kvm: x86: use correct privilege level for sgdt/sidt/fxsave/fxrstor access

commit 3c9fa24ca7c9c47605672916491f79e8ccacb9e6 upstream.

The functions that were used in the emulation of fxrstor, fxsave, sgdt and
sidt were originally meant for task switching, and as such they did not
check privilege levels.  This is very bad when the same functions are used
in the emulation of unprivileged instructions.  This is CVE-2018-10853.

The obvious fix is to add a new argument to ops->read_std and ops->write_std,
which decides whether the access is a "system" access or should use the
processor's CPL.

Fixes: 129a72a0d3c8 ("KVM: x86: Introduce segmented_write_std", 2017-01-12)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 20 insertions, 10 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index c27e09ee8669..f1507626ed36 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -793,14 +793,14 @@ static inline int jmp_rel(struct x86_emulate_ctxt *ctxt, int rel)
 static int linear_read_system(struct x86_emulate_ctxt *ctxt, ulong linear,
 			      void *data, unsigned size)
 {
-	return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception);
+	return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception, true);
 }
 
 static int linear_write_system(struct x86_emulate_ctxt *ctxt,
 			       ulong linear, void *data,
 			       unsigned int size)
 {
-	return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception);
+	return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception, true);
 }
 
 static int segmented_read_std(struct x86_emulate_ctxt *ctxt,
@@ -814,7 +814,7 @@ static int segmented_read_std(struct x86_emulate_ctxt *ctxt,
 	rc = linearize(ctxt, addr, size, false, &linear);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
-	return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception);
+	return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception, false);
 }
 
 static int segmented_write_std(struct x86_emulate_ctxt *ctxt,
@@ -828,7 +828,7 @@ static int segmented_write_std(struct x86_emulate_ctxt *ctxt,
 	rc = linearize(ctxt, addr, size, true, &linear);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
-	return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception);
+	return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception, false);
 }
 
 /*
@@ -2900,12 +2900,12 @@ static bool emulator_io_port_access_allowed(struct x86_emulate_ctxt *ctxt,
 #ifdef CONFIG_X86_64
 	base |= ((u64)base3) << 32;
 #endif
-	r = ops->read_std(ctxt, base + 102, &io_bitmap_ptr, 2, NULL);
+	r = ops->read_std(ctxt, base + 102, &io_bitmap_ptr, 2, NULL, true);
 	if (r != X86EMUL_CONTINUE)
 		return false;
 	if (io_bitmap_ptr + port/8 > desc_limit_scaled(&tr_seg))
 		return false;
-	r = ops->read_std(ctxt, base + io_bitmap_ptr + port/8, &perm, 2, NULL);
+	r = ops->read_std(ctxt, base + io_bitmap_ptr + port/8, &perm, 2, NULL, true);
 	if (r != X86EMUL_CONTINUE)
 		return false;
 	if ((perm >> bit_idx) & mask)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b6238ce9806d..53d43d22a84b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4258,10 +4258,15 @@ EXPORT_SYMBOL_GPL(kvm_read_guest_virt);
 
 static int emulator_read_std(struct x86_emulate_ctxt *ctxt,
 			     gva_t addr, void *val, unsigned int bytes,
-			     struct x86_exception *exception)
+			     struct x86_exception *exception, bool system)
 {
 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
-	return kvm_read_guest_virt_helper(addr, val, bytes, vcpu, 0, exception);
+	u32 access = 0;
+
+	if (!system && kvm_x86_ops->get_cpl(vcpu) == 3)
+		access |= PFERR_USER_MASK;
+
+	return kvm_read_guest_virt_helper(addr, val, bytes, vcpu, access, exception);
 }
 
 static int kvm_read_guest_phys_system(struct x86_emulate_ctxt *ctxt,
@@ -4305,12 +4310,17 @@ out:
 }
 
 static int emulator_write_std(struct x86_emulate_ctxt *ctxt, gva_t addr, void *val,
-			      unsigned int bytes, struct x86_exception *exception)
+			      unsigned int bytes, struct x86_exception *exception,
+			      bool system)
 {
 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
+	u32 access = PFERR_WRITE_MASK;
+
+	if (!system && kvm_x86_ops->get_cpl(vcpu) == 3)
+		access |= PFERR_USER_MASK;
 
 	return kvm_write_guest_virt_helper(addr, val, bytes, vcpu,
-					   PFERR_WRITE_MASK, exception);
+					   access, exception);
 }
 
 int kvm_write_guest_virt_system(struct kvm_vcpu *vcpu, gva_t addr, void *val,