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Subject: Re: KASAN: out-of-bounds Read in kvm_arch_hardware_setup
To:     syzbot <syzbot+e0240f9c36530bda7130@syzkaller.appspotmail.com>,
        bp@alien8.de, hpa@zytor.com, jmattson@google.com, joro@8bytes.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        mingo@redhat.com, sean.j.christopherson@intel.com,
        syzkaller-bugs@googlegroups.com, tglx@linutronix.de,
        vkuznets@redhat.com, wanpengli@tencent.com, x86@kernel.org
References: <000000000000a0784a05a916495e@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <04786ba2-4934-c544-63d1-4d5d36dc5822@redhat.com>
Date:   Mon, 29 Jun 2020 17:25:28 +0200
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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?

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
> ==================================================================