Re: [PATCH] netfilter: ebtables: compat: fix a memory leak bug

Wenwen Wang
2019-07-22T13:10:48Z

On Mon, Jul 22, 2019 at 7:34 AM Liam R. Howlett <Liam.Howlett@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@umn.edu> [190720 08:23]:
> > From: Wenwen Wang <wenwen@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@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