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>

4 files changed, 22 insertions, 2 deletions

diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index 29fbb554af5c..bb66d8b8efdf 100644
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/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;
diff --git a/arch/powerpc/include/uapi/asm/kvm_para.h b/arch/powerpc/include/uapi/asm/kvm_para.h
index e3af3286a068..91e42f09b323 100644
--- a/arch/powerpc/include/uapi/asm/kvm_para.h
+++ b/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__ */
diff --git a/arch/powerpc/kvm/book3s_64_mmu.c b/arch/powerpc/kvm/book3s_64_mmu.c
index 278729f4df80..774a253ca4e1 100644
--- a/arch/powerpc/kvm/book3s_64_mmu.c
+++ b/arch/powerpc/kvm/book3s_64_mmu.c
@@ -313,6 +313,9 @@ do_second:
 	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;
 
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index b4e15bf3ff88..154f352c39ae 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/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;