Re: [PATCH 1/1] KVM/x86: Check input paging mode when cs.l is set

[Paolo Bonzini]

On 13/12/2017 05:17, Lan Tianyu wrote:
> Reported by syzkaller:
>     WARNING: CPU: 0 PID: 27962 at arch/x86/kvm/emulate.c:5631 x86_emulate_insn+0x557/0x15f0 [kvm]
>     Modules linked in: kvm_intel kvm [last unloaded: kvm]
>     CPU: 0 PID: 27962 Comm: syz-executor Tainted: G    B   W        4.15.0-rc2-next-20171208+ #32
>     Hardware name: Intel Corporation S1200SP/S1200SP, BIOS S1200SP.86B.01.03.0006.040720161253 04/07/2016
>     RIP: 0010:x86_emulate_insn+0x557/0x15f0 [kvm]
>     RSP: 0018:ffff8807234476d0 EFLAGS: 00010282
>     RAX: 0000000000000000 RBX: ffff88072d0237a0 RCX: ffffffffa0065c4d
>     RDX: 1ffff100e5a046f9 RSI: 0000000000000003 RDI: ffff88072d0237c8
>     RBP: ffff880723447728 R08: ffff88072d020000 R09: ffffffffa008d240
>     R10: 0000000000000002 R11: ffffed00e7d87db3 R12: ffff88072d0237c8
>     R13: ffff88072d023870 R14: ffff88072d0238c2 R15: ffffffffa008d080
>     FS:  00007f8a68666700(0000) GS:ffff880802200000(0000) knlGS:0000000000000000
>     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>     CR2: 000000002009506c CR3: 000000071fec4005 CR4: 00000000003626f0
>     Call Trace:
>      x86_emulate_instruction+0x3bc/0xb70 [kvm]
>      ? reexecute_instruction.part.162+0x130/0x130 [kvm]
>      vmx_handle_exit+0x46d/0x14f0 [kvm_intel]
>      ? trace_event_raw_event_kvm_entry+0xe7/0x150 [kvm]
>      ? handle_vmfunc+0x2f0/0x2f0 [kvm_intel]
>      ? wait_lapic_expire+0x25/0x270 [kvm]
>      vcpu_enter_guest+0x720/0x1ef0 [kvm]
>      ...
> 
> When cs.l is set, vcpu should run in the 64 bit paging mode.
> Current kvm set_sregs function doesn't have such check when userspace
> inputs sreg values. This will lead unexpected behavior. This patch
> is to add such check.
> 
> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Tianyu Lan <tianyu.lan@xxxxxxxxx>
> Signed-off-by: Lan Tianyu <tianyu.lan@xxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1e1e617..07d6d6c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7485,6 +7485,18 @@ int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int idt_index,
>  }
>  EXPORT_SYMBOL_GPL(kvm_task_switch);
>  
> +int kvm_valid_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
> +{
> +	/* When cs.l is set, vcpu should run in 64-bit mode */
> +	if (sregs->cs.l)
> +		if (!((sregs->cr0 & X86_CR0_PG_BIT) &&
> +		      (sregs->cr4 & X86_CR4_PAE_BIT) &&
> +		      (sregs->efer & EFER_LME)))
> +			return -EINVAL;
> +
> +	return 0;
> +}
> +
>  int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
>  				  struct kvm_sregs *sregs)
>  {
> @@ -7497,6 +7509,9 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
>  			(sregs->cr4 & X86_CR4_OSXSAVE))
>  		return -EINVAL;
>  
> +	if (kvm_valid_sregs(vcpu, sregs))
> +		return -EINVAL;
> +
>  	apic_base_msr.data = sregs->apic_base;
>  	apic_base_msr.host_initiated = true;
>  	if (kvm_set_apic_base(vcpu, &apic_base_msr))
> 

Thanks, this is the right direction.  However, there are many more
checks that are missing and can trigger more unexpected behavior.  In
particular:

- if EFER.LME=1 and CR0.PG=1, EFER.LMA and CR4.PAE must be 1

- otherwise, EFER.LMA and CS.L must be 0

Thanks,

Paolo