KMSAN: kernel-infoleak in kvm_arch_vcpu_ioctl

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    006aa39cddee kmsan: don't instrument fixup_bad_iret()
git tree:       https://github.com/google/kmsan.git/master
console output: https://syzkaller.appspot.com/x/log.txt?x=101dcb0b400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f388ea1732f3c473
dashboard link: https://syzkaller.appspot.com/bug?extid=cfbc368e283d381f8cef
compiler:       clang version 8.0.0 (trunk 343298)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10c56fbd400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=153c8a47400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+cfbc368e283d381f8cef@xxxxxxxxxxxxxxxxxxxxxxxxx

IPVS: ftp: loaded support on port[0] = 21
L1TF CPU bug present and SMT on, data leak possible. See CVE-2018-3646 and  
https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html for details.
==================================================================
BUG: KMSAN: kernel-infoleak in _copy_to_user+0x19a/0x230 lib/usercopy.c:31
CPU: 0 PID: 6697 Comm: syz-executor853 Not tainted 4.20.0-rc2+ #85
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+0x32d/0x480 lib/dump_stack.c:113
 kmsan_report+0x19f/0x300 mm/kmsan/kmsan.c:911
 kmsan_internal_check_memory+0x35b/0x3b0 mm/kmsan/kmsan.c:993
 kmsan_copy_to_user+0x7c/0xe0 mm/kmsan/kmsan_hooks.c:552
 _copy_to_user+0x19a/0x230 lib/usercopy.c:31
 copy_to_user include/linux/uaccess.h:183 [inline]
 kvm_vcpu_ioctl_enable_cap arch/x86/kvm/x86.c:3834 [inline]
 kvm_arch_vcpu_ioctl+0x5dee/0x7680 arch/x86/kvm/x86.c:4132
 kvm_vcpu_ioctl+0xca3/0x1f90 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2748
 do_vfs_ioctl+0xfbc/0x2f70 fs/ioctl.c:46
 ksys_ioctl fs/ioctl.c:713 [inline]
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl+0x1da/0x270 fs/ioctl.c:718
 __x64_sys_ioctl+0x4a/0x70 fs/ioctl.c:718
 do_syscall_64+0xcf/0x110 arch/x86/entry/common.c:291
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
RIP: 0033:0x4471b9
Code: e8 fc b9 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 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 3b 07 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f1e22946da8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000006f0038 RCX: 00000000004471b9
RDX: 0000000020000000 RSI: 000000004068aea3 RDI: 0000000000000005
RBP: 00000000006f0030 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006f003c
R13: 6d766b2f7665642f R14: 00007f1e229479c0 R15: 00000000000003e8

Local variable description: ----__pu_val@kvm_arch_vcpu_ioctl
Variable was created at:
 kvm_arch_vcpu_ioctl+0x29d/0x7680 arch/x86/kvm/x86.c:3848
 kvm_vcpu_ioctl+0xca3/0x1f90 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2748

Bytes 0-1 of 2 are uninitialized
Memory access of size 2 starts at ffff8881967ffbb0
Data copied to user address 0000000000706000
==================================================================


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