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

[Michael S. Tsirkin]

On Mon, Aug 18, 2014 at 02:05:46PM -0400, Luiz Capitulino wrote:
> 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...


I agree generally, it's just that I don't understand this function.
For example, doesn't it leak memory? When
monitor_fdset_dup_fd_find_remove drops a MonFdsetFd entry from list, it does
not seem to free it.



> > +                } 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;
> >  }