Re: [PATCH AUTOSEL for 4.14 006/110] KVM/x86: Check input paging mode when cs.l is set

[Eric Biggers]

On Sat, Feb 03, 2018 at 06:00:29PM +0000, Sasha Levin wrote:
> From: Lan Tianyu <tianyu.lan@xxxxxxxxx>
> 
> [ Upstream commit f29810335965ac1f7bcb501ee2af5f039f792416 ]
> 
> 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 checks for CS.L, EFER.LME, EFER.LMA and
> CR4.PAE when get SREG inputs from userspace in order to avoid
> unexpected behavior.
> 
> 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: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8c28023a43b1..ad0f18107c74 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7473,6 +7473,29 @@ 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)
> +{
> +	if ((sregs->efer & EFER_LME) && (sregs->cr0 & X86_CR0_PG_BIT)) {
> +		/*
> +		 * When EFER.LME and CR0.PG are set, the processor is in
> +		 * 64-bit mode (though maybe in a 32-bit code segment).
> +		 * CR4.PAE and EFER.LMA must be set.
> +		 */
> +		if (!(sregs->cr4 & X86_CR4_PAE_BIT)
> +		    || !(sregs->efer & EFER_LMA))
> +			return -EINVAL;
> +	} else {
> +		/*
> +		 * Not in 64-bit mode: EFER.LMA is clear and the code
> +		 * segment cannot be 64-bit.
> +		 */
> +		if (sregs->efer & EFER_LMA || sregs->cs.l)
> +			return -EINVAL;
> +	}
> +
> +	return 0;
> +}

This commit is broken and there was a fix for it merged: 37b95951c58 ("KVM/x86:
Fix wrong macro references of X86_CR0_PG_BIT and X86_CR4_PAE_BIT in
kvm_valid_sregs()").  Shouldn't your script have picked that up too?

- Eric