Date:   Wed, 7 Nov 2018 13:10:01 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     syzbot+ded1696f6b50b615b630@...kaller.appspotmail.com
Cc:     kvm@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        pbonzini@...hat.com, rkrcmar@...hat.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: KMSAN: kernel-infoleak in kvm_vcpu_write_guest_page

On Wed, Nov 7, 2018 at 2:38 AM syzbot
<syzbot+ded1696f6b50b615b630@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    88b95ef4c780 kmsan: use MSan assembly instrumentation
> git tree:       https://github.com/google/kmsan.git/master
> console output: https://syzkaller.appspot.com/x/log.txt?x=12505e33400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8df5fc509a1b351b
> dashboard link: https://syzkaller.appspot.com/bug?extid=ded1696f6b50b615b630
> compiler:       clang version 8.0.0 (trunk 343298)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ce62f5400000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=174efca3400000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+ded1696f6b50b615b630@...kaller.appspotmail.com
>
> ==================================================================
> BUG: KMSAN: kernel-infoleak in __copy_to_user include/linux/uaccess.h:121
> [inline]
> BUG: KMSAN: kernel-infoleak in __kvm_write_guest_page
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:1849 [inline]
> BUG: KMSAN: kernel-infoleak in kvm_vcpu_write_guest_page+0x39a/0x510
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:1870
> CPU: 0 PID: 7918 Comm: syz-executor542 Not tainted 4.19.0+ #77
> 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+0x1a2/0x2e0 mm/kmsan/kmsan.c:911
>   kmsan_internal_check_memory+0x34c/0x430 mm/kmsan/kmsan.c:991
>   kmsan_copy_to_user+0x85/0xe0 mm/kmsan/kmsan_hooks.c:552
>   __copy_to_user include/linux/uaccess.h:121 [inline]
>   __kvm_write_guest_page arch/x86/kvm/../../../virt/kvm/kvm_main.c:1849
> [inline]
>   kvm_vcpu_write_guest_page+0x39a/0x510
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:1870
>   nested_release_vmcs12 arch/x86/kvm/vmx.c:8441 [inline]
>   handle_vmptrld+0x2384/0x26b0 arch/x86/kvm/vmx.c:8907
>   vmx_handle_exit+0x1e81/0xbac0 arch/x86/kvm/vmx.c:10128
>   vcpu_enter_guest arch/x86/kvm/x86.c:7667 [inline]
>   vcpu_run arch/x86/kvm/x86.c:7730 [inline]
>   kvm_arch_vcpu_ioctl_run+0xac32/0x11d80 arch/x86/kvm/x86.c:7930
>   kvm_vcpu_ioctl+0xfb1/0x1f90 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2590
>   do_vfs_ioctl+0xf77/0x2d30 fs/ioctl.c:46
>   ksys_ioctl fs/ioctl.c:702 [inline]
>   __do_sys_ioctl fs/ioctl.c:709 [inline]
>   __se_sys_ioctl+0x1da/0x270 fs/ioctl.c:707
>   __x64_sys_ioctl+0x4a/0x70 fs/ioctl.c:707
>   do_syscall_64+0xcf/0x110 arch/x86/entry/common.c:291
>   entry_SYSCALL_64_after_hwframe+0x63/0xe7
> RIP: 0033:0x44b6e9
> Code: e8 dc e6 ff ff 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 2b ff fb ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007f096b292ce8 EFLAGS: 00000206 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 00000000006e3c48 RCX: 000000000044b6e9
> RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005
> RBP: 00000000006e3c40 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000206 R12: 00000000006e3c4c
> R13: 00007ffd978aeb2f R14: 00007f096b2939c0 R15: 00000000006e3d4c
>
> Uninit was created at:
>   kmsan_save_stack_with_flags mm/kmsan/kmsan.c:252 [inline]
>   kmsan_internal_poison_shadow+0xc8/0x1e0 mm/kmsan/kmsan.c:177
>   kmsan_kmalloc+0x98/0x110 mm/kmsan/kmsan_hooks.c:104
>   __kmalloc+0x14c/0x4d0 mm/slub.c:3789
>   kmalloc include/linux/slab.h:518 [inline]
>   enter_vmx_operation+0x980/0x1a90 arch/x86/kvm/vmx.c:8278
>   vmx_set_nested_state+0xc3a/0x1530 arch/x86/kvm/vmx.c:14045
>   kvm_arch_vcpu_ioctl+0x4fc9/0x73a0 arch/x86/kvm/x86.c:4057
>   kvm_vcpu_ioctl+0xca3/0x1f90 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2742
>   do_vfs_ioctl+0xf77/0x2d30 fs/ioctl.c:46
>   ksys_ioctl fs/ioctl.c:702 [inline]
>   __do_sys_ioctl fs/ioctl.c:709 [inline]
>   __se_sys_ioctl+0x1da/0x270 fs/ioctl.c:707
>   __x64_sys_ioctl+0x4a/0x70 fs/ioctl.c:707
>   do_syscall_64+0xcf/0x110 arch/x86/entry/common.c:291
>   entry_SYSCALL_64_after_hwframe+0x63/0xe7
>
> Bytes 1000-4095 of 4096 are uninitialized
> Memory access of size 4096 starts at ffff8801b5157000
> ==================================================================
This appears to be a real bug in KVM.
Please see a simplified reproducer attached.

>
> ---
> This bug is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
>
> syzbot will keep track of this bug report. See:
> https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with
> syzbot.
> syzbot can test patches for this bug, for details see:
> https://goo.gl/tpsmEJ#testing-patches
>
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-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

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View attachment "kvm-infoleak.c" of type "text/x-csrc" (27803 bytes)

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