[PATCH V2] x86/split_lock: Add sysctl to control the misery mode

Guilherme G. Piccoli
2022-10-14T18:05:52Z

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 is understandable that a malicious application could try a 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@intel.com/

[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: Andre Almeida <andrealmeid@igalia.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Joshua Ashton <joshua@froggi.es>
Cc: Melissa Wen <mwen@igalia.com>
Cc: Paul Gofman <pgofman@codeweavers.com>
Cc: Pavel Machek <pavel@denx.de>
Cc: Pierre-Loup Griffais <pgriffais@valvesoftware.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Zebediah Figura <zfigura@codeweavers.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---

V2:
- Switched to sysctl approach following Thomas' suggestion (thanks!).

Andre tested the patch and will comment in this thread - seems everything is
working as expected and we can enable/disable that, affecting the misery mode
as one expects.

I've tried to keep the semaphore's up()/down() calls in-sync/paired, hence
my approach of two delayed tasks, with and without misery.

Reviews / comments are greatly appreciated.
Thanks,

Guilherme

Documentation/admin-guide/sysctl/kernel.rst | 18 ++++++
arch/x86/kernel/cpu/intel.c | 61 +++++++++++++++++----
2 files changed, 69 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index ee6572b1edad..508952e42914 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1298,6 +1298,24 @@ 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)
+=============
+
+For x86 CPUs supporting the split lock detection mechanism, this parameter
+allows the users to turn off what is called "the misery mode", which
+introduces intentional delay in userspace applications that split locks.
+The goal of the misery mode is to prevent using such unaligned access to
+DoS the system dropping the performance overall, but some of these split
+locking programs are legacy and/or proprietary software that cannot be fixed,
+so using this sysctl is a way to allow them to run with a decent performance.
+
+= ===================================================================
+0 Disables the misery mode - just warns the split lock on kernel log.
+1 Enables the misery 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..2aacf9d6c723 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;
@@ -1146,11 +1170,18 @@ 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_sem(struct work_struct *work)
{
sld_update_msr(true);
up(&buslock_sem);
}
+static DECLARE_DELAYED_WORK(split_lock_reenable_sem, __split_lock_reenable_sem);
+
+static void __split_lock_reenable(struct work_struct *work)
+{
+ sld_update_msr(true);
+}
+static DECLARE_DELAYED_WORK(split_lock_reenable, __split_lock_reenable);

/*
* If a CPU goes offline with pending delayed work to re-enable split lock
@@ -1169,10 +1200,9 @@ 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 *wk;
int cpu;

if (!current->reported_split_lock)
@@ -1180,14 +1210,25 @@ 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.
+ */
+ wk = &split_lock_reenable_sem;
+ if (down_interruptible(&buslock_sem) == -EINTR)
+ return;
+ } else
+ wk = &split_lock_reenable;
+
cpu = get_cpu();
- schedule_delayed_work_on(cpu, &split_lock_reenable, 2);
+ schedule_delayed_work_on(cpu, wk, 2);

/* Disable split lock detection on this CPU to make progress */
sld_update_msr(false);
--
2.38.0