linux-kernel - [PATCH 2/2] KVM: X86: Fix load RFLAGS w/o the fixed bit

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Date:   Mon,  4 Dec 2017 22:21:31 -0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     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: [PATCH 2/2] KVM: X86: Fix load RFLAGS w/o the fixed bit

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 catching userspace set RFLAGS w/o the fixes 
bit and bailing out immediately.

Suggested-by: Jim Mattson <jmattson@...gle.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>
---
 virt/kvm/kvm_main.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c01cff0..7100833 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2602,6 +2602,11 @@ static long kvm_vcpu_ioctl(struct file *filp,
 			r = PTR_ERR(kvm_regs);
 			goto out;
 		}
+		r = -EINVAL;
+		if (!(kvm_regs->rflags & X86_EFLAGS_FIXED)) {
+			kfree(kvm_regs);
+			goto out;
+		}
 		r = kvm_arch_vcpu_ioctl_set_regs(vcpu, kvm_regs);
 		kfree(kvm_regs);
 		break;
-- 
2.7.4