Re: [PATCH] security/selinux: fix potential memleak

[Ondrej Mosnacek]

On Mon, Dec 6, 2021 at 4:12 AM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> On Wed, Dec 1, 2021 at 10:35 PM Bernard Zhao <bernard@xxxxxxxx> wrote:
> >
> > This patch try to fix potential memleak in function
> > selinux_fs_context_dup`s error branch.
> >
> > Signed-off-by: Bernard Zhao <bernard@xxxxxxxx>
> > ---
> >  security/selinux/hooks.c | 22 ++++++++++++++++++----
> >  1 file changed, 18 insertions(+), 4 deletions(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 62d30c0a30c2..36d7fc373839 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -2856,24 +2856,38 @@ static int selinux_fs_context_dup(struct fs_context *fc,
> >         if (src->fscontext) {
> >                 opts->fscontext = kstrdup(src->fscontext, GFP_KERNEL);
> >                 if (!opts->fscontext)
> > -                       return -ENOMEM;
> > +                       goto err_fscontext;
> >         }
> >         if (src->context) {
> >                 opts->context = kstrdup(src->context, GFP_KERNEL);
> >                 if (!opts->context)
> > -                       return -ENOMEM;
> > +                       goto err_context;
> >         }
> >         if (src->rootcontext) {
> >                 opts->rootcontext = kstrdup(src->rootcontext, GFP_KERNEL);
> >                 if (!opts->rootcontext)
> > -                       return -ENOMEM;
> > +                       goto err_rootcontext;
> >         }
> >         if (src->defcontext) {
> >                 opts->defcontext = kstrdup(src->defcontext, GFP_KERNEL);
> >                 if (!opts->defcontext)
> > -                       return -ENOMEM;
> > +                       goto err_defcontext;
> >         }
> >         return 0;
> > +
> > +err_defcontext:
> > +       if (src->rootcontext)
> > +               kfree(opts->rootcontext);
> > +err_rootcontext:
> > +       if (src->context)
> > +               kfree(opts->context);
> > +err_context:
> > +       if (src->fscontext)
> > +               kfree(opts->fscontext);
> > +err_fscontext:
> > +       kfree(fc->security);

Also here you need to be careful to not double-free fc->security.
(Paul's pseudocode below already correctly resets it to NULL after
freeing.)

> > +
> > +       return -ENOMEM;
> >  }
>
> Thanks for catching this a providing a patch, however I think the
> memory cleanup can be made simpler, see the pseudo code below:
>
> static int selinux_fs_context_dup(...)
> {
>
>   fc->security = kzalloc(...);
>   if (!fc->security)
>     return -ENOMEM;
>
>   opts = fc->security;
>
>   if (src->fscontext) {
>     opts->fscontext = kstrdup(...);
>     if (!opts->fscontext)
>       goto err;
>   }
>
>   /* ... */
>
>   return 0;
>
>   err:
>     kfree(opts->fscontext);
>     kfree(opts->context);
>     kfree(opts->rootcontext);
>     kfree(opts->defcontext);
>     kfree(opts);
>     fc->security = NULL;
>     return -ENOMEM;
> }
>
> --
> paul moore
> www.paul-moore.com
>

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.