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Sean Christopherson f8730d6335 KVM: Always flush async #PF workqueue when vCPU is being destroyed 
[ Upstream commit 3d75b8aa5c ]

Always flush the per-vCPU async #PF workqueue when a vCPU is clearing its
completion queue, e.g. when a VM and all its vCPUs is being destroyed.
KVM must ensure that none of its workqueue callbacks is running when the
last reference to the KVM _module_ is put.  Gifting a reference to the
associated VM prevents the workqueue callback from dereferencing freed
vCPU/VM memory, but does not prevent the KVM module from being unloaded
before the callback completes.

Drop the misguided VM refcount gifting, as calling kvm_put_kvm() from
async_pf_execute() if kvm_put_kvm() flushes the async #PF workqueue will
result in deadlock.  async_pf_execute() can't return until kvm_put_kvm()
finishes, and kvm_put_kvm() can't return until async_pf_execute() finishes:

 WARNING: CPU: 8 PID: 251 at virt/kvm/kvm_main.c:1435 kvm_put_kvm+0x2d/0x320 [kvm]
 Modules linked in: vhost_net vhost vhost_iotlb tap kvm_intel kvm irqbypass
 CPU: 8 PID: 251 Comm: kworker/8:1 Tainted: G        W          6.6.0-rc1-e7af8d17224a-x86/gmem-vm #119
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
 Workqueue: events async_pf_execute [kvm]
 RIP: 0010:kvm_put_kvm+0x2d/0x320 [kvm]
 Call Trace:
  <TASK>
  async_pf_execute+0x198/0x260 [kvm]
  process_one_work+0x145/0x2d0
  worker_thread+0x27e/0x3a0
  kthread+0xba/0xe0
  ret_from_fork+0x2d/0x50
  ret_from_fork_asm+0x11/0x20
  </TASK>
 ---[ end trace 0000000000000000 ]---
 INFO: task kworker/8:1:251 blocked for more than 120 seconds.
       Tainted: G        W          6.6.0-rc1-e7af8d17224a-x86/gmem-vm #119
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 task:kworker/8:1     state:D stack:0     pid:251   ppid:2      flags:0x00004000
 Workqueue: events async_pf_execute [kvm]
 Call Trace:
  <TASK>
  __schedule+0x33f/0xa40
  schedule+0x53/0xc0
  schedule_timeout+0x12a/0x140
  __wait_for_common+0x8d/0x1d0
  __flush_work.isra.0+0x19f/0x2c0
  kvm_clear_async_pf_completion_queue+0x129/0x190 [kvm]
  kvm_arch_destroy_vm+0x78/0x1b0 [kvm]
  kvm_put_kvm+0x1c1/0x320 [kvm]
  async_pf_execute+0x198/0x260 [kvm]
  process_one_work+0x145/0x2d0
  worker_thread+0x27e/0x3a0
  kthread+0xba/0xe0
  ret_from_fork+0x2d/0x50
  ret_from_fork_asm+0x11/0x20
  </TASK>

If kvm_clear_async_pf_completion_queue() actually flushes the workqueue,
then there's no need to gift async_pf_execute() a reference because all
invocations of async_pf_execute() will be forced to complete before the
vCPU and its VM are destroyed/freed.  And that in turn fixes the module
unloading bug as __fput() won't do module_put() on the last vCPU reference
until the vCPU has been freed, e.g. if closing the vCPU file also puts the
last reference to the KVM module.

Note that kvm_check_async_pf_completion() may also take the work item off
the completion queue and so also needs to flush the work queue, as the
work will not be seen by kvm_clear_async_pf_completion_queue().  Waiting
on the workqueue could theoretically delay a vCPU due to waiting for the
work to complete, but that's a very, very small chance, and likely a very
small delay.  kvm_arch_async_page_present_queued() unconditionally makes a
new request, i.e. will effectively delay entering the guest, so the
remaining work is really just:

        trace_kvm_async_pf_completed(addr, cr2_or_gpa);

        __kvm_vcpu_wake_up(vcpu);

        mmput(mm);

and mmput() can't drop the last reference to the page tables if the vCPU is
still alive, i.e. the vCPU won't get stuck tearing down page tables.

Add a helper to do the flushing, specifically to deal with "wakeup all"
work items, as they aren't actually work items, i.e. are never placed in a
workqueue.  Trying to flush a bogus workqueue entry rightly makes
__flush_work() complain (kudos to whoever added that sanity check).

Note, commit 5f6de5cbeb ("KVM: Prevent module exit until all VMs are
freed") *tried* to fix the module refcounting issue by having VMs grab a
reference to the module, but that only made the bug slightly harder to hit
as it gave async_pf_execute() a bit more time to complete before the KVM
module could be unloaded.

Fixes: af585b921e ("KVM: Halt vcpu if page it tries to access is swapped out")
Cc: stable@vger.kernel.org
Cc: David Matlack <dmatlack@google.com>
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240110011533.503302-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
 		2024-04-13 12:58:04 +02:00
Documentation x86/cpu: Support AMD Automatic IBRS 2024-04-13 12:57:56 +02:00
LICENSES LICENSES/deprecated: add Zlib license text 2020-09-16 14:33:49 +02:00
arch arm: dts: marvell: Fix maxium->maxim typo in brownstone dts 2024-04-13 12:58:00 +02:00
block block: add a new set_read_only method 2024-03-26 18:21:48 -04:00
certs certs/blacklist_hashes.c: fix const confusion in certs blacklist 2022-06-22 14:13:17 +02:00
crypto crypto: api - Disallow identical driver names 2024-02-23 08:41:52 +01:00
drivers media: xc4000: Fix atomicity violation in xc4000_get_frequency 2024-04-13 12:58:03 +02:00
fs afs: Revert "afs: Hide silly-rename files from userspace" 2024-03-26 18:22:01 -04:00
include timers: Rename del_timer_sync() to timer_delete_sync() 2024-04-13 12:57:58 +02:00
init rootfs: Fix support for rootfstype= when root= is given 2024-01-25 14:37:52 -08:00
io_uring io_uring: don't save/restore iowait state 2024-03-26 18:22:02 -04:00
ipc ipc/sem: Fix dangling sem_array access in semtimedop race 2022-12-08 11:24:00 +01:00
kernel timers: Rename del_timer_sync() to timer_delete_sync() 2024-04-13 12:57:58 +02:00
lib net: blackhole_dev: fix build warning for ethh set but not used 2024-03-26 18:21:50 -04:00
mm mm/hugetlb: change hugetlb_reserve_pages() to type bool 2024-03-15 10:48:22 -04:00
net netfilter: nf_tables: do not compare internal table flags on updates 2024-03-26 18:22:04 -04:00
samples samples/hw_breakpoint: fix building without module unloading 2023-09-23 11:01:09 +02:00
scripts kconfig: fix infinite loop when expanding a macro at the end of file 2024-03-26 18:22:02 -04:00
security smack: Handle SMACK64TRANSMUTE in smack_inode_setsecurity() 2024-04-13 12:58:00 +02:00
sound ALSA: usb-audio: Stop parsing channels bits when all channels are found. 2024-03-26 18:22:00 -04:00
tools perf stat: Avoid metric-only segv 2024-03-26 18:21:57 -04:00
usr usr/include/Makefile: add linux/nfc.h to the compile-test coverage 2022-02-01 17:25:48 +01:00
virt KVM: Always flush async #PF workqueue when vCPU is being destroyed 2024-04-13 12:58:04 +02:00
.clang-format RDMA 5.10 pull request 2020-10-17 11:18:18 -07:00
.cocciconfig
…
.get_maintainer.ignore
…
.gitattributes
…
.gitignore kbuild: generate Module.symvers only when vmlinux exists 2021-05-19 10:12:59 +02:00
.mailmap mailmap: add two more addresses of Uwe Kleine-König 2020-12-06 10:19:07 -08:00
COPYING
…
CREDITS MAINTAINERS: Move Jason Cooper to CREDITS 2020-11-30 10:20:34 +01:00
Kbuild
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Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Remove DECnet support from kernel 2023-06-21 15:45:38 +02:00
Makefile Linux 5.10.214 2024-03-26 18:23:07 -04:00
README
…

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.