KVM: Fix stack-out-of-bounds read in write_mmio

Reported by syzkaller:

BUG: KASAN: stack-out-of-bounds in write_mmio+0x11e/0x270 [kvm]
Read of size 8 at addr ffff8803259df7f8 by task syz-executor/32298

CPU: 6 PID: 32298 Comm: syz-executor Tainted: G OE 4.15.0-rc2+ #18
Hardware name: LENOVO ThinkCentre M8500t-N000/SHARKBAY, BIOS FBKTC1AUS 02/16/2016
Call Trace:
dump_stack+0xab/0xe1
print_address_description+0x6b/0x290
kasan_report+0x28a/0x370
write_mmio+0x11e/0x270 [kvm]
emulator_read_write_onepage+0x311/0x600 [kvm]
emulator_read_write+0xef/0x240 [kvm]
emulator_fix_hypercall+0x105/0x150 [kvm]
em_hypercall+0x2b/0x80 [kvm]
x86_emulate_insn+0x2b1/0x1640 [kvm]
x86_emulate_instruction+0x39a/0xb90 [kvm]
handle_exception+0x1b4/0x4d0 [kvm_intel]
vcpu_enter_guest+0x15a0/0x2640 [kvm]
kvm_arch_vcpu_ioctl_run+0x549/0x7d0 [kvm]
kvm_vcpu_ioctl+0x479/0x880 [kvm]
do_vfs_ioctl+0x142/0x9a0
SyS_ioctl+0x74/0x80
entry_SYSCALL_64_fastpath+0x23/0x9a

The path of patched vmmcall will patch 3 bytes opcode 0F 01 C1(vmcall)
to the guest memory, however, write_mmio tracepoint always prints 8 bytes
through *(u64 *)val since kvm splits the mmio access into 8 bytes. This
leaks 5 bytes from the kernel stack (CVE-2017-17741). This patch fixes
it by just accessing the bytes which we operate on.

Before patch:

syz-executor-5567 [007] .... 51370.561696: kvm_mmio: mmio write len 3 gpa 0x10 val 0x1ffff10077c1010f

After patch:

syz-executor-13416 [002] .... 51302.299573: kvm_mmio: mmio write len 3 gpa 0x10 val 0xc1010f

Reported-by: Dmitry Vyukov <[e-mail]>
Reviewed-by: Darren Kenny <[e-mail]>
Reviewed-by: Marc Zyngier <[e-mail]>
Tested-by: Marc Zyngier <[e-mail]>
Cc: Paolo Bonzini <[e-mail]>
Cc: Radim Krčmář <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: Christoffer Dall <[e-mail]>
Signed-off-by: Wanpeng Li <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/x86.c

···
 					 addr, n, v))
 		    && kvm_io_bus_read(vcpu, KVM_MMIO_BUS, addr, n, v))
 			break;
-		trace_kvm_mmio(KVM_TRACE_MMIO_READ, n, addr, *(u64 *)v);
+		trace_kvm_mmio(KVM_TRACE_MMIO_READ, n, addr, v);
 		handled += n;
 		addr += n;
 		len -= n;
···
 {
 	if (vcpu->mmio_read_completed) {
 		trace_kvm_mmio(KVM_TRACE_MMIO_READ, bytes,
-			       vcpu->mmio_fragments[0].gpa, *(u64 *)val);
+			       vcpu->mmio_fragments[0].gpa, val);
 		vcpu->mmio_read_completed = 0;
 		return 1;
 	}
···
 
 static int write_mmio(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes, void *val)
 {
-	trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, bytes, gpa, *(u64 *)val);
+	trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, bytes, gpa, val);
 	return vcpu_mmio_write(vcpu, gpa, bytes, val);
 }
 
 static int read_exit_mmio(struct kvm_vcpu *vcpu, gpa_t gpa,
 			  void *val, int bytes)
 {
-	trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, bytes, gpa, 0);
+	trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, bytes, gpa, NULL);
 	return X86EMUL_IO_NEEDED;
 }
 

include/trace/events/kvm.h

···
 	{ KVM_TRACE_MMIO_WRITE, "write" }
 
 TRACE_EVENT(kvm_mmio,
-	TP_PROTO(int type, int len, u64 gpa, u64 val),
+	TP_PROTO(int type, int len, u64 gpa, void *val),
 	TP_ARGS(type, len, gpa, val),
 
 	TP_STRUCT__entry(
···
 		__entry->type		= type;
 		__entry->len		= len;
 		__entry->gpa		= gpa;
-		__entry->val		= val;
+		__entry->val		= 0;
+		if (val)
+			memcpy(&__entry->val, val,
+			       min_t(u32, sizeof(__entry->val), len));
 	),
 
 	TP_printk("mmio %s len %u gpa 0x%llx val 0x%llx",

virt/kvm/arm/mmio.c

···
 		}
 
 		trace_kvm_mmio(KVM_TRACE_MMIO_READ, len, run->mmio.phys_addr,
-			       data);
+			       &data);
 		data = vcpu_data_host_to_guest(vcpu, data, len);
 		vcpu_set_reg(vcpu, vcpu->arch.mmio_decode.rt, data);
 	}
···
 		data = vcpu_data_guest_to_host(vcpu, vcpu_get_reg(vcpu, rt),
 					       len);
 
-		trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, len, fault_ipa, data);
+		trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, len, fault_ipa, &data);
 		kvm_mmio_write_buf(data_buf, len, data);
 
 		ret = kvm_io_bus_write(vcpu, KVM_MMIO_BUS, fault_ipa, len,
 				       data_buf);
 	} else {
 		trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, len,
-			       fault_ipa, 0);
+			       fault_ipa, NULL);
 
 		ret = kvm_io_bus_read(vcpu, KVM_MMIO_BUS, fault_ipa, len,
 				      data_buf);