[PATCH v4] kernel: add panic_on_taint

[Rafael Aquini]

Analogously to the introduction of panic_on_warn, this patch
introduces a kernel option named panic_on_taint in order to
provide a simple and generic way to stop execution and catch
a coredump when the kernel gets tainted by any given taint flag.

This is useful for debugging sessions as it avoids rebuilding
the kernel to explicitly add calls to panic() or BUG() into
code sites that introduce the taint flags of interest.
For instance, if one is interested in following up with
a post mortem analysis at the point a code path is hitting
a bad page (i.e. unaccount_page_cache_page(), or slab_bug()),
a crashdump could be collected by rebooting the kernel with
'panic_on_taint=0x20' amended to the command line string.

Another, perhaps less frequent, use for this option would be
as a mean for assuring a security policy case where only a
subset of taints, or no single taint (in paranoid mode),
is allowed for the running system.
The optional switch 'nousertaint' is handy in this particular
scenario as it will avoid userspace induced crashes by writes
to /proc/sys/kernel/tainted causing false positive hits for
such policies.

Suggested-by: Qian Cai <cai@xxxxxx>
Signed-off-by: Rafael Aquini <aquini@xxxxxxxxxx>
---
Changelog:
* v2: get rid of unnecessary/misguided compiler hints		(Luis)
      enhance documentation text for the new kernel parameter	(Randy)
* v3: drop sysctl interface, keep it only as a kernel parameter (Luis)
* v4: change panic_on_taint input from alphabetical taint flags
      to hexadecimal bitmasks, for clarity and extendability	(Luis)

 Documentation/admin-guide/kdump/kdump.rst     |  7 ++++
 .../admin-guide/kernel-parameters.txt         | 13 +++++++
 include/linux/kernel.h                        |  4 +++
 kernel/panic.c                                | 34 +++++++++++++++++++
 kernel/sysctl.c                               | 11 +++++-
 5 files changed, 68 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index ac7e131d2935..2707de840fd3 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -521,6 +521,13 @@ will cause a kdump to occur at the panic() call.  In cases where a user wants
 to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
 to achieve the same behaviour.
 
+Trigger Kdump on add_taint()
+============================
+
+The kernel parameter panic_on_taint facilitates calling panic() from within
+add_taint() whenever the value set in this bitmask matches with the bit flag
+being set by add_taint(). This will cause a kdump to occur at the panic() call.
+
 Contact
 =======
 
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 7bc83f3d9bdf..ce17fdbec7d1 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3401,6 +3401,19 @@
 			bit 4: print ftrace buffer
 			bit 5: print all printk messages in buffer
 
+	panic_on_taint=	Bitmask for conditionally call panic() in add_taint()
+			Format: <hex>[,nousertaint]
+			Hexadecimal bitmask representing the set of TAINT flags
+			that will cause the kernel to panic when add_taint() is
+			called with any of the flags in this set.
+			The optional switch "nousertaint" can be utilized to
+			prevent userland forced crashes by writing to sysctl
+			/proc/sys/kernel/tainted any flagset matching with the
+			bitmask set on panic_on_taint.
+			See Documentation/admin-guide/tainted-kernels.rst for
+			extra details on the taint flags that users can pick
+			to compose the bitmask to assign to panic_on_taint.
+
 	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 9b7a8d74a9d6..70712944dffc 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -528,6 +528,8 @@ extern int panic_on_oops;
 extern int panic_on_unrecovered_nmi;
 extern int panic_on_io_nmi;
 extern int panic_on_warn;
+extern unsigned long panic_on_taint;
+extern bool panic_on_taint_nousertaint;
 extern int sysctl_panic_on_rcu_stall;
 extern int sysctl_panic_on_stackoverflow;
 
@@ -597,6 +599,8 @@ extern enum system_states {
 #define TAINT_RANDSTRUCT		17
 #define TAINT_FLAGS_COUNT		18
 
+#define TAINT_FLAGS_MAX			((1UL << TAINT_FLAGS_COUNT) - 1)
+
 struct taint_flag {
 	char c_true;	/* character printed when tainted */
 	char c_false;	/* character printed when not tainted */
diff --git a/kernel/panic.c b/kernel/panic.c
index b69ee9e76cb2..94b5c973770c 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -44,6 +44,8 @@ static int pause_on_oops_flag;
 static DEFINE_SPINLOCK(pause_on_oops_lock);
 bool crash_kexec_post_notifiers;
 int panic_on_warn __read_mostly;
+unsigned long panic_on_taint;
+bool panic_on_taint_nousertaint = false;
 
 int panic_timeout = CONFIG_PANIC_TIMEOUT;
 EXPORT_SYMBOL_GPL(panic_timeout);
@@ -434,6 +436,11 @@ void add_taint(unsigned flag, enum lockdep_ok lockdep_ok)
 		pr_warn("Disabling lock debugging due to kernel taint\n");
 
 	set_bit(flag, &tainted_mask);
+
+	if (tainted_mask & panic_on_taint) {
+		panic_on_taint = 0;
+		panic("panic_on_taint set ...");
+	}
 }
 EXPORT_SYMBOL(add_taint);
 
@@ -686,3 +693,30 @@ static int __init oops_setup(char *s)
 	return 0;
 }
 early_param("oops", oops_setup);
+
+static int __init panic_on_taint_setup(char *s)
+{
+	char *taint_str;
+
+	if (!s)
+		return -EINVAL;
+
+	taint_str = strsep(&s, ",");
+	if (kstrtoul(taint_str, 16, &panic_on_taint))
+		return -EINVAL;
+
+	/* make sure panic_on_taint doesn't hold out-of-range TAINT flags */
+	panic_on_taint &= TAINT_FLAGS_MAX;
+
+	if (!panic_on_taint)
+		return -EINVAL;
+
+	if (s && !strcmp(s, "nousertaint"))
+		panic_on_taint_nousertaint = true;
+
+	pr_info("panic_on_taint: bitmask=0x%lx nousertaint_mode=%sabled\n",
+		panic_on_taint, panic_on_taint_nousertaint ? "en" : "dis");
+
+	return 0;
+}
+early_param("panic_on_taint", panic_on_taint_setup);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 8a176d8727a3..e257c965683a 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2623,11 +2623,20 @@ static int proc_taint(struct ctl_table *table, int write,
 		return err;
 
 	if (write) {
+		int i;
+
+		/*
+		 * If we are relying on panic_on_taint not producing
+		 * false positives due to userland input, bail out
+		 * before setting the requested taint flags.
+		 */
+		if (panic_on_taint_nousertaint && (tmptaint & panic_on_taint))
+			return -EINVAL;
+
 		/*
 		 * Poor man's atomic or. Not worth adding a primitive
 		 * to everyone's atomic.h for this
 		 */
-		int i;
 		for (i = 0; i < BITS_PER_LONG && tmptaint >> i; i++) {
 			if ((tmptaint >> i) & 1)
 				add_taint(i, LOCKDEP_STILL_OK);
-- 
2.25.4