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References: <20211206071859.324729-1-bernard@vivo.com>
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From:   Ondrej Mosnacek <omosnace@redhat.com>
Date:   Mon, 6 Dec 2021 10:11:28 +0100
Message-ID: <CAFqZXNs1QzHh6Kekg76PkgUHk1a8_awB0xBqD-=Y9sH=cYz5UQ@mail.gmail.com>
Subject: Re: [PATCH v2] security/selinux: fix potential memleak in error branch
To:     Bernard Zhao <bernard@vivo.com>
Cc:     Paul Moore <paul@paul-moore.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
        Eric Paris <eparis@parisplace.org>,
        SElinux list <selinux@vger.kernel.org>,
        Linux kernel mailing list <linux-kernel@vger.kernel.org>
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On Mon, Dec 6, 2021 at 8:19 AM Bernard Zhao <bernard@vivo.com> wrote:
> This patch try to fix potential memleak in error branch.
>
> Signed-off-by: Bernard Zhao <bernard@vivo.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).

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