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

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Date:   Tue, 7 Dec 2021 12:05:49 +0000
From:   赵军奎 <bernard@...o.com>
To:     Ondrej Mosnacek <omosnace@...hat.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: 答复: [PATCH v2] security/selinux: fix potential memleak in error branch

-----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!

BR//Bernard
--
>Ondrej Mosnacek
>Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.