L1TF CPU bug present and SMT on, data leak possible. See CVE-2018-3646 and https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html for details.
==================================================================
BUG: KASAN: use-after-free in instrument_atomic_write include/linux/instrumented.h:71 [inline]
BUG: KASAN: use-after-free in paging32_cmpxchg_gpte arch/x86/kvm/mmu/paging_tmpl.h:181 [inline]
BUG: KASAN: use-after-free in paging32_update_accessed_dirty_bits arch/x86/kvm/mmu/paging_tmpl.h:287 [inline]
BUG: KASAN: use-after-free in paging32_walk_addr_generic+0xdda/0x1480 arch/x86/kvm/mmu/paging_tmpl.h:457
Write of size 4 at addr ffff888000105000 by task syz-executor832/6828

CPU: 1 PID: 6828 Comm: syz-executor832 Not tainted 5.8.0-syzkaller #0
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+0x1f0/0x31e lib/dump_stack.c:118
 print_address_description+0x66/0x620 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report+0x132/0x1d0 mm/kasan/report.c:530
 check_memory_region_inline mm/kasan/generic.c:183 [inline]
 check_memory_region+0x2b5/0x2f0 mm/kasan/generic.c:192
 instrument_atomic_write include/linux/instrumented.h:71 [inline]
 paging32_cmpxchg_gpte arch/x86/kvm/mmu/paging_tmpl.h:181 [inline]
 paging32_update_accessed_dirty_bits arch/x86/kvm/mmu/paging_tmpl.h:287 [inline]
 paging32_walk_addr_generic+0xdda/0x1480 arch/x86/kvm/mmu/paging_tmpl.h:457
 paging32_walk_addr arch/x86/kvm/mmu/paging_tmpl.h:514 [inline]
 paging32_gva_to_gpa+0x6d/0x110 arch/x86/kvm/mmu/paging_tmpl.h:958
 vcpu_mmio_gva_to_gpa arch/x86/kvm/x86.c:5758 [inline]
 emulator_read_write_onepage+0x420/0x900 arch/x86/kvm/x86.c:5874
 emulator_read_write+0x1e7/0x590 arch/x86/kvm/x86.c:5934
 emulator_write_emulated arch/x86/kvm/x86.c:5971 [inline]
 emulator_fix_hypercall+0xf6/0x120 arch/x86/kvm/x86.c:7763
 em_hypercall+0x56/0xf0 arch/x86/kvm/emulate.c:3803
 x86_emulate_insn+0xe9a/0x6890 arch/x86/kvm/emulate.c:5676
 x86_emulate_instruction+0xe98/0x2e20 arch/x86/kvm/x86.c:7017
 kvm_emulate_instruction arch/x86/kvm/x86.c:7085 [inline]
 handle_ud+0x2fb/0x320 arch/x86/kvm/x86.c:5718
 vcpu_enter_guest+0x6682/0x89f0 arch/x86/kvm/x86.c:8641
 vcpu_run+0x332/0xc00 arch/x86/kvm/x86.c:8706
 kvm_arch_vcpu_ioctl_run+0x451/0x8f0 arch/x86/kvm/x86.c:8923
 kvm_vcpu_ioctl+0x64f/0xa50 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3229
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0xfb/0x170 fs/ioctl.c:739
 do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x443639
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 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 0f 83 9b 0b fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff1eefeb88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fff1eefeb90 RCX: 0000000000443639
RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000006
RBP: 0000000000000000 R08: 0000000000000000 R09: 00000000004011b0
R10: 0000000000000012 R11: 0000000000000246 R12: 0000000000404660
R13: 00000000004046f0 R14: 0000000000000000 R15: 0000000000000000

The buggy address belongs to the page:
page:00000000d4b4eedb refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x105
flags: 0x7ffe0000000000()
raw: 007ffe0000000000 ffffea0000004148 ffffea0000004148 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888000104f00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888000104f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888000105000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff888000105080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888000105100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================