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

commit e39d200fa5bf5b94a0948db0dae44c1b73b84a56 upstream.

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 <dvyukov@google.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.2:
 - Drop ARM changes
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

2 files changed, 9 insertions, 6 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4baab231292f..7e6094b0308b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3920,7 +3920,7 @@ static int vcpu_mmio_read(struct kvm_vcpu *vcpu, gpa_t addr, int len, void *v)
 		      !kvm_iodevice_read(&vcpu->arch.apic->dev, addr, n, v))
 		    && kvm_io_bus_read(vcpu->kvm, 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;
@@ -4152,7 +4152,7 @@ static int read_prepare(struct kvm_vcpu *vcpu, void *val, int bytes)
 	if (vcpu->mmio_read_completed) {
 		memcpy(val, vcpu->mmio_data, bytes);
 		trace_kvm_mmio(KVM_TRACE_MMIO_READ, bytes,
-			       vcpu->mmio_phys_addr, *(u64 *)val);
+			       vcpu->mmio_phys_addr, val);
 		vcpu->mmio_read_completed = 0;
 		return 1;
 	}
@@ -4174,14 +4174,14 @@ static int write_emulate(struct kvm_vcpu *vcpu, gpa_t gpa,
 
 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;
 }
 
diff --git a/include/trace/events/kvm.h b/include/trace/events/kvm.h
index 46e3cd8e197a..215a904f5e41 100644
--- a/include/trace/events/kvm.h
+++ b/include/trace/events/kvm.h
@@ -152,7 +152,7 @@ TRACE_EVENT(kvm_ack_irq,
 	{ 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(
@@ -166,7 +166,10 @@ TRACE_EVENT(kvm_mmio,
 		__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",