linux-kernel - [PATCH 3.16 112/254] KVM: X86: Fix load RFLAGS w/o the fixed bit

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Message-ID: <lsq.1519831218.690452039@decadent.org.uk>
Date:   Wed, 28 Feb 2018 15:20:18 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Radim Krčmář" <rkrcmar@...hat.com>,
        "David Hildenbrand" <david@...hat.com>,
        "Wanpeng Li" <wanpeng.li@...mail.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Quan Xu" <quan.xu0@...il.com>, "Jim Mattson" <jmattson@...gle.com>
Subject: [PATCH 3.16 112/254] KVM: X86: Fix load RFLAGS w/o the fixed bit

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Wanpeng Li <wanpeng.li@...mail.com>

commit d73235d17ba63b53dc0e1051dbc10a1f1be91b71 upstream.

 *** 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>
Reviewed-by: Quan Xu <quan.xu0@...il.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>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6471,7 +6471,7 @@ int kvm_arch_vcpu_ioctl_set_regs(struct
 #endif
 
 	kvm_rip_write(vcpu, regs->rip);
-	kvm_set_rflags(vcpu, regs->rflags);
+	kvm_set_rflags(vcpu, regs->rflags | X86_EFLAGS_FIXED);
 
 	vcpu->arch.exception.pending = false;