Re: [PATCH] KVM: VMX: Fix rflags cache during vCPU reset

[Paolo Bonzini]

On 20/11/2017 10:17, Wanpeng Li wrote:
>    ------------[ cut here ]------------
>    WARNING: CPU: 6 PID: 24431 at /home/kernel/linux/arch/x86/kvm//x86.c:7302 kvm_arch_vcpu_ioctl_run+0x651/0x2ea0 [kvm]
>    CPU: 6 PID: 24431 Comm: reprotest Tainted: G        W  OE   4.14.0+ #26
>    RIP: 0010:kvm_arch_vcpu_ioctl_run+0x651/0x2ea0 [kvm]
>    RSP: 0018:ffff880291d179e0 EFLAGS: 00010202
>    Call Trace:
>     ? synchronize_rcu.part.59+0x9b/0xf0
>     ? synchronize_rcu_expedited+0xa0/0xa0
>     ? kfree_call_rcu+0x20/0x20
>     ? trace_raw_output_rcu_utilization+0x70/0x70
>     ? kvm_arch_vcpu_runnable+0x220/0x220 [kvm]
>     ? wait_for_completion+0x222/0x2a0
>     ? __wait_rcu_gp+0x1d3/0x230
>     ? kvm_vcpu_ioctl+0x479/0x880 [kvm]
>     kvm_vcpu_ioctl+0x479/0x880 [kvm]
>     ? kvm_dev_ioctl+0xba0/0xba0 [kvm]
>     ? pick_next_task_fair+0x629/0xce0
>     ? sched_clock+0x1b/0x20
>     ? check_chain_key+0x131/0x1e0
>     ? lock_acquire+0x2c0/0x2c0
>     ? check_flags.part.44+0x62/0x250
>     ? check_chain_key+0x131/0x1e0
>     ? __lock_acquire+0xd2e/0x1cb0
>     do_vfs_ioctl+0x142/0x9a0
>     ? ioctl_preallocate+0x150/0x150
>     ? lock_acquire+0x2c0/0x2c0
>     ? lock_acquire+0x118/0x2c0
>     ? __lock_is_held+0x4b/0xe0
>     ? __fget+0x16a/0x250
>     SyS_ioctl+0x74/0x80
>     entry_SYSCALL_64_fastpath+0x23/0x9a

Please leave out the "?" lines of the backtrace, and if possible include
the C code for the testcase.

> The syzkaller testcase tries to setup the processor specific debug registers 
> and configure vCPU for handling guest debug events through KVM_SET_GUEST_DEBUG.
> The KVM_SET_GUEST_DEBUG ioctl will get and set rflags in order to set TF bit 
> if single step is needed. All regs' caches are reset to avail and GUEST_RFLAGS 
> vmcs field is reset to 0x2 during vCPU reset. However, the cache of rflags is 
> not reset during vCPU reset. The function vmx_get_rflags() returns an unreset 
> rflags cache value since the cache is marked avail, it is 0 after boot. Vmentry 
> fails if the rflags reserved bit 1 is 0.
> 
> This patch fixes it by resetting both the GUEST_RFLAGS vmcs field and its cache 
> to 0x2 during vCPU reset.
> 
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b348920..131fa1c 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5590,6 +5590,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  		vmcs_write64(GUEST_IA32_DEBUGCTL, 0);
>  	}
>  
> +	vmx->rflags = 0x02;
>  	vmcs_writel(GUEST_RFLAGS, 0x02);
>  	kvm_rip_write(vcpu, 0xfff0);
>  
> 

Maybe this should be just kvm_set_rflags(vcpu, 2); just like in svm.c's
init_vmcb?

Thanks,

Paolo