KASAN: null-ptr-deref Read in kvm_vfio_set_attr

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    770fbb32 Add linux-next specific files for 20200228
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=16974813e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=576314276bce4ad5
dashboard link: https://syzkaller.appspot.com/bug?extid=c8f52ab16178cebd5f5c
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14fa5e75e00000

Bisection is inconclusive: the bug happens on the oldest tested release.

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=14eeae75e00000
final crash:    https://syzkaller.appspot.com/x/report.txt?x=16eeae75e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=12eeae75e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+c8f52ab16178cebd5f5c@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: null-ptr-deref in kvm_vfio_set_group arch/x86/kvm/../../../virt/kvm/vfio.c:235 [inline]
BUG: KASAN: null-ptr-deref in kvm_vfio_set_attr+0x7da/0xa30 arch/x86/kvm/../../../virt/kvm/vfio.c:337
Read of size 8 at addr 0000000000000000 by task syz-executor.2/9641

CPU: 0 PID: 9641 Comm: syz-executor.2 Not tainted 5.6.0-rc3-next-20200228-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 __kasan_report.cold+0x5/0x32 mm/kasan/report.c:510
 kasan_report+0xe/0x20 mm/kasan/common.c:618
 kvm_vfio_set_group arch/x86/kvm/../../../virt/kvm/vfio.c:235 [inline]
 kvm_vfio_set_attr+0x7da/0xa30 arch/x86/kvm/../../../virt/kvm/vfio.c:337
 </IRQ>
==================================================================
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 9641 Comm: syz-executor.2 Tainted: G    B             5.6.0-rc3-next-20200228-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 panic+0x2e3/0x75c kernel/panic.c:221
 end_report+0x43/0x49 mm/kasan/report.c:96
 __kasan_report.cold+0xd/0x32 mm/kasan/report.c:513
 kasan_report+0xe/0x20 mm/kasan/common.c:618
 kvm_vfio_set_group arch/x86/kvm/../../../virt/kvm/vfio.c:235 [inline]
 kvm_vfio_set_attr+0x7da/0xa30 arch/x86/kvm/../../../virt/kvm/vfio.c:337
 </IRQ>
Shutting down cpus with NMI
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches