Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid

[Jim Mattson]

It looks like userspace can possibly induce this by providing guest
CPUID information with a "physical address width" of 64 in leaf
0x80000008.

Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of
CPUID.80000008H:EAX? Better would be to return an error for
out-of-range values, but I understand that the kvm community's stance
is that, in general, guest CPUID information should not be validated
by kvm.

On Tue, Dec 22, 2020 at 12:36 AM syzbot
<syzbot+e87846c48bf72bc85311@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    5e60366d Merge tag 'fallthrough-fixes-clang-5.11-rc1' of g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11c7046b500000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=db720fe37a6a41d8
> dashboard link: https://syzkaller.appspot.com/bug?extid=e87846c48bf72bc85311
> compiler:       gcc (GCC) 10.1.0-syz 20200507
> userspace arch: i386
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+e87846c48bf72bc85311@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ================================================================================
> UBSAN: shift-out-of-bounds in arch/x86/kvm/mmu.h:52:16
> shift exponent 64 is too large for 64-bit type 'long long unsigned int'
> CPU: 1 PID: 11156 Comm: syz-executor.1 Not tainted 5.10.0-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:79 [inline]
>  dump_stack+0x107/0x163 lib/dump_stack.c:120
>  ubsan_epilogue+0xb/0x5a lib/ubsan.c:148
>  __ubsan_handle_shift_out_of_bounds.cold+0xb1/0x181 lib/ubsan.c:395
>  rsvd_bits arch/x86/kvm/mmu.h:52 [inline]
>  kvm_vcpu_after_set_cpuid.cold+0x35/0x3a arch/x86/kvm/cpuid.c:181
>  kvm_vcpu_ioctl_set_cpuid+0x28e/0x970 arch/x86/kvm/cpuid.c:273
>  kvm_arch_vcpu_ioctl+0x1091/0x2d70 arch/x86/kvm/x86.c:4699
>  kvm_vcpu_ioctl+0x7b9/0xdb0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3386
>  kvm_vcpu_compat_ioctl+0x1a2/0x340 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3430
>  __do_compat_sys_ioctl+0x1d3/0x230 fs/ioctl.c:842
>  do_syscall_32_irqs_on arch/x86/entry/common.c:78 [inline]
>  __do_fast_syscall_32+0x56/0x80 arch/x86/entry/common.c:137
>  do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:160
>  entry_SYSENTER_compat_after_hwframe+0x4d/0x5c
> RIP: 0023:0xf7fe8549
> Code: b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 00 00 00 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
> RSP: 002b:00000000f55e20cc EFLAGS: 00000296 ORIG_RAX: 0000000000000036
> RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 000000004008ae8a
> RDX: 00000000200000c0 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> ================================================================================
>
>
> ---
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