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-rw-r--r--Documentation/admin-guide/sysctl/kernel.rst23
-rw-r--r--arch/x86/kernel/cpu/intel.c63
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);