From: Guilherme G. Piccoli gpiccoli@igalia.com
[ Upstream commit 727209376f4998bc84db1d5d8af15afea846a92b ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- 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 ee6572b1edad..66d1b23ca64f 100644 --- a/Documentation/admin-guide/sysctl/kernel.rst +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -1298,6 +1298,29 @@ 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);