[Qemu-block] [PATCH v4 04/11] nbd: Improve server handling of bogus commands

[Eric Blake]

We have a few bugs in how we handle invalid client commands:

- A client can send an NBD_CMD_DISC where from + len overflows,
convincing us to reply with an error and stay connected, even
though the protocol requires us to silently disconnect. Fix by
hoisting the special case sooner.

- A client can send an NBD_CMD_WRITE with bogus from and len,
where we reply to the client with EINVAL without consuming the
payload; this will normally cause us to fail if the next thing
read is not the right magic, but in rare cases, could cause us
to interpret the data payload as valid commands and do things
not requested by the client. Fix by adding a complete flag to
track whether we are in sync or must disconnect.

- If we report an error to NBD_CMD_READ, we are not writing out
any data payload; but the protocol says that a client can expect
to read the payload no matter what (and must instead ignore it),
which means the client will start reading our next replies as
its data payload. Fix by disconnecting (an alternative fix of
sending bogus payload would be trickier to implement).

Furthermore, we have split the checks for bogus from/len across
two functions, when it is easier to do it all at once.

Signed-off-by: Eric Blake <address@hidden>
---
 nbd/server.c | 67 +++++++++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 46 insertions(+), 21 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 53507c5..9ac7e01 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -52,6 +52,7 @@ struct NBDRequest {
     QSIMPLEQ_ENTRY(NBDRequest) entry;
     NBDClient *client;
     uint8_t *data;
+    bool complete;
 };

 struct NBDExport {
@@ -989,7 +990,13 @@ static ssize_t nbd_co_send_reply(NBDRequest *req, struct 
nbd_reply *reply,
     return rc;
 }

-static ssize_t nbd_co_receive_request(NBDRequest *req, struct nbd_request 
*request)
+/* Collect a client request.  Return 0 if request looks valid, -EAGAIN
+ * to keep trying the collection, -EIO to drop connection right away,
+ * and any other negative value to report an error to the client
+ * (although the caller may still need to disconnect after reporting
+ * the error).  */
+static ssize_t nbd_co_receive_request(NBDRequest *req,
+                                      struct nbd_request *request)
 {
     NBDClient *client = req->client;
     uint32_t command;
@@ -1007,16 +1014,26 @@ static ssize_t nbd_co_receive_request(NBDRequest *req, 
struct nbd_request *reque
         goto out;
     }

-    if ((request->from + request->len) < request->from) {
-        LOG("integer overflow detected! "
-            "you're probably being attacked");
-        rc = -EINVAL;
-        goto out;
-    }
-
     TRACE("Decoding type");

     command = request->type & NBD_CMD_MASK_COMMAND;
+    if (command == NBD_CMD_DISC) {
+        /* Special case: we're going to disconnect without a reply,
+         * whether or not flags, from, or len are bogus */
+        TRACE("Request type is DISCONNECT");
+        rc = -EIO;
+        goto out;
+    }
+
+    /* Check for sanity in the parameters, part 1.  Defer as many
+     * checks as possible until after reading any NBD_CMD_WRITE
+     * payload, so we can try and keep the connection alive.  */
+    if ((request->from + request->len) < request->from) {
+        LOG("integer overflow detected, you're probably being attacked");
+        rc = -EINVAL;
+        goto out;
+    }
+
     if (command == NBD_CMD_READ || command == NBD_CMD_WRITE) {
         if (request->len > NBD_MAX_BUFFER_SIZE) {
             LOG("len (%" PRIu32" ) is larger than max len (%u)",
@@ -1039,7 +1056,18 @@ static ssize_t nbd_co_receive_request(NBDRequest *req, 
struct nbd_request *reque
             rc = -EIO;
             goto out;
         }
+        req->complete = true;
     }
+
+    /* Sanity checks, part 2. */
+    if (request->from + request->len > client->exp->size) {
+        LOG("operation past EOF; From: %" PRIu64 ", Len: %" PRIu32
+            ", Size: %" PRIu64, request->from, request->len,
+            (uint64_t)client->exp->size);
+        rc = -EINVAL;
+        goto out;
+    }
+
     rc = 0;

 out:
@@ -1082,14 +1110,6 @@ static void nbd_trip(void *opaque)
         goto error_reply;
     }
     command = request.type & NBD_CMD_MASK_COMMAND;
-    if (command != NBD_CMD_DISC && (request.from + request.len) > exp->size) {
-            LOG("From: %" PRIu64 ", Len: %" PRIu32", Size: %" PRIu64
-                ", Offset: %" PRIu64 "\n",
-                request.from, request.len,
-                (uint64_t)exp->size, (uint64_t)exp->dev_offset);
-        LOG("requested operation past EOF--bad client?");
-        goto invalid_request;
-    }

     if (client->closing) {
         /*
@@ -1151,10 +1171,11 @@ static void nbd_trip(void *opaque)
             goto out;
         }
         break;
+
     case NBD_CMD_DISC:
-        TRACE("Request type is DISCONNECT");
-        errno = 0;
-        goto out;
+        /* unreachable, thanks to special case in nbd_co_receive_request() */
+        abort();
+
     case NBD_CMD_FLUSH:
         TRACE("Request type is FLUSH");

@@ -1182,10 +1203,14 @@ static void nbd_trip(void *opaque)
         break;
     default:
         LOG("invalid request type (%" PRIu32 ") received", request.type);
-    invalid_request:
         reply.error = EINVAL;
     error_reply:
-        if (nbd_co_send_reply(req, &reply, 0) < 0) {
+        /* We must disconnect after replying with an error to
+         * NBD_CMD_READ, since we choose not to send bogus filler
+         * data; likewise after NBD_CMD_WRITE if we did not read the
+         * payload. */
+        if (nbd_co_send_reply(req, &reply, 0) < 0 || command == NBD_CMD_READ ||
+            (command == NBD_CMD_WRITE && !req->complete)) {
             goto out;
         }
         break;
-- 
2.5.5