KVM: PPC: Disable NX for old magic page using guests

Old guests try to use the magic page, but map their trampoline code inside
of an NX region.

Since we can't fix those old kernels, try to detect whether the guest is sane
or not. If not, just disable NX functionality in KVM so that old guests at
least work at all. For newer guests, add a bit that we can set to keep NX
functionality available.
Signed-off-by: Alexander Graf <agraf@suse.de>

Documentation/virtual/kvm/ppc-pv.txt

@@ -94,10 +94,24 @@ a bitmap of available features inside the magic page.
 The following enhancements to the magic page are currently available:
 
   KVM_MAGIC_FEAT_SR		Maps SR registers r/w in the magic page
+  KVM_MAGIC_FEAT_MAS0_TO_SPRG7	Maps MASn, ESR, PIR and high SPRGs
 
 For enhanced features in the magic page, please check for the existence of the
 feature before using them!
 
+Magic page flags
+================
+
+In addition to features that indicate whether a host is capable of a particular
+feature we also have a channel for a guest to tell the guest whether it's capable
+of something. This is what we call "flags".
+
+Flags are passed to the host in the low 12 bits of the Effective Address.
+
+The following flags are currently available for a guest to expose:
+
+  MAGIC_PAGE_FLAG_NOT_MAPPED_NX Guest handles NX bits correclty wrt magic page
+
 MSR bits
 ========
 

arch/powerpc/include/asm/kvm_host.h

@@ -631,6 +631,7 @@ struct kvm_vcpu_arch {
 #endif
 	unsigned long magic_page_pa; /* phys addr to map the magic page to */
 	unsigned long magic_page_ea; /* effect. addr to map the magic page to */
+	bool disable_kernel_nx;
 
 	int irq_type;		/* one of KVM_IRQ_* */
 	int irq_cpu_id;

arch/powerpc/include/uapi/asm/kvm_para.h

@@ -82,10 +82,16 @@ struct kvm_vcpu_arch_shared {
 
 #define KVM_FEATURE_MAGIC_PAGE	1
 
+/* Magic page flags from host to guest */
+
 #define KVM_MAGIC_FEAT_SR		(1 << 0)
 
 /* MASn, ESR, PIR, and high SPRGs */
 #define KVM_MAGIC_FEAT_MAS0_TO_SPRG7	(1 << 1)
 
+/* Magic page flags from guest to host */
+
+#define MAGIC_PAGE_FLAG_NOT_MAPPED_NX	(1 << 0)
+
 
 #endif /* _UAPI__POWERPC_KVM_PARA_H__ */

arch/powerpc/kvm/book3s_64_mmu.c

@@ -313,6 +313,9 @@ static int kvmppc_mmu_book3s_64_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
 	gpte->raddr = (r & HPTE_R_RPN & ~eaddr_mask) | (eaddr & eaddr_mask);
 	gpte->page_size = pgsize;
 	gpte->may_execute = ((r & HPTE_R_N) ? false : true);
+	if (unlikely(vcpu->arch.disable_kernel_nx) &&
+	    !(kvmppc_get_msr(vcpu) & MSR_PR))
+		gpte->may_execute = true;
 	gpte->may_read = false;
 	gpte->may_write = false;
 

arch/powerpc/kvm/powerpc.c

@@ -177,8 +177,18 @@ int kvmppc_kvm_pv(struct kvm_vcpu *vcpu)
 		vcpu->arch.shared_big_endian = shared_big_endian;
 #endif
 
-		vcpu->arch.magic_page_pa = param1;
-		vcpu->arch.magic_page_ea = param2;
+		if (!(param2 & MAGIC_PAGE_FLAG_NOT_MAPPED_NX)) {
+			/*
+			 * Older versions of the Linux magic page code had
+			 * a bug where they would map their trampoline code
+			 * NX. If that's the case, remove !PR NX capability.
+			 */
+			vcpu->arch.disable_kernel_nx = true;
+			kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
+		}
+
+		vcpu->arch.magic_page_pa = param1 & ~0xfffULL;
+		vcpu->arch.magic_page_ea = param2 & ~0xfffULL;
 
 		r2 = KVM_MAGIC_FEAT_SR | KVM_MAGIC_FEAT_MAS0_TO_SPRG7;