x86/split_lock: Add sysctl to control the misery mode

Commit b041b525dab9 ("x86/split_lock: Make life miserable for split lockers")
changed the way the split lock detector works when in "warn" mode;
basically, it not only 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.

This slowdown is enough to totally break some proprietary (aka.
unfixable) userspace[1].

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 slows down 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].

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 observing 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/

[ dhansen: minor changelog tweaks, including clarifying the actual
  	   problem ]

Fixes: b041b525dab9 ("x86/split_lock: Make life miserable for split lockers")
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Tested-by: Andre Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/all/20221024200254.635256-1-gpiccoli%40igalia.com

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 98d1b198b2b4c1040172761c88c7ee956c1edb12..c2c64c1b706ff6be4859cf1be24bbb6cc0364a22 100644 (file)
--- 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 2d7ea5480ec3394d7422b7c06df98d759ed8ad84..42789965048339786387c0cb4691d62bedbc5b4d 100644 (file)
--- 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);