linux-kernel - Re: [PATCH v3] KVM: X86: Fix load RFLAGS w/o the fixed bit

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Date:   Thu, 7 Dec 2017 17:24:28 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v3] KVM: X86: Fix load RFLAGS w/o the fixed bit



On 2017/12/07 16:30, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
>
>   *** Guest State ***
>   CR0: actual=0x0000000000000030, shadow=0x0000000060000010, gh_mask=fffffffffffffff7
>   CR4: actual=0x0000000000002050, shadow=0x0000000000000000, gh_mask=ffffffffffffe871
>   CR3 = 0x00000000fffbc000
>   RSP = 0x0000000000000000  RIP = 0x0000000000000000
>   RFLAGS=0x00000000         DR7 = 0x0000000000000400
>          ^^^^^^^^^^
>
> The failed vmentry is triggered by the following testcase when ept=Y:
>
>      #include <unistd.h>
>      #include <sys/syscall.h>
>      #include <string.h>
>      #include <stdint.h>
>      #include <linux/kvm.h>
>      #include <fcntl.h>
>      #include <sys/ioctl.h>
>      
>      long r[5];
>      int main()
>      {
>      	r[2] = open("/dev/kvm", O_RDONLY);
>      	r[3] = ioctl(r[2], KVM_CREATE_VM, 0);
>      	r[4] = ioctl(r[3], KVM_CREATE_VCPU, 7);
>      	struct kvm_regs regs = {
>      		.rflags = 0,
>      	};
>      	ioctl(r[4], KVM_SET_REGS, &regs);
>      	ioctl(r[4], KVM_RUN, 0);
>      }
>
> X86 RFLAGS bit 1 is fixed set, userspace can simply clearing bit 1
> of RFLAGS with KVM_SET_REGS ioctl which results in vmentry fails.
> This patch fixes it by oring X86_EFLAGS_FIXED during ioctl.
>
> Suggested-by: Jim Mattson <jmattson@...gle.com>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Jim Mattson <jmattson@...gle.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
   Reviewed-by: Quan Xu <quan.xu0@...il.com>