[PATCH V3] x86/split_lock: Add sysctl to control the misery mode

[Guilherme G. Piccoli]

Commit b041b525dab9 ("x86/split_lock: Make life miserable for split lockers")
changed the way the split lock detector works when in "warn" mode;
basically, not only it shows the warn message, but also intentionally
introduces a slowdown (through sleeping plus serialization mechanism)
on such task. Based on discussions in [0], seems the warning alone
wasn't enough motivation for userspace developers to fix their
applications.

Happens that originally the proposal in [0] was to add a new mode
which would warns + slowdown the "split locking" task, keeping the
old warn mode untouched. In the end, that idea was discarded and
the regular/default "warn" mode now slowdowns the applications. This
is quite aggressive with regards proprietary/legacy programs that
basically are unable to properly run in kernel with this change.
While it is understandable that a malicious application could DoS
by split locking, it seems unacceptable to regress old/proprietary
userspace programs through a default configuration that previously
worked. An example of such breakage was reported in [1].

So let's add a sysctl to allow controlling the "misery mode" behavior,
as per Thomas suggestion on [2]. This way, users running legacy and/or
proprietary software are allowed to still execute them with a decent
performance while still observe the warning messages on kernel log.

[0] https://lore.kernel.org/lkml/20220217012721.9694-1-tony.luck@xxxxxxxxx/

[1] https://github.com/doitsujin/dxvk/issues/2938

[2] https://lore.kernel.org/lkml/87pmf4bter.ffs@tglx/

Fixes: b041b525dab9 ("x86/split_lock: Make life miserable for split lockers")
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Joshua Ashton <joshua@xxxxxxxxx>
Cc: Melissa Wen <mwen@xxxxxxxxxx>
Cc: Paul Gofman <pgofman@xxxxxxxxxxxxxxx>
Cc: Pavel Machek <pavel@xxxxxxx>
Cc: Pierre-Loup Griffais <pgriffais@xxxxxxxxxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Zebediah Figura <zfigura@xxxxxxxxxxxxxxx>
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Andre Almeida <andrealmeid@xxxxxxxxxx>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>
---


V3:
- Reworked with the suggestions from Bagas and Dave - thanks!
- Rebased to v6.1-rc2.

Dave, in the end I kept the 2 delayed workers, seems easier than
encompass the worker in a new struct and have a bool embedded there
(we can't use data directly in the work struct, since it keeps the
workqueue state). Lemme know if you (or others) really prefer the
other approach and I can rework if required.

Thanks again for comments/reviews!
Cheers,

Guilherme


 Documentation/admin-guide/sysctl/kernel.rst | 23 ++++++++
 arch/x86/kernel/cpu/intel.c                 | 63 +++++++++++++++++----
 2 files changed, 76 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 98d1b198b2b4..c2c64c1b706f 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1314,6 +1314,29 @@ watchdog work to be queued by the watchdog timer function, otherwise the NMI
 watchdog — if enabled — can detect a hard lockup condition.
 
 
+split_lock_mitigate (x86 only)
+==============================
+
+On x86, each "split lock" imposes a system-wide performance penalty. On larger
+systems, large numbers of split locks from unprivileged users can result in
+denials of service to well-behaved and potentially more important users.
+
+The kernel mitigates these bad users by detecting split locks and imposing
+penalties: forcing them to wait and only allowing one core to execute split
+locks at a time.
+
+These mitigations can make those bad applications unbearably slow. Setting
+split_lock_mitigate=0 may restore some application performance, but will also
+increase system exposure to denial of service attacks from split lock users.
+
+= ===================================================================
+0 Disable the mitigation mode - just warns the split lock on kernel log
+  and exposes the system to denials of service from the split lockers.
+1 Enable the mitigation mode (this is the default) - penalizes the split
+  lockers with intentional performance degradation.
+= ===================================================================
+
+
 stack_erasing
 =============
 
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 2d7ea5480ec3..427899650483 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1034,8 +1034,32 @@ static const struct {
 
 static struct ratelimit_state bld_ratelimit;
 
+static unsigned int sysctl_sld_mitigate = 1;
 static DEFINE_SEMAPHORE(buslock_sem);
 
+#ifdef CONFIG_PROC_SYSCTL
+static struct ctl_table sld_sysctls[] = {
+	{
+		.procname       = "split_lock_mitigate",
+		.data           = &sysctl_sld_mitigate,
+		.maxlen         = sizeof(unsigned int),
+		.mode           = 0644,
+		.proc_handler	= proc_douintvec_minmax,
+		.extra1         = SYSCTL_ZERO,
+		.extra2         = SYSCTL_ONE,
+	},
+	{}
+};
+
+static int __init sld_mitigate_sysctl_init(void)
+{
+	register_sysctl_init("kernel", sld_sysctls);
+	return 0;
+}
+
+late_initcall(sld_mitigate_sysctl_init);
+#endif
+
 static inline bool match_option(const char *arg, int arglen, const char *opt)
 {
 	int len = strlen(opt), ratelimit;
@@ -1146,12 +1170,20 @@ static void split_lock_init(void)
 		split_lock_verify_msr(sld_state != sld_off);
 }
 
-static void __split_lock_reenable(struct work_struct *work)
+static void __split_lock_reenable_unlock(struct work_struct *work)
 {
 	sld_update_msr(true);
 	up(&buslock_sem);
 }
 
+static DECLARE_DELAYED_WORK(sl_reenable_unlock, __split_lock_reenable_unlock);
+
+static void __split_lock_reenable(struct work_struct *work)
+{
+	sld_update_msr(true);
+}
+static DECLARE_DELAYED_WORK(sl_reenable, __split_lock_reenable);
+
 /*
  * If a CPU goes offline with pending delayed work to re-enable split lock
  * detection then the delayed work will be executed on some other CPU. That
@@ -1169,10 +1201,9 @@ static int splitlock_cpu_offline(unsigned int cpu)
 	return 0;
 }
 
-static DECLARE_DELAYED_WORK(split_lock_reenable, __split_lock_reenable);
-
 static void split_lock_warn(unsigned long ip)
 {
+	struct delayed_work *work;
 	int cpu;
 
 	if (!current->reported_split_lock)
@@ -1180,14 +1211,26 @@ static void split_lock_warn(unsigned long ip)
 				    current->comm, current->pid, ip);
 	current->reported_split_lock = 1;
 
-	/* misery factor #1, sleep 10ms before trying to execute split lock */
-	if (msleep_interruptible(10) > 0)
-		return;
-	/* Misery factor #2, only allow one buslocked disabled core at a time */
-	if (down_interruptible(&buslock_sem) == -EINTR)
-		return;
+	if (sysctl_sld_mitigate) {
+		/*
+		 * misery factor #1:
+		 * sleep 10ms before trying to execute split lock.
+		 */
+		if (msleep_interruptible(10) > 0)
+			return;
+		/*
+		 * Misery factor #2:
+		 * only allow one buslocked disabled core at a time.
+		 */
+		if (down_interruptible(&buslock_sem) == -EINTR)
+			return;
+		work = &sl_reenable_unlock;
+	} else {
+		work = &sl_reenable;
+	}
+
 	cpu = get_cpu();
-	schedule_delayed_work_on(cpu, &split_lock_reenable, 2);
+	schedule_delayed_work_on(cpu, work, 2);
 
 	/* Disable split lock detection on this CPU to make progress */
 	sld_update_msr(false);
-- 
2.38.0