Re: [PATCH] tests/9p: fix potential leak in v9fs_rreaddir()

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Saturday, April 29, 2023 2:04:30 PM CEST Greg Kurz wrote:
> Hi Christian !

Hi there, it's been a while! :)

> On Sat, 29 Apr 2023 11:25:33 +0200
> Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> 
> > Free allocated directory entries in v9fs_rreaddir() if argument
> > `entries` was passed as NULL, to avoid a memory leak. It is
> > explicitly allowed by design for `entries` to be NULL. [1]
> > 
> > [1] https://lore.kernel.org/all/1690923.g4PEXVpXuU@silver
> > 
> > Reported-by: Coverity (CID 1487558)
> > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > ---
> 
> Good catch Coverity ! :-)

Yeah, this Coverity report is actually from March and I ignored it so far,
because the reported leak could never happen with current test code. But Paolo
brought it up this week, so ...

> Reviewed-by: Greg Kurz <groug@kaod.org>
> 
> I still have a suggestion. See below.
> 
> >  tests/qtest/libqos/virtio-9p-client.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/tests/qtest/libqos/virtio-9p-client.c b/tests/qtest/libqos/virtio-9p-client.c
> > index e4a368e036..b8adc8d4b9 100644
> > --- a/tests/qtest/libqos/virtio-9p-client.c
> > +++ b/tests/qtest/libqos/virtio-9p-client.c
> > @@ -594,6 +594,8 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t *nentries,
> >  {
> >      uint32_t local_count;
> >      struct V9fsDirent *e = NULL;
> > +    /* only used to avoid a leak if entries was NULL */
> > +    struct V9fsDirent *unused_entries = NULL;
> >      uint16_t slen;
> >      uint32_t n = 0;
> >  
> > @@ -612,6 +614,8 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t *nentries,
> >              e = g_new(struct V9fsDirent, 1);
> >              if (entries) {
> >                  *entries = e;
> > +            } else {
> > +                unused_entries = e;
> >              }
> >          } else {
> >              e = e->next = g_new(struct V9fsDirent, 1);
> 
> This is always allocating and chaining a new entry even
> though it isn't needed in the entries == NULL case.
> 
> > @@ -628,6 +632,7 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t *nentries,
> >          *nentries = n;
> >      }
> >  
> > +    v9fs_free_dirents(unused_entries);
> 
> This is going to loop again on all entries to free them.
> 
> >      v9fs_req_free(req);
> >  }
> >  
> 
> If this function is to be called one day with an enormous
> number of entries and entries == NULL case, this might
> not scale well.
> 
> What about only allocating a single entry in this case ?
> 
> E.g.
> 
> @@ -593,7 +593,7 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t *nentries,
>                     struct V9fsDirent **entries)
>  {
>      uint32_t local_count;
> -    struct V9fsDirent *e = NULL;
> +    g_autofree struct V9fsDirent *e = NULL;
>      uint16_t slen;
>      uint32_t n = 0;
>  
> @@ -611,10 +611,12 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t *nentries,
>          if (!e) {
>              e = g_new(struct V9fsDirent, 1);
>              if (entries) {
> -                *entries = e;
> +                *entries = g_steal_pointer(e);

g_steal_pointer(e) just sets `e` to NULL and returns its old value, so ...

>              }
>          } else {
> -            e = e->next = g_new(struct V9fsDirent, 1);
> +            if (entries) {
> +                e = e->next = g_new(struct V9fsDirent, 1);
> +            }

... this `else` block would never be reached and no list assembled.

>          }
>          e->next = NULL;
>          /* qid[13] offset[8] type[1] name[s] */

And even if above's issue was fixed, then it would cause a use-after-free for
the last element in the list if entries != NULL and caller trying to access
the last element afterwards. So you would still need a separate g_autofree
pointer instead of tagging `e` directly, or something like this after loop
end:

  if (entries)
    g_steal_pointer(e);

Which would somehow defeat the purpose of using g_autofree though.

I mean, yes this could be addressed, but is it worth it? I don't know. Even
this reported leak is a purely theoretical one, but I understand if people
want to silence a warning.

Best regards,
Christian Schoenebeck