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Maksim Davydov 8dfff85d5d x86/split_lock: Fix the delayed detection logic 
commit c929d08df8 upstream.

If the warning mode with disabled mitigation mode is used, then on each
CPU where the split lock occurred detection will be disabled in order to
make progress and delayed work will be scheduled, which then will enable
detection back.

Now it turns out that all CPUs use one global delayed work structure.
This leads to the fact that if a split lock occurs on several CPUs
at the same time (within 2 jiffies), only one CPU will schedule delayed
work, but the rest will not.

The return value of schedule_delayed_work_on() would have shown this,
but it is not checked in the code.

A diagram that can help to understand the bug reproduction:

 - sld_update_msr() enables/disables SLD on both CPUs on the same core

 - schedule_delayed_work_on() internally checks WORK_STRUCT_PENDING_BIT.
   If a work has the 'pending' status, then schedule_delayed_work_on()
   will return an error code and, most importantly, the work will not
   be placed in the workqueue.

Let's say we have a multicore system on which split_lock_mitigate=0 and
a multithreaded application is running that calls splitlock in multiple
threads. Due to the fact that sld_update_msr() affects the entire core
(both CPUs), we will consider 2 CPUs from different cores. Let the 2
threads of this application schedule to CPU0 (core 0) and to CPU 2
(core 1), then:

|                                 ||                                   |
|             CPU 0 (core 0)      ||          CPU 2 (core 1)           |
|_________________________________||___________________________________|
|                                 ||                                   |
| 1) SPLIT LOCK occured           ||                                   |
|                                 ||                                   |
| 2) split_lock_warn()            ||                                   |
|                                 ||                                   |
| 3) sysctl_sld_mitigate == 0     ||                                   |
|    (work = &sl_reenable)        ||                                   |
|                                 ||                                   |
| 4) schedule_delayed_work_on()   ||                                   |
|    (reenable will be called     ||                                   |
|     after 2 jiffies on CPU 0)   ||                                   |
|                                 ||                                   |
| 5) disable SLD for core 0       ||                                   |
|                                 ||                                   |
|    -------------------------    ||                                   |
|                                 ||                                   |
|                                 || 6) SPLIT LOCK occured             |
|                                 ||                                   |
|                                 || 7) split_lock_warn()              |
|                                 ||                                   |
|                                 || 8) sysctl_sld_mitigate == 0       |
|                                 ||    (work = &sl_reenable,          |
|                                 ||     the same address as in 3) )   |
|                                 ||                                   |
|            2 jiffies            || 9) schedule_delayed_work_on()     |
|                                 ||    fials because the work is in   |
|                                 ||    the pending state since 4).    |
|                                 ||    The work wasn't placed to the  |
|                                 ||    workqueue. reenable won't be   |
|                                 ||    called on CPU 2                |
|                                 ||                                   |
|                                 || 10) disable SLD for core 0        |
|                                 ||                                   |
|                                 ||     From now on SLD will          |
|                                 ||     never be reenabled on core 1  |
|                                 ||                                   |
|    -------------------------    ||                                   |
|                                 ||                                   |
|    11) enable SLD for core 0 by ||                                   |
|        __split_lock_reenable    ||                                   |
|                                 ||                                   |

If the application threads can be scheduled to all processor cores,
then over time there will be only one core left, on which SLD will be
enabled and split lock will be able to be detected; and on all other
cores SLD will be disabled all the time.

Most likely, this bug has not been noticed for so long because
sysctl_sld_mitigate default value is 1, and in this case a semaphore
is used that does not allow 2 different cores to have SLD disabled at
the same time, that is, strictly only one work is placed in the
workqueue.

In order to fix the warning mode with disabled mitigation mode,
delayed work has to be per-CPU. Implement it.

Fixes: 727209376f ("x86/split_lock: Add sysctl to control the misery mode")
Signed-off-by: Maksim Davydov <davydov-max@yandex-team.ru>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20250115131704.132609-1-davydov-max@yandex-team.ru
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2025-04-25 10:44:01 +02:00
arch x86/split_lock: Fix the delayed detection logic 2025-04-25 10:44:01 +02:00
block block, bfq: fix re-introduced UAF in bic_set_bfqq() 2025-03-28 21:59:02 +01:00
certs certs: Fix build error when PKCS#11 URI contains semicolon 2023-02-09 11:28:11 +01:00
crypto crypto: api - Add crypto_clone_tfm 2024-12-14 19:53:51 +01:00
Documentation sched/isolation: Prevent boot crash when the boot CPU is nohz_full 2025-03-28 21:58:48 +01:00
drivers misc: pci_endpoint_test: Fix 'irq_type' to convey the correct type 2025-04-25 10:44:01 +02:00
fs virtiofs: add filesystem context source name check 2025-04-25 10:43:55 +02:00
include nfs: add missing selections of CONFIG_CRC32 2025-04-25 10:43:52 +02:00
init rust: Disallow BTF generation with Rust + LTO 2025-03-28 21:58:57 +01:00
io_uring io_uring/net: fix accept multishot handling 2025-04-25 10:43:58 +02:00
ipc ipc: fix memleak if msg_init_ns failed in create_ipc_ns 2024-12-14 19:54:06 +01:00
kernel tracing: Fix filter string testing 2025-04-25 10:43:55 +02:00
lib string: Add load_unaligned_zeropad() code path to sized_strscpy() 2025-04-25 10:43:55 +02:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm: Fix is_zero_page() usage in try_grab_page() 2025-04-25 10:44:01 +02:00
net mptcp: sockopt: fix getting freebind & transparent 2025-04-25 10:44:00 +02:00
rust scripts: generate_rust_analyzer: provide cfgs for core and alloc 2025-03-28 21:58:57 +01:00
samples samples/landlock: Fix possible NULL dereference in parse_path() 2025-02-21 13:49:03 +01:00
scripts selinux: Chain up tool resolving errors in install_policy.sh 2025-04-10 14:33:30 +02:00
security smack: dont compile ipv6 code unless ipv6 is configured 2025-04-10 14:33:30 +02:00
sound ASoC: codecs:lpass-wsa-macro: Fix logic of enabling vi channels 2025-04-25 10:43:52 +02:00
tools test suite: use %zu to print size_t 2025-04-25 10:43:50 +02:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt KVM: Fix a data race on last_boosted_vcpu in kvm_vcpu_on_spin() 2024-06-27 13:46:21 +02:00
.clang-format inet: ping: use hlist_nulls rcu iterator during lookup 2022-12-01 12:42:46 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Remove *.orig pattern from .gitignore 2024-10-17 15:21:15 +02:00
.mailmap 9 hotfixes. 6 for MM, 3 for other areas. Four of these patches address 2022-12-10 17:10:52 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove Michal Marek from Kbuild maintainers 2022-11-16 14:53:00 +09:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: add leah to 6.1 MAINTAINERS file 2024-05-17 11:56:16 +02:00
Makefile kbuild: Add '-fno-builtin-wcslen' 2025-04-25 10:44:00 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.