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SubjectKASAN: use-after-free Read in finish_task_switch

syzbot found the following crash on:

HEAD commit: 8c8399e0a3fb Add linux-next specific files for 20180806
git tree: linux-next
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 8.0.1 20180413 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:

BUG: KASAN: use-after-free in __fire_sched_in_preempt_notifiers
kernel/sched/core.c:2481 [inline]
BUG: KASAN: use-after-free in fire_sched_in_preempt_notifiers
kernel/sched/core.c:2487 [inline]
BUG: KASAN: use-after-free in finish_task_switch+0x544/0x870
Read of size 8 at addr ffff880193b21d18 by task syz-executor3/4885

CPU: 0 PID: 4885 Comm: syz-executor3 Not tainted 4.18.0-rc8-next-20180806+
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
print_address_description+0x6c/0x20b mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report.cold.7+0x242/0x30d mm/kasan/report.c:412
__asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
__fire_sched_in_preempt_notifiers kernel/sched/core.c:2481 [inline]
fire_sched_in_preempt_notifiers kernel/sched/core.c:2487 [inline]
finish_task_switch+0x544/0x870 kernel/sched/core.c:2679
context_switch kernel/sched/core.c:2826 [inline]
__schedule+0x884/0x1ec0 kernel/sched/core.c:3471
preempt_schedule_common+0x22/0x60 kernel/sched/core.c:3595
_cond_resched+0x1d/0x30 kernel/sched/core.c:4961
zap_pmd_range mm/memory.c:1449 [inline]
zap_pud_range mm/memory.c:1476 [inline]
zap_p4d_range mm/memory.c:1497 [inline]
unmap_page_range+0x11d2/0x2100 mm/memory.c:1518
unmap_single_vma+0x1a0/0x310 mm/memory.c:1563
unmap_vmas+0x125/0x200 mm/memory.c:1593
exit_mmap+0x2c2/0x590 mm/mmap.c:3093
__mmput kernel/fork.c:1003 [inline]
mmput+0x265/0x620 kernel/fork.c:1024
exit_mm kernel/exit.c:544 [inline]
do_exit+0xec6/0x2760 kernel/exit.c:856
do_group_exit+0x177/0x440 kernel/exit.c:972
get_signal+0x88e/0x1970 kernel/signal.c:2467
do_signal+0x9c/0x21c0 arch/x86/kernel/signal.c:816
exit_to_usermode_loop+0x2e5/0x380 arch/x86/entry/common.c:162
prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293
RIP: 0033:0x456cb9
Code: fd b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 0f 83 cb b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f24f212ecf8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 00000000009300a8 RCX: 0000000000456cb9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000009300a8
RBP: 00000000009300a0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000009300ac
R13: 00007ffe23b48b7f R14: 00007f24f212f9c0 R15: 0000000000000000

Allocated by task 4885:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:553
kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:490
kmem_cache_alloc+0x12e/0x760 mm/slab.c:3554
kmem_cache_zalloc include/linux/slab.h:697 [inline]
vmx_create_vcpu+0xcf/0x2980 arch/x86/kvm/vmx.c:10656
kvm_arch_vcpu_create+0xe5/0x220 arch/x86/kvm/x86.c:8398
kvm_vm_ioctl_create_vcpu arch/x86/kvm/../../../virt/kvm/kvm_main.c:2476
kvm_vm_ioctl+0x488/0x1d80 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2977
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:501 [inline]
do_vfs_ioctl+0x1de/0x1720 fs/ioctl.c:685
ksys_ioctl+0xa9/0xd0 fs/ioctl.c:702
__do_sys_ioctl fs/ioctl.c:709 [inline]
__se_sys_ioctl fs/ioctl.c:707 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:707
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290

Freed by task 4884:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
__kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:521
kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
__cache_free mm/slab.c:3498 [inline]
kmem_cache_free+0x86/0x2d0 mm/slab.c:3756
vmx_free_vcpu+0x26b/0x300 arch/x86/kvm/vmx.c:10650
kvm_arch_vcpu_free arch/x86/kvm/x86.c:8384 [inline]
kvm_free_vcpus arch/x86/kvm/x86.c:8833 [inline]
kvm_arch_destroy_vm+0x365/0x7c0 arch/x86/kvm/x86.c:8930
kvm_destroy_vm arch/x86/kvm/../../../virt/kvm/kvm_main.c:752 [inline]
kvm_put_kvm+0x73f/0x1060 arch/x86/kvm/../../../virt/kvm/kvm_main.c:773
kvm_vcpu_release+0x7b/0xa0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2407
__fput+0x376/0x8a0 fs/file_table.c:279
____fput+0x15/0x20 fs/file_table.c:312
task_work_run+0x1e8/0x2a0 kernel/task_work.c:113
tracehook_notify_resume include/linux/tracehook.h:193 [inline]
exit_to_usermode_loop+0x318/0x380 arch/x86/entry/common.c:166
prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293

The buggy address belongs to the object at ffff880193b21d00
which belongs to the cache kvm_vcpu of size 23808
The buggy address is located 24 bytes inside of
23808-byte region [ffff880193b21d00, ffff880193b27a00)
The buggy address belongs to the page:
page:ffffea00064ec800 count:1 mapcount:0 mapping:ffff8801d4c35dc0 index:0x0
compound_mapcount: 0
flags: 0x2fffc0000008100(slab|head)
raw: 02fffc0000008100 ffffea000657de08 ffff8801d4c29648 ffff8801d4c35dc0
raw: 0000000000000000 ffff880193b21d00 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff880193b21c00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff880193b21c80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff880193b21d00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff880193b21d80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff880193b21e00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

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