KVM: PPC: mmio: Reject instructions that access more than mmio.data size

The MMIO interface between the kernel and userspace uses a structure
that supports a maximum of 8-bytes of data. Instructions that access
more than that need to be emulated in parts.

We currently don't have generic support for splitting the emulation in
parts and each set of instructions needs to be explicitly included.

There's already an error message being printed when a load or store
exceeds the mmio.data buffer but we don't fail the emulation until
later at kvmppc_complete_mmio_load and even then we allow userspace to
make a partial copy of the data, which ends up overwriting some fields
of the mmio structure.

This patch makes the emulation fail earlier at kvmppc_handle_load|store,
which will send a Program interrupt to the guest. This is better than
allowing the guest to proceed with partial data.

Note that this was caught in a somewhat artificial scenario using
quadword instructions (lq/stq), there's no account of an actual guest
in the wild running instructions that are not properly emulated.

(While here, remove the "bad MMIO" messages. The caller already has an
error message.)

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220125215655.1026224-4-farosas@linux.ibm.com

Author: farosas

arch/powerpc/kvm/powerpc.c

@@ -1114,10 +1114,8 @@ static void kvmppc_complete_mmio_load(struct kvm_vcpu *vcpu)
 	struct kvm_run *run = vcpu->run;
 	u64 gpr;
 
-	if (run->mmio.len > sizeof(gpr)) {
-		printk(KERN_ERR "bad MMIO length: %d\n", run->mmio.len);
+	if (run->mmio.len > sizeof(gpr))
 		return;
-	}
 
 	if (!vcpu->arch.mmio_host_swabbed) {
 		switch (run->mmio.len) {
@@ -1236,10 +1234,8 @@ static int __kvmppc_handle_load(struct kvm_vcpu *vcpu,
 		host_swabbed = !is_default_endian;
 	}
 
-	if (bytes > sizeof(run->mmio.data)) {
-		printk(KERN_ERR "%s: bad MMIO length: %d\n", __func__,
-		       run->mmio.len);
-	}
+	if (bytes > sizeof(run->mmio.data))
+		return EMULATE_FAIL;
 
 	run->mmio.phys_addr = vcpu->arch.paddr_accessed;
 	run->mmio.len = bytes;
@@ -1325,10 +1321,8 @@ int kvmppc_handle_store(struct kvm_vcpu *vcpu,
 		host_swabbed = !is_default_endian;
 	}
 
-	if (bytes > sizeof(run->mmio.data)) {
-		printk(KERN_ERR "%s: bad MMIO length: %d\n", __func__,
-		       run->mmio.len);
-	}
+	if (bytes > sizeof(run->mmio.data))
+		return EMULATE_FAIL;
 
 	run->mmio.phys_addr = vcpu->arch.paddr_accessed;
 	run->mmio.len = bytes;