linux-kernel - Re: [PATCH v2] security/selinux: fix potential memleak in error branch

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Date:   Thu, 9 Dec 2021 10:59:41 +0100
From:   Ondrej Mosnacek <omosnace@...hat.com>
To:     赵军奎 <bernard@...o.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        SElinux list <selinux@...r.kernel.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] security/selinux: fix potential memleak in error branch

On Tue, Dec 7, 2021 at 1:05 PM 赵军奎 <bernard@...o.com> wrote:
> -----邮件原件-----
> 发件人: bernard@...o.com <bernard@...o.com> 代表 Ondrej Mosnacek
> 发送时间: 2021年12月6日 17:11
> 收件人: 赵军奎 <bernard@...o.com>
> 抄送: Paul Moore <paul@...l-moore.com>; Stephen Smalley <stephen.smalley.work@...il.com>; Eric Paris <eparis@...isplace.org>; SElinux list <selinux@...r.kernel.org>; Linux kernel mailing list <linux-kernel@...r.kernel.org>
> 主题: Re: [PATCH v2] security/selinux: fix potential memleak in error branch
>
> On Mon, Dec 6, 2021 at 8:19 AM Bernard Zhao <bernard@...o.com> wrote:
> > This patch try to fix potential memleak in error branch.
> >
> > Signed-off-by: Bernard Zhao <bernard@...o.com>
> > ---
> >  security/selinux/hooks.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index
> > 62d30c0a30c2..8dc140399a23 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -983,18 +983,22 @@ static int selinux_sb_clone_mnt_opts(const
> > struct super_block *oldsb,  static int selinux_add_opt(int token,
> > const char *s, void **mnt_opts)  {
> >         struct selinux_mnt_opts *opts = *mnt_opts;
> > +       bool is_alloc_opts = false;
> >
> >         if (token == Opt_seclabel)      /* eaten and completely ignored */
> >                 return 0;
> >
> > +       if (!s)
> > +               return -ENOMEM;
> > +
> >         if (!opts) {
> >                 opts = kzalloc(sizeof(struct selinux_mnt_opts), GFP_KERNEL);
> >                 if (!opts)
> >                         return -ENOMEM;
> >                 *mnt_opts = opts;
> > +               is_alloc_opts = true;
> >         }
> > -       if (!s)
> > -               return -ENOMEM;
> > +
> >         switch (token) {
> >         case Opt_context:
> >                 if (opts->context || opts->defcontext) @@ -1019,6
> > +1023,8 @@ static int selinux_add_opt(int token, const char *s, void **mnt_opts)
> >         }
> >         return 0;
> >  Einval:
> > +       if (is_alloc_opts)
> > +               kfree(opts);
> >         pr_warn(SEL_MOUNT_FAIL_MSG);
> >         return -EINVAL;
> >  }
> > --
> > 2.33.1
>
> >The problem is a bit more tricky... As is, this patch will lead to double frees in some cases. Look at security_sb_eat_lsm_opts() callers, for example - some of them don't do anything when error is returned, some call
> >security_free_mnt_opts() on the opts regardless, some will let it store the opts in fc->security, where
> >put_fs_context() will eventually call security_free_mnt_opts() on them.
>
> >You need to at least *mnt_opts = NULL after kfree(opts), but it would be also nice to make the LSM hook callers more consistent in what they do in the error path and document the fact that *mnt_opts will be NULL
> >on error in lsm_hooks.h (in case of the sb_eat_lsm_opts hook).
> Hi Ondrej Mosnacek:
>
> Thanks for your comments!
> I am not sure if there is some gap, for this part " it would be also nice to make the LSM hook callers more consistent in what they do in the error path and document the fact that *mnt_opts will be NULL
> on error in lsm_hooks.h (in case of the sb_eat_lsm_opts hook)"
> I am not sure if this is OK:
> 116   * @sb_eat_lsm_opts:
> 117   *         Eat (scan @orig options) and save them in @mnt_opts.
> 118   *     If error is returned, then the *mnt_opts will be NULL.
> Please help to double check, thanks!

I'd prefer something like:

If the hook returns 0, the caller is responsible for destroying the
returned @mnt_opts using the @sb_free_mnt_opts hook. The LSMs must not
expect the callers to destroy @mnt_opts if the hook returns an error
and should always set it to NULL in such case.

(You may want to double-check that the other implementations of this
hook (i.e. security/smack/smack_lsm.c) follow that contract and fix
them if necessary.)

Thanks for your efforts to improve this!

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