[Libguestfs] [libnbd PATCH 1/2] api: Tighter checking of structured read replies

[Nir Soffer]

On Tue, May 31, 2022 at 5:15 PM Eric Blake <eblake at redhat.com> wrote:
>
> Now that we allow clients to bypass buffer pre-initialization, it
> becomes more important to detect when a buggy server using structured
> replies does not send us enough bytes to cover the requested read
> size.  Our check is not perfect (a server that duplicates reply chunks
> for byte 0 and omits byte 1 gets past our check), but this is a
> tighter sanity check so that we are less likely to report a successful
> read containing uninitialized memory on a buggy server.

Nice!

> Because we have a maximum read buffer size of 64M, and first check
> that the server's chunk fits bounds, we don't have to worry about
> overflowing a uint32_t, even if a server sends enough duplicate
> responses that an actual sum would overflow.
> ---
>  lib/internal.h                      | 2 +-
>  generator/states-reply-simple.c     | 4 ++--
>  generator/states-reply-structured.c | 6 ++++--
>  lib/aio.c                           | 7 +++++--
>  4 files changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/lib/internal.h b/lib/internal.h
> index 885cee1..4121a5c 100644
> --- a/lib/internal.h
> +++ b/lib/internal.h
> @@ -352,8 +352,8 @@ struct command {
>    void *data; /* Buffer for read/write */
>    struct command_cb cb;
>    enum state state; /* State to resume with on next POLLIN */
> -  bool data_seen; /* For read, true if at least one data chunk seen */
>    bool initialized; /* For read, true if getting a hole may skip memset */
> +  uint32_t data_seen; /* For read, cumulative size of data chunks seen */
>    uint32_t error; /* Local errno value */
>  };
>
> diff --git a/generator/states-reply-simple.c b/generator/states-reply-simple.c
> index 7dc26fd..2a7b9a9 100644
> --- a/generator/states-reply-simple.c
> +++ b/generator/states-reply-simple.c
> @@ -1,5 +1,5 @@
>  /* nbd client library in userspace: state machine
> - * Copyright (C) 2013-2019 Red Hat Inc.
> + * Copyright (C) 2013-2022 Red Hat Inc.
>   *
>   * This library is free software; you can redistribute it and/or
>   * modify it under the terms of the GNU Lesser General Public
> @@ -40,7 +40,7 @@ STATE_MACHINE {
>    if (cmd->error == 0 && cmd->type == NBD_CMD_READ) {
>      h->rbuf = cmd->data;
>      h->rlen = cmd->count;
> -    cmd->data_seen = true;
> +    cmd->data_seen = cmd->count;
>      SET_NEXT_STATE (%RECV_READ_PAYLOAD);
>    }
>    else {
> diff --git a/generator/states-reply-structured.c b/generator/states-reply-structured.c
> index 12c24f5..cabd543 100644
> --- a/generator/states-reply-structured.c
> +++ b/generator/states-reply-structured.c
> @@ -354,7 +354,6 @@ STATE_MACHINE {
>      assert (cmd); /* guaranteed by CHECK */
>
>      assert (cmd->data && cmd->type == NBD_CMD_READ);
> -    cmd->data_seen = true;
>
>      /* Length of the data following. */
>      length -= 8;
> @@ -364,6 +363,8 @@ STATE_MACHINE {
>        SET_NEXT_STATE (%.DEAD);
>        return 0;
>      }
> +    if (cmd->data_seen <= cmd->count)
> +      cmd->data_seen += length;

This does not feel right. if you received more data, it should be counted,
and if this causes data_seen to be bigger than cmd->count, isn't this a fatal
error?

>      /* Now this is the byte offset in the read buffer. */
>      offset -= cmd->offset;
>
> @@ -422,13 +423,14 @@ STATE_MACHINE {
>      assert (cmd); /* guaranteed by CHECK */
>
>      assert (cmd->data && cmd->type == NBD_CMD_READ);
> -    cmd->data_seen = true;
>
>      /* Is the data within bounds? */
>      if (! structured_reply_in_bounds (offset, length, cmd)) {
>        SET_NEXT_STATE (%.DEAD);
>        return 0;
>      }
> +    if (cmd->data_seen <= cmd->count)
> +      cmd->data_seen += length;

Same here

>      /* Now this is the byte offset in the read buffer. */
>      offset -= cmd->offset;
>
> diff --git a/lib/aio.c b/lib/aio.c
> index 9744840..dc01f90 100644
> --- a/lib/aio.c
> +++ b/lib/aio.c
> @@ -1,5 +1,5 @@
>  /* NBD client library in userspace
> - * Copyright (C) 2013-2019 Red Hat Inc.
> + * Copyright (C) 2013-2022 Red Hat Inc.
>   *
>   * This library is free software; you can redistribute it and/or
>   * modify it under the terms of the GNU Lesser General Public
> @@ -91,8 +91,11 @@ nbd_unlocked_aio_command_completed (struct nbd_handle *h,
>    assert (cmd->type != NBD_CMD_DISC);
>    /* The spec states that a 0-length read request is unspecified; but
>     * it is easy enough to treat it as successful as an extension.
> +   * Conversely, make sure a server sending structured replies sent
> +   * enough data chunks to cover the overall count (although we do not
> +   * detect if it duplicated some bytes while omitting others).
>     */
> -  if (type == NBD_CMD_READ && !cmd->data_seen && cmd->count && !error)
> +  if (type == NBD_CMD_READ && cmd->data_seen != cmd->count && !error)
>      error = EIO;
>
>    /* Retire it from the list and free it. */
> --
> 2.36.1
>
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Nir