[PATCH 1/2] KVM: MMU: Fix infinite loop when there is no available mmu page

[PATCH 1/2] KVM: MMU: Fix infinite loop when there is no available mmu page

[Wanpeng Li]

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

The below test case can cause infinite loop in kvm when ept=0.

    #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);
    	ioctl(r[4], KVM_RUN, 0);
    }

It doesn't setup the memory regions, mmu_alloc_shadow/direct_roots() in 
kvm return 1 when kvm fails to allocate root page table which can result 
in beblow infinite loop:

    vcpu_run() {
    	for (;;) {
	    	r = vcpu_enter_guest()::kvm_mmu_reload() returns 1 
	    	if (r <= 0)
	    		break;
	    	if (need_resched())
	    		cond_resched();
      }
    }

This patch fixes it by returning -ENOSPC when there is no available kvm mmu 
page for root page table.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Fixes: 26eeb53cf0f (KVM: MMU: Bail out immediately if there is no available mmu page)
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 arch/x86/kvm/mmu.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index c9aaa18..89da688 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3395,7 +3395,7 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
 		spin_lock(&vcpu->kvm->mmu_lock);
 		if(make_mmu_pages_available(vcpu) < 0) {
 			spin_unlock(&vcpu->kvm->mmu_lock);
-			return 1;
+			return -ENOSPC;
 		}
 		sp = kvm_mmu_get_page(vcpu, 0, 0,
 				vcpu->arch.mmu.shadow_root_level, 1, ACC_ALL);
@@ -3410,7 +3410,7 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
 			spin_lock(&vcpu->kvm->mmu_lock);
 			if (make_mmu_pages_available(vcpu) < 0) {
 				spin_unlock(&vcpu->kvm->mmu_lock);
-				return 1;
+				return -ENOSPC;
 			}
 			sp = kvm_mmu_get_page(vcpu, i << (30 - PAGE_SHIFT),
 					i << 30, PT32_ROOT_LEVEL, 1, ACC_ALL);
@@ -3450,7 +3450,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
 		spin_lock(&vcpu->kvm->mmu_lock);
 		if (make_mmu_pages_available(vcpu) < 0) {
 			spin_unlock(&vcpu->kvm->mmu_lock);
-			return 1;
+			return -ENOSPC;
 		}
 		sp = kvm_mmu_get_page(vcpu, root_gfn, 0,
 				vcpu->arch.mmu.shadow_root_level, 0, ACC_ALL);
@@ -3487,7 +3487,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
 		spin_lock(&vcpu->kvm->mmu_lock);
 		if (make_mmu_pages_available(vcpu) < 0) {
 			spin_unlock(&vcpu->kvm->mmu_lock);
-			return 1;
+			return -ENOSPC;
 		}
 		sp = kvm_mmu_get_page(vcpu, root_gfn, i << 30, PT32_ROOT_LEVEL,
 				      0, ACC_ALL);
-- 
2.7.4

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

[Wanpeng Li]

From: Wanpeng Li <wanpeng.li@hotmail.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@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.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

Re: [PATCH 1/2] KVM: MMU: Fix infinite loop when there is no available mmu page

[Paolo Bonzini]

On 05/12/2017 07:21, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
> 
> The below test case can cause infinite loop in kvm when ept=0.
> 
>     #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);
>     	ioctl(r[4], KVM_RUN, 0);
>     }
> 
> It doesn't setup the memory regions, mmu_alloc_shadow/direct_roots() in 
> kvm return 1 when kvm fails to allocate root page table which can result 
> in beblow infinite loop:
> 
>     vcpu_run() {
>     	for (;;) {
> 	    	r = vcpu_enter_guest()::kvm_mmu_reload() returns 1 
> 	    	if (r <= 0)
> 	    		break;
> 	    	if (need_resched())
> 	    		cond_resched();
>       }
>     }
> 
> This patch fixes it by returning -ENOSPC when there is no available kvm mmu 
> page for root page table.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Fixes: 26eeb53cf0f (KVM: MMU: Bail out immediately if there is no available mmu page)
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
>  arch/x86/kvm/mmu.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index c9aaa18..89da688 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3395,7 +3395,7 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>  		spin_lock(&vcpu->kvm->mmu_lock);
>  		if(make_mmu_pages_available(vcpu) < 0) {
>  			spin_unlock(&vcpu->kvm->mmu_lock);
> -			return 1;
> +			return -ENOSPC;
>  		}
>  		sp = kvm_mmu_get_page(vcpu, 0, 0,
>  				vcpu->arch.mmu.shadow_root_level, 1, ACC_ALL);
> @@ -3410,7 +3410,7 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>  			spin_lock(&vcpu->kvm->mmu_lock);
>  			if (make_mmu_pages_available(vcpu) < 0) {
>  				spin_unlock(&vcpu->kvm->mmu_lock);
> -				return 1;
> +				return -ENOSPC;
>  			}
>  			sp = kvm_mmu_get_page(vcpu, i << (30 - PAGE_SHIFT),
>  					i << 30, PT32_ROOT_LEVEL, 1, ACC_ALL);
> @@ -3450,7 +3450,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>  		spin_lock(&vcpu->kvm->mmu_lock);
>  		if (make_mmu_pages_available(vcpu) < 0) {
>  			spin_unlock(&vcpu->kvm->mmu_lock);
> -			return 1;
> +			return -ENOSPC;
>  		}
>  		sp = kvm_mmu_get_page(vcpu, root_gfn, 0,
>  				vcpu->arch.mmu.shadow_root_level, 0, ACC_ALL);
> @@ -3487,7 +3487,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>  		spin_lock(&vcpu->kvm->mmu_lock);
>  		if (make_mmu_pages_available(vcpu) < 0) {
>  			spin_unlock(&vcpu->kvm->mmu_lock);
> -			return 1;
> +			return -ENOSPC;
>  		}
>  		sp = kvm_mmu_get_page(vcpu, root_gfn, i << 30, PT32_ROOT_LEVEL,
>  				      0, ACC_ALL);
> 

Queued, thanks.