bhyve: Fully reset the fwctl state machine if the guest requests a reset.

If a guest tries to reset the fwctl device while a pending request was
in flight, the fwctl state machine can be left in an incomplete state.
Specifically, rinfo is not cleared.

Normally the state machine for fwctl alternates between REQ (receiving
request) and RESP (sending response) and ignores port writes while in
RESP or port reads while in REQ.  Once a guest completes the writes to
the port to send a request, the state machine transitions to RESP and
ignores future writes.

However, if a guest writes a full request and then resets the fwctl
device, the state would transition to REQ without draining the pending
response or discarding the received request.  Instead, additional
port writes after the reset were treated as new payload bytes, but
were appended to the previously-received request and could overflow
the fget_str buffer.

To fix, fully reset the fwctl state machine if the guest requests a
reset.

admbugs:	998
Approved by:	so
Reviewed by:	markj
Reported by:	Omri Ben Bassat <t-benbassato@microsoft.com>
Security:	FreeBSD-SA-23:07.bhyve
Security:	CVE-2023-3494

1 files changed, 27 insertions, 9 deletions

diff --git a/usr.sbin/bhyve/fwctl.c b/usr.sbin/bhyve/fwctl.c
index 5227c06f0da7..24fedfab8885 100644
--- a/usr.sbin/bhyve/fwctl.c
+++ b/usr.sbin/bhyve/fwctl.c
@@ -67,12 +67,11 @@ __FBSDID("$FreeBSD$");
  * Back-end state-machine
  */
 static enum state {
-	DORMANT,
 	IDENT_WAIT,
 	IDENT_SEND,
 	REQ,
 	RESP
-} be_state = DORMANT;
+} be_state;
 
 static uint8_t sig[] = { 'B', 'H', 'Y', 'V' };
 static u_int ident_idx;
@@ -200,6 +199,7 @@ static void
 fget_data(uint32_t data, uint32_t len __unused)
 {
 
+	assert(fget_cnt + sizeof(uint32_t) <= sizeof(fget_str));
 	memcpy(&fget_str[fget_cnt], &data, sizeof(data));
 	fget_cnt += sizeof(uint32_t);
 }
@@ -344,7 +344,8 @@ static int
 fwctl_request_data(uint32_t value)
 {
 
-	/* Make sure remaining size is >= 0 */
+	/* Make sure remaining size is > 0 */
+	assert(rinfo.req_size > 0);
 	if (rinfo.req_size <= sizeof(uint32_t))
 		rinfo.req_size = 0;
 	else
@@ -441,6 +442,28 @@ fwctl_response(uint32_t *retval)
 	return (0);
 }
 
+static void
+fwctl_reset(void)
+{
+
+	switch (be_state) {
+	case RESP:
+		/* If a response was generated but not fully read, discard it. */
+		fwctl_response_done();
+		break;
+	case REQ:
+		/* Discard partially-received request. */
+		memset(&rinfo, 0, sizeof(rinfo));
+		break;
+	case IDENT_WAIT:
+	case IDENT_SEND:
+		break;
+	}
+
+	be_state = IDENT_SEND;
+	ident_idx = 0;
+}
+
 
 /*
  * i/o port handling.
@@ -468,18 +491,13 @@ fwctl_inb(void)
 static void
 fwctl_outw(uint16_t val)
 {
-	if (be_state == DORMANT) {
-		return;
-	}
-
 	if (val == 0) {
 		/*
 		 * The guest wants to read the signature. It's possible that the
 		 * guest is unaware of the fwctl state at this moment. For that
 		 * reason, reset the state machine unconditionally.
 		 */
-		be_state = IDENT_SEND;
-		ident_idx = 0;
+		fwctl_reset();
 	}
 }