bug.h                   | 12 ++++++++---
 include/linux/panic.h                       |  3 +++
 kernel/panic.c                              | 22 ++++++++++++++++++++-
 kernel/sysctl.c                             |  9 +++++++++
 lib/bug.c                                   |  3 +++
 6 files changed, 59 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 426162009ce9..5faf395fdf8f 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -921,6 +921,20 @@ lives in) pid namespace. When selecting a pid for a next task on fork
 kernel tries to allocate a number starting from this one.
 
 
+pkill_on_warn
+=============
+
+Kills all threads in a process that provoked a kernel warning.
+That allows the kernel to stop the process when the first signs
+of wrong behavior are detected.
+
+= =====================================================================
+0 Allows a process to proceed execution after hitting a kernel warning,
+  this is the default behavior.
+1 Kills all threads in a process that provoked a kernel warning.
+= =====================================================================
+
+
 powersave-nap (PPC only)
 ========================
 
diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 881aeaf5a2d5..959000b5856a 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -94,8 +94,10 @@ void warn_slowpath_fmt(const char *file, const int line, unsigned taint,
 #ifndef WARN_ON_ONCE
 #define WARN_ON_ONCE(condition) ({					\
 	int __ret_warn_on = !!(condition);				\
-	if (unlikely(__ret_warn_on))					\
+	if (unlikely(__ret_warn_on)) {					\
 		DO_ONCE_LITE(__WARN_printf, TAINT_WARN, NULL);		\
+		do_pkill_on_warn();					\
+	}								\
 	unlikely(__ret_warn_on);					\
 })
 #endif
@@ -151,15 +153,19 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 
 #define WARN_ONCE(condition, format...) ({				\
 	int __ret_warn_on = !!(condition);				\
-	if (unlikely(__ret_warn_on))					\
+	if (unlikely(__ret_warn_on)) {					\
 		DO_ONCE_LITE(__WARN_printf, TAINT_WARN, format);	\
+		do_pkill_on_warn();					\
+	}								\
 	unlikely(__ret_warn_on);					\
 })
 
 #define WARN_TAINT_ONCE(condition, taint, format...) ({			\
 	int __ret_warn_on = !!(condition);				\
-	if (unlikely(__ret_warn_on))					\
+	if (unlikely(__ret_warn_on)) {					\
 		DO_ONCE_LITE(__WARN_printf, taint, format);		\
+		do_pkill_on_warn();					\
+	}								\
 	unlikely(__ret_warn_on);					\
 })
 
diff --git a/include/linux/panic.h b/include/linux/panic.h
index f5844908a089..f79c69279859 100644
--- a/include/linux/panic.h
+++ b/include/linux/panic.h
@@ -27,6 +27,9 @@ extern int panic_on_oops;
 extern int panic_on_unrecovered_nmi;
 extern int panic_on_io_nmi;
 extern int panic_on_warn;
+extern int pkill_on_warn;
+
+extern void do_pkill_on_warn(void);
 
 extern unsigned long panic_on_taint;
 extern bool panic_on_taint_nousertaint;
diff --git a/kernel/panic.c b/kernel/panic.c
index cefd7d82366f..1323c9e2630f 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -53,6 +53,7 @@ static int pause_on_oops_flag;
 static DEFINE_SPINLOCK(pause_on_oops_lock);
 bool crash_kexec_post_notifiers;
 int panic_on_warn __read_mostly;
+int pkill_on_warn __read_mostly;
 unsigned long panic_on_taint;
 bool panic_on_taint_nousertaint = false;
 
@@ -625,13 +626,16 @@ void warn_slowpath_fmt(const char *file, int line, unsigned taint,
 	if (!fmt) {
 		__warn(file, line, __builtin_return_address(0), taint,
 		       NULL, NULL);
-		return;
+		goto out;
 	}
 
 	args.fmt = fmt;
 	va_start(args.args, fmt);
 	__warn(file, line, __builtin_return_address(0), taint, NULL, &args);
 	va_end(args.args);
+
+out:
+	do_pkill_on_warn();
 }
 EXPORT_SYMBOL(warn_slowpath_fmt);
 #else
@@ -732,3 +736,19 @@ static int __init panic_on_taint_setup(char *s)
 	return 0;
 }
 early_param("panic_on_taint", panic_on_taint_setup);
+
+void do_pkill_on_warn(void)
+{
+	if (!pkill_on_warn)
+		return;
+
+	if (is_global_init(current))
+		return;
+
+	if (current->flags & PF_KTHREAD)
+		return;
+
+	if (system_state >= SYSTEM_RUNNING)
+		do_send_sig_info(SIGKILL, SEND_SIG_PRIV, current, PIDTYPE_TGID);
+}
+EXPORT_SYMBOL(do_pkill_on_warn);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 083be6af29d7..7fe6f0aaad2b 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2656,6 +2656,15 @@ static struct ctl_table kern_table[] = {
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_ONE,
 	},
+		{
+		.procname	= "pkill_on_warn",
+		.data		= &pkill_on_warn,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
 #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
 	{
 		.procname	= "timer_migration",
diff --git a/lib/bug.c b/lib/bug.c
index 1a91f01412b8..28cc8a5b2ee0 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -214,6 +214,9 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
 	bug_type = BUG_TRAP_TYPE_BUG;
 
 out:
+	if (bug_type == BUG_TRAP_TYPE_WARN)
+		do_pkill_on_warn();
+
 	return bug_type;
 }
 
-- 
2.31.1