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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Mon, 29 Jun 2020 17:29:56 +0200
Message-ID: <CAAeHK+yPbp_e9_DfRP4ikvfyMcfaveTwoVUyA+2=K3g+MvW-fA@mail.gmail.com>
Subject: Re: KASAN: out-of-bounds Read in kvm_arch_hardware_setup
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     syzbot <syzbot+e0240f9c36530bda7130@syzkaller.appspotmail.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Jim Mattson <jmattson@google.com>, joro@8bytes.org,
        KVM list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        sean.j.christopherson@intel.com,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>, vkuznets@redhat.com,
        wanpengli@tencent.com, "the arch/x86 maintainers" <x86@kernel.org>
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On Mon, Jun 29, 2020 at 5:25 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> The reproducer has nothing to do with KVM:
>
>         # https://syzkaller.appspot.com/bug?id=c356395d480ca736b00443ad89cd76fd7d209013
>         # See https://goo.gl/kgGztJ for information about syzkaller reproducers.
>         #{"repeat":true,"procs":1,"sandbox":"","fault_call":-1,"close_fds":false,"segv":true}
>         r0 = openat$fb0(0xffffffffffffff9c, &(0x7f0000000180)='/dev/fb0\x00', 0x0, 0x0)
>         ioctl$FBIOPUT_VSCREENINFO(r0, 0x4601, &(0x7f0000000000)={0x0, 0x80, 0xc80, 0x0, 0x2, 0x1, 0x4, 0x0, {0x0, 0x0, 0x1}, {0x0, 0x0, 0xfffffffc}, {}, {}, 0x0, 0x40})
>
> but the stack trace does.  On the other hand, the address seems okay:
>
>         kvm_cpu_caps+0x24/0x50
>
> and there are tons of other kvm_cpu_cap_get calls that aren't causing
> KASAN to complain.  The variable is initialized from
>
>         kvm_arch_hardware_setup
>           hardware_setup (in arch/x86/kvm/vmx/vmx.c)
>             vmx_set_cpu_caps
>               kvm_set_cpu_caps
>
> with a simple memcpy that writes the entire array.  Does anyone understand
> what is going on here?

Most likely a bug in /dev/fb handlers caused a memory corruption in
kvm related memory.

>
> Paolo
>
> On 27/06/20 22:01, syzbot wrote:
> > BUG: KASAN: out-of-bounds in kvm_cpu_cap_get arch/x86/kvm/cpuid.h:292 [inline]
> > BUG: KASAN: out-of-bounds in kvm_cpu_cap_has arch/x86/kvm/cpuid.h:297 [inline]
> > BUG: KASAN: out-of-bounds in kvm_init_msr_list arch/x86/kvm/x86.c:5362 [inline]
> > BUG: KASAN: out-of-bounds in kvm_arch_hardware_setup+0xb05/0xf40 arch/x86/kvm/x86.c:9802
> > Read of size 4 at addr ffffffff896c3134 by task syz-executor614/6786
> >
> > CPU: 1 PID: 6786 Comm: syz-executor614 Not tainted 5.7.0-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+0x1e9/0x30e lib/dump_stack.c:118
> >  print_address_description+0x66/0x5a0 mm/kasan/report.c:383
> >  __kasan_report mm/kasan/report.c:513 [inline]
> >  kasan_report+0x132/0x1d0 mm/kasan/report.c:530
> >  kvm_cpu_cap_get arch/x86/kvm/cpuid.h:292 [inline]
> >  kvm_cpu_cap_has arch/x86/kvm/cpuid.h:297 [inline]
> >  kvm_init_msr_list arch/x86/kvm/x86.c:5362 [inline]
> >  kvm_arch_hardware_setup+0xb05/0xf40 arch/x86/kvm/x86.c:9802
> >  </IRQ>
> >
> > The buggy address belongs to the variable:
> >  kvm_cpu_caps+0x24/0x50
> >
> > Memory state around the buggy address:
> >  ffffffff896c3000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >  ffffffff896c3080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> ffffffff896c3100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >                                         ^
> >  ffffffff896c3180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >  ffffffff896c3200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > ==================================================================
>
> --
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