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Carlos López 0ca49fbd28 KVM: x86: hyper-v: Clamp stimer deadline to avoid livelock
Fix an issue where userspace or the guest can program an Hyper-V
synthetic timer to have a deadline in the past via integer overflow,
preventing the CPU from making progress and triggering an RCU stall.

Hyper-V's SynIC exposes 4 per-vCPU synthetic timers to the
guest, which are emulated by KVM. Each is programmed through the
HV_X64_MSR_STIMERi_CONFIG and HV_X64_MSR_STIMERi_COUNT MSRs. Depending
on CONFIG, COUNT represents either the absolute expiration time or the
period of a periodic timer, both expressed in 100ns ticks. These timers
may be set both by the guest (WRMSR) and the host (KVM_SET_MSRS).

When the timer is enabled, stimer_start() translates COUNT to an
absolute monotonic deadline and arms an hrtimer. If COUNT is set to a
value close to U64_MAX, the deadline calculation can overflow.

    ktime_add_ns(ktime_now, 100 * (stimer->exp_time - time_now))

This can result in a CPU livelock. stimer_start() arms the timer
via hrtimer_start() with a deadline in the past, which causes it to
immediately fire. The stimer callback then raises KVM_RQ_HV_STIMER, with
the intention of causing KVM to deliver a synthetic interrupt on the
next vCPU guest enter.

Then, once userspace issues KVM_RUN, vcpu_enter_guest() consumes the
request, calling kvm_hv_process_stimers(). This would normally disable
the timer via stimer_expiration() once the deadline is in the past.
However, the deadline comparison is done between the KVM reference
counter and stime->exp_time, which is a big value close to U64_MAX, so
this never happens for a few thousand years.

kvm_hv_process_timers() then re-arms the timer via stimer_start(), since
it was not disabled, which again fires immediately. Before entering
the guest, kvm_vcpu_exit_request() checks kvm_request_pending(),
which returns true due to the newly raised KVM_REQ_HV_STIMER. Then
vcpu_enter_guest() aborts the guest entry, returning early into
vcpu_run(), which loops back again into vcpu_enter_guest(), restarting
the cycle.

Since there are no manual yields in this loop, a task with SCHED_FIFO
may starve RCU grace-period kthreads, which exposes the stalls found
by syzcaller:

    rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
    rcu:    (detected by 1, t=10502 jiffies, g=14269, q=1142 ncpus=2)
    rcu: All QSes seen, last rcu_preempt kthread activity 10500 (4294965239-4294954739), jiffies_till_next_fqs=1, root ->qsmask 0x0
    rcu: rcu_preempt kthread starved for 10500 jiffies! g14269 f0x2 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0
    rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
        ( ... )
    Call Trace:
     <IRQ>
     __run_hrtimer kernel/time/hrtimer.c:1773 [inline]
     __hrtimer_run_queues+0x408/0xc30 kernel/time/hrtimer.c:1841
     hrtimer_interrupt+0x45b/0xaa0 kernel/time/hrtimer.c:1903
     local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1045 [inline]
     __sysvec_apic_timer_interrupt+0x102/0x3e0 arch/x86/kernel/apic/apic.c:1062
     instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1056 [inline]
     sysvec_apic_timer_interrupt+0xa1/0xc0 arch/x86/kernel/apic/apic.c:1056
     </IRQ>
     <TASK>
     asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
    RIP: 0010:__raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:152 [inline]
    RIP: 0010:_raw_spin_unlock_irqrestore+0xa8/0x110 kernel/locking/spinlock.c:194
    Code: 74 05 e8 0b f4 5f f6 48 c7 44 24 20 00 00 00 00 9c 8f 44 24 20 f6 44 24 21 02 75 4f f7 c3 00 02 00 00 74 01 fb bf 01 00 00 00 <e8> 23 6b 27 f6 65 8b 05 7c 60 5a 07 85 c0 74 40 48 c7 04 24 0e 36
    RSP: 0018:ffffc900040a7320 EFLAGS: 00000206
    RAX: 5de15cb931505900 RBX: 0000000000000a06 RCX: 5de15cb931505900
    RDX: 0000000000000007 RSI: ffffffff8daa9dc3 RDI: 0000000000000001
    RBP: ffffc900040a73b0 R08: ffffffff8fc3d077 R09: 1ffffffff1f87a0e
    R10: dffffc0000000000 R11: fffffbfff1f87a0f R12: dffffc0000000000
    R13: 0000000000000000 R14: ffff8880b8628240 R15: 1ffff92000814e64
     hrtimer_start include/linux/hrtimer.h:259 [inline]
     stimer_start arch/x86/kvm/hyperv.c:682 [inline]
     kvm_hv_process_stimers+0xd0a/0x16a0 arch/x86/kvm/hyperv.c:893
     vcpu_enter_guest arch/x86/kvm/x86.c:11193 [inline]
     vcpu_run+0x2240/0x76b0 arch/x86/kvm/x86.c:11639
     kvm_arch_vcpu_ioctl_run+0x1148/0x1c90 arch/x86/kvm/x86.c:11984
     kvm_vcpu_ioctl+0x99a/0xed0 virt/kvm/kvm_main.c:4492
     vfs_ioctl fs/ioctl.c:51 [inline]
     __do_sys_ioctl fs/ioctl.c:597 [inline]
     __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
     do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
     do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
     entry_SYSCALL_64_after_hwframe+0x77/0x7f
    RIP: 0033:0x7f635278f749
    Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
    RSP: 002b:00007f635365c038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
    RAX: ffffffffffffffda RBX: 00007f63529e5fa0 RCX: 00007f635278f749
    RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005
    RBP: 00007f6352813f91 R08: 0000000000000000 R09: 0000000000000000
    R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
    R13: 00007f63529e6038 R14: 00007f63529e5fa0 R15: 00007ffd5b219358
     </TASK>

Fix this by clamping the deadline computation to KTIME_MAX, which
preserves the intent of arming a timer very far in the future.
ktime_add_safe() already does this type of clamping, so use it after
checking that that multiplying by the 100ns time tick also does not
overflow.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reported-by: syzbot+3d5461510f8dc4adfe30@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3d5461510f8dc4adfe30
Fixes: 1f4b34f825 ("kvm/x86: Hyper-V SynIC timers")
Cc: stable@vger.kernel.org
Signed-off-by: Carlos López <clopez@suse.de>
Link: https://patch.msgid.link/20260714133212.3916611-3-clopez@suse.de
[sean: tag for stable]
Signed-off-by: Sean Christopherson <seanjc@google.com>
2026-07-22 13:35:21 -07:00
arch KVM: x86: hyper-v: Clamp stimer deadline to avoid livelock 2026-07-22 13:35:21 -07:00
block block-7.1-20260529 2026-05-29 10:41:07 -07:00
certs Clang build fixes for 7.1 2026-04-24 09:29:51 -07:00
crypto crypto/krb5, rxrpc: Fix lack of pre-decrypt/pre-verify length checks 2026-05-20 16:36:45 -07:00
Documentation KVM: s390: Fix S390_USER_OPEREXEC and more gmap fixes 2026-06-24 13:41:41 +02:00
drivers KVM: s390: Fix S390_USER_OPEREXEC and more gmap fixes 2026-06-24 13:41:41 +02:00
fs namespace: restrict OPEN_TREE_NAMESPACE/FSMOUNT_NAMESPACE to directories 2026-06-10 07:09:20 -07:00
include KVM: x86: Treat a vCPU as unreachable if its index is invalid 2026-07-08 13:41:13 -07:00
init memblock: updates for 7.0-rc1 2026-04-18 11:29:14 -07:00
io_uring io_uring-7.1-20260611 2026-06-12 10:49:59 -07:00
ipc ipc/shm: serialize orphan cleanup with shm_nattch updates 2026-06-03 16:25:49 -07:00
kernel dma-mapping fixes for Linux 7.1 2026-06-11 14:22:26 -07:00
lib - Fix potential debugobjects deadlock on PREEMPT_RT kernels 2026-06-13 08:23:36 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm 11 hotfixes. 9 are for MM. 8 are cc:stable and the remaining 3 2026-06-09 08:24:25 -07:00
net netfilter pull request 26-06-10 2026-06-11 12:30:00 +02:00
rust rust: i2c: fix I2cAdapter refcount double increment 2026-06-08 09:47:50 +02:00
samples soc: drivers for 7.1 2026-04-16 20:34:34 -07:00
scripts Rust fixes for v7.1 (2nd) 2026-06-06 09:44:42 -07:00
security security/keys: fix missed RCU read section on lookup 2026-05-28 11:45:41 -07:00
sound ASoC: Fixes for v7.1 2026-06-11 21:29:47 +02:00
tools KVM selftests for 7.3, early edition 2026-06-24 12:01:00 -04:00
usr kbuild: uapi: also test UAPI headers against C++ compilers 2026-03-25 13:24:42 +01:00
virt KVM: Remove kvm_debugfs_dir on kvm_init() error paths 2026-07-08 13:41:15 -07:00
.clang-format Devicetree updates for v7.0: 2026-02-11 18:27:08 -08:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add myself 2026-04-02 16:48:25 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: rust: provide an option to inline C helpers into Rust 2026-03-30 02:03:52 +02:00
.mailmap MAINTAINERS: update Baoquan He's email address 2026-06-03 16:25:50 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING
CREDITS Delete some obsolete networking code 2026-04-24 09:41:58 -07:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS KVM: s390: Fix S390_USER_OPEREXEC and more gmap fixes 2026-06-24 13:41:41 +02:00
Makefile Linux 7.1 2026-06-14 15:58:38 +01:00
README docs: add AI Coding Assistants documentation 2026-01-06 14:55:06 -07:00

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

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


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============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
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* AI Coding Assistant - LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
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Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
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* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

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--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
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* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
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---------------

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* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


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=========================

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