Re: [Qemu-devel] [PATCH] monitor: fix use after free

[Luiz Capitulino]

On Sun, 17 Aug 2014 11:45:17 +0200
"Michael S. Tsirkin" <address@hidden> wrote:

> The function monitor_fdset_dup_fd_find_remove() references member of
> 'mon_fdset' which - when remove flag is set - may be freed in function
> monitor_fdset_cleanup().
> remove is set by monitor_fdset_dup_fd_remove which in practice
> does not need the returned value, so make it void,
> and return -1 from monitor_fdset_dup_fd_find_remove.
> 
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> Reported-by: zhanghailiang <address@hidden>

I've applied this one to the qmp tree, but I have one comment below.

> ---
>  include/monitor/monitor.h | 2 +-
>  monitor.c                 | 8 +++++---
>  stubs/fdset-remove-fd.c   | 3 +--
>  3 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/include/monitor/monitor.h b/include/monitor/monitor.h
> index 3d6929d..78a5fc8 100644
> --- a/include/monitor/monitor.h
> +++ b/include/monitor/monitor.h
> @@ -64,7 +64,7 @@ AddfdInfo *monitor_fdset_add_fd(int fd, bool has_fdset_id, 
> int64_t fdset_id,
>                                  Error **errp);
>  int monitor_fdset_get_fd(int64_t fdset_id, int flags);
>  int monitor_fdset_dup_fd_add(int64_t fdset_id, int dup_fd);
> -int monitor_fdset_dup_fd_remove(int dup_fd);
> +void monitor_fdset_dup_fd_remove(int dup_fd);
>  int monitor_fdset_dup_fd_find(int dup_fd);
>  
>  #endif /* !MONITOR_H */
> diff --git a/monitor.c b/monitor.c
> index cdbaa60..ba1908f 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -2542,8 +2542,10 @@ static int monitor_fdset_dup_fd_find_remove(int 
> dup_fd, bool remove)
>                      if (QLIST_EMPTY(&mon_fdset->dup_fds)) {
>                          monitor_fdset_cleanup(mon_fdset);
>                      }
> +                    return -1;

Returning -1 here looks wrong. A better fix is probably to split this
function into two functions as it's doing two unrelated things currently.

I've applied the fix anyway because the only user of remove=true doesn't
care about the return value and also because the bad semantic is less worse
than the use after free...

> +                } else {
> +                    return mon_fdset->id;
>                  }
> -                return mon_fdset->id;
>              }
>          }
>      }
> @@ -2555,9 +2557,9 @@ int monitor_fdset_dup_fd_find(int dup_fd)
>      return monitor_fdset_dup_fd_find_remove(dup_fd, false);
>  }
>  
> -int monitor_fdset_dup_fd_remove(int dup_fd)
> +void monitor_fdset_dup_fd_remove(int dup_fd)
>  {
> -    return monitor_fdset_dup_fd_find_remove(dup_fd, true);
> +    monitor_fdset_dup_fd_find_remove(dup_fd, true);
>  }
>  
>  int monitor_handle_fd_param(Monitor *mon, const char *fdname)
> diff --git a/stubs/fdset-remove-fd.c b/stubs/fdset-remove-fd.c
> index b3886d9..7f6d61e 100644
> --- a/stubs/fdset-remove-fd.c
> +++ b/stubs/fdset-remove-fd.c
> @@ -1,7 +1,6 @@
>  #include "qemu-common.h"
>  #include "monitor/monitor.h"
>  
> -int monitor_fdset_dup_fd_remove(int dupfd)
> +void monitor_fdset_dup_fd_remove(int dupfd)
>  {
> -    return -1;
>  }