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x86/split_lock: Add sysctl to control the misery mode

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Commit b041b525da ("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: b041b525da ("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
This commit is contained in:
Guilherme G. Piccoli 2022-10-24 17:02:54 -03:00 committed by Dave Hansen
parent f0c4d9fc9c
commit 727209376f
2 changed files with 76 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
Documentation/admin-guide/sysctl/kernel.rst
View File

@ -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
=============

63
arch/x86/kernel/cpu/intel.c
View File

@ -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);