Linux Ethernet Bridging - Jul 2019 - [Bridge] [PATCH] netfilter: ebtables: compat: fix a memory leak bug

On Mon, Jul 22, 2019 at 7:34 AM Liam R. Howlett <Liam.Howlett at
oracle.com> wrote:
>
> Nice catch.  The code that exists is confusing due to newinfo->entries
> being overwritten and then freed in the existing code path as you state
> in your commit log.
>
> * Wenwen Wang <wang6495 at umn.edu> [190720 08:23]:
> > From: Wenwen Wang <wenwen at cs.uga.edu>
> >
> > In compat_do_replace(), a temporary buffer is allocated through
vmalloc()
> > to hold entries copied from the user space. The buffer address is
firstly
> > saved to 'newinfo->entries', and later on assigned to
'entries_tmp'. Then
> > the entries in this temporary buffer is copied to the internal kernel
> > structure through compat_copy_entries(). If this copy process fails,
> > compat_do_replace() should be terminated. However, the allocated
temporary
> > buffer is not freed on this path, leading to a memory leak.
> >
> > To fix the bug, free the buffer before returning from
compat_do_replace().
> >
> > Signed-off-by: Wenwen Wang <wenwen at cs.uga.edu>
> > ---
> >  net/bridge/netfilter/ebtables.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/bridge/netfilter/ebtables.c
b/net/bridge/netfilter/ebtables.c
> > index 963dfdc..fd84b48e 100644
> > --- a/net/bridge/netfilter/ebtables.c
> > +++ b/net/bridge/netfilter/ebtables.c
> > @@ -2261,8 +2261,10 @@ static int compat_do_replace(struct net *net,
void __user *user,
> >       state.buf_kern_len = size64;
> >
> >       ret = compat_copy_entries(entries_tmp, tmp.entries_size,
&state);
> > -     if (WARN_ON(ret < 0))
> > +     if (WARN_ON(ret < 0)) {
> > +             vfree(entries_tmp);
> >               goto out_unlock;
> > +     }
>
>
> Would it be worth adding a new goto label above out_unlock and free this
> entries_tmp?  It could then be used in previous failure path as well.
Yes, that would make the code much clearer and easier to understand.

Thanks!
Wenwen