[Libguestfs] [PATCH v7 01/12] nbd/server: Support a request payload

Eric Blake
2023-09-25T19:22:31Z

Upcoming additions to support NBD 64-bit effect lengths allow for the
possibility to distinguish between payload length (capped at 32M) and
effect length (64 bits, although we generally assume 63 bits because
of off_t limitations).  Without that extension, only the NBD_CMD_WRITE
request has a payload; but with the extension, it makes sense to allow
at least NBD_CMD_BLOCK_STATUS to have both a payload and effect length
in a future patch (where the payload is a limited-size struct that in
turn gives the real effect length as well as a subset of known ids for
which status is requested).  Other future NBD commands may also have a
request payload, so the 64-bit extension introduces a new
NBD_CMD_FLAG_PAYLOAD_LEN that distinguishes between whether the header
length is a payload length or an effect length, rather than
hard-coding the decision based on the command.

According to the spec, a client should never send a command with a
payload without the negotiation phase proving such extension is
available.  So in the unlikely event the bit is set or cleared
incorrectly, the client is already at fault; if the client then
provides the payload, we can gracefully consume it off the wire and
fail the command with NBD_EINVAL (subsequent checks for magic numbers
ensure we are still in sync), while if the client fails to send
payload we block waiting for it (basically deadlocking our connection
to the bad client, but not negatively impacting our ability to service
other clients, so not a security risk).  Note that we do not support
the payload version of BLOCK_STATUS yet.

Signed-off-by: Eric Blake <eblake at redhat.com>
---

v7: another try at better logic [Vladimir]

v5: retitled from v4 13/24, rewrite on top of previous patch's switch
statement [Vladimir]

v4: less indentation on several 'if's [Vladimir]
---
 nbd/server.c     | 37 +++++++++++++++++++++++++++++++++----
 nbd/trace-events |  1 +
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 7a6f95071f8..1eabcfc908d 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -2322,9 +2322,11 @@ static int coroutine_fn
nbd_co_receive_request(NBDRequestData *req,
                                                Error **errp)
 {
     NBDClient *client = req->client;
+    bool extended_with_payload;
     bool check_length = false;
     bool check_rofs = false;
     bool allocate_buffer = false;
+    bool payload_okay = false;
     unsigned payload_len = 0;
     int valid_flags = NBD_CMD_FLAG_FUA;
     int ret;
@@ -2338,6 +2340,13 @@ static int coroutine_fn
nbd_co_receive_request(NBDRequestData *req,

     trace_nbd_co_receive_request_decode_type(request->cookie,
request->type,
                                              nbd_cmd_lookup(request->type));
+    extended_with_payload = client->mode >= NBD_MODE_EXTENDED &&
+        request->flags & NBD_CMD_FLAG_PAYLOAD_LEN;
+    if (extended_with_payload) {
+        payload_len = request->len;
+        check_length = true;
+    }
+
     switch (request->type) {
     case NBD_CMD_DISC:
         /* Special case: we're going to disconnect without a reply,
@@ -2354,6 +2363,15 @@ static int coroutine_fn
nbd_co_receive_request(NBDRequestData *req,
         break;

     case NBD_CMD_WRITE:
+        if (client->mode >= NBD_MODE_EXTENDED) {
+            if (!extended_with_payload) {
+                /* The client is noncompliant. Trace it, but proceed. */
+                trace_nbd_co_receive_ext_payload_compliance(request->from,
+                                                            request->len);
+            }
+            valid_flags |= NBD_CMD_FLAG_PAYLOAD_LEN;
+        }
+        payload_okay = true;
         payload_len = request->len;
         check_length = true;
         allocate_buffer = true;
@@ -2395,6 +2413,14 @@ static int coroutine_fn
nbd_co_receive_request(NBDRequestData *req,
                    request->len, NBD_MAX_BUFFER_SIZE);
         return -EINVAL;
     }
+    if (payload_len && !payload_okay) {
+        /*
+         * For now, we don't support payloads on other commands; but
+         * we can keep the connection alive by ignoring the payload.
+         */
+        assert(request->type != NBD_CMD_WRITE);
+        request->len = 0;
+    }
     if (allocate_buffer) {
         /* READ, WRITE */
         req->data = blk_try_blockalign(client->exp->common.blk,
@@ -2405,10 +2431,13 @@ static int coroutine_fn
nbd_co_receive_request(NBDRequestData *req,
         }
     }
     if (payload_len) {
-        /* WRITE */
-        assert(req->data);
-        ret = nbd_read(client->ioc, req->data, payload_len,
-                       "CMD_WRITE data", errp);
+        if (payload_okay) {
+            assert(req->data);
+            ret = nbd_read(client->ioc, req->data, payload_len,
+                           "CMD_WRITE data", errp);
+        } else {
+            ret = nbd_drop(client->ioc, payload_len, errp);
+        }
         if (ret < 0) {
             return -EIO;
         }
diff --git a/nbd/trace-events b/nbd/trace-events
index f9dccfcfb44..c1a3227613f 100644
--- a/nbd/trace-events
+++ b/nbd/trace-events
@@ -71,6 +71,7 @@ nbd_co_send_extents(uint64_t cookie, unsigned int extents,
uint32_t id, uint64_t
 nbd_co_send_chunk_error(uint64_t cookie, int err, const char *errname, const
char *msg) "Send structured error reply: cookie = %" PRIu64 ",
error = %d (%s), msg = '%s'"
 nbd_co_receive_request_decode_type(uint64_t cookie, uint16_t type, const char
*name) "Decoding type: cookie = %" PRIu64 ", type = %"
PRIu16 " (%s)"
 nbd_co_receive_request_payload_received(uint64_t cookie, uint64_t len)
"Payload received: cookie = %" PRIu64 ", len = %" PRIu64
+nbd_co_receive_ext_payload_compliance(uint64_t from, uint64_t len) "client
sent non-compliant write without payload flag: from=0x%" PRIx64 ",
len=0x%" PRIx64
 nbd_co_receive_align_compliance(const char *op, uint64_t from, uint64_t len,
uint32_t align) "client sent non-compliant unaligned %s request:
from=0x%" PRIx64 ", len=0x%" PRIx64 ", align=0x%"
PRIx32
 nbd_trip(void) "Reading request"

--
2.41.0