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

[Greg Kurz]

On Sat, 29 Apr 2023 15:20:12 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> 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.
> 

Yeah you're right.

Cheers,

--
Greg

> Best regards,
> Christian Schoenebeck
> 
>