migration: fix the memory overwriting risk in add_to_iovec

migration: fix the memory overwriting risk in add_to_iovec

When testing migration, a Segmentation fault qemu core is generated.
0  error_free (err=0x1)
1  0x00007f8b862df647 in qemu_fclose (f=f@entry=0x55e06c247640)
2  0x00007f8b8516d59a in migrate_fd_cleanup (s=s@entry=0x55e06c0e1ef0)
3  0x00007f8b8516d66c in migrate_fd_cleanup_bh (opaque=0x55e06c0e1ef0)
4  0x00007f8b8626a47f in aio_bh_poll (ctx=ctx@entry=0x55e06b5a16d0)
5  0x00007f8b8626e71f in aio_dispatch (ctx=0x55e06b5a16d0)
6  0x00007f8b8626a33d in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>)
7  0x00007f8b866bdba4 in g_main_context_dispatch ()
8  0x00007f8b8626cde9 in glib_pollfds_poll ()
9  0x00007f8b8626ce62 in os_host_main_loop_wait (timeout=<optimized out>)
10 0x00007f8b8626cffd in main_loop_wait (nonblocking=nonblocking@entry=0)
11 0x00007f8b862ef01f in main_loop ()
Using gdb print the struct QEMUFile f = {
  ...,
  iovcnt = 65, last_error = 21984,
  last_error_obj = 0x1, shutdown = true
}
Well iovcnt is overflow, because the max size of MAX_IOV_SIZE is 64.
struct QEMUFile {
    ...;
    struct iovec iov[MAX_IOV_SIZE];
    unsigned int iovcnt;
    int last_error;
    Error *last_error_obj;
    bool shutdown;
};
iovcnt and last_error is overwrited by add_to_iovec().
Right now, add_to_iovec() increase iovcnt before check the limit.
And it seems that add_to_iovec() assumes that iovcnt will set to zero
in qemu_fflush(). But qemu_fflush() will directly return when f->shutdown
is true.

The situation may occur when libvirtd restart during migration, after
f->shutdown is set, before calling qemu_file_set_error() in
qemu_file_shutdown().

So the safiest way is checking the iovcnt before increasing it.

Signed-off-by: Feng Lin <linfeng23@huawei.com>
Message-Id: <20210625062138.1899-1-linfeng23@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Fix typo in 'writeable' which is actually misnamed 'writable'