linux-kernel - Re: [PATCH v3] selinux: lsm: fix a missing-check bug in selinux_sb_eat_lsm

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Message-ID: <CAFqZXNvBpmxNYjZx6YcH5Q-u4Tkwhfyzu_8VmEe8O7r9CCsvNg@mail.gmail.com>
Date:   Mon, 3 Jun 2019 09:23:32 +0200
From:   Ondrej Mosnacek <omosnace@...hat.com>
To:     Gen Zhang <blackgod016574@...il.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v3] selinux: lsm: fix a missing-check bug in selinux_sb_eat_lsm_opts()

On Sat, Jun 1, 2019 at 4:15 AM Gen Zhang <blackgod016574@...il.com> wrote:
> In selinux_sb_eat_lsm_opts(), 'arg' is allocated by kmemdup_nul(). It
> returns NULL when fails. So 'arg' should be checked. And 'mnt_opts'
> should be freed when error.
>
> Signed-off-by: Gen Zhang <blackgod016574@...il.com>
> Reviewed-by: Ondrej Mosnacek <omosnace@...hat.com>

It looks like you're new to the kernel development community, so let
me give you a bit of friendly advice for the future :)

You don't need to repost the patch when people give you
Acked-by/Reviewed-by/Tested-by (unless there is a different reason to
respin/repost the patches). The maintainer goes over the replies when
applying the final patch and adds Acked-by/Reviewed-by/... on his/her
own.

If you *do* need to respin a path for which you have received A/R/T,
then you need to distinguish between two cases:
1. Only trivial changes to the patch (only fixed typos, edited commit
message, removed empty line, etc. - for example, v1 -> v2 of this
patch falls into this category) - in this case you can collect the
A/R/T yourself and add them to the new version. This saves the
maintainer and the reviewers from redundant work, since the patch is
still semantically the same and the A/R/T from the last version still
apply.
2. Non-trivial changes to the patch (as is the case for this patch) -
in this case your patch needs to be reviewed again and you should
disregard all A/R/T from the previous version. You can easily piss
someone off if you add their Reviewed-by to a patch they haven't
actually reviewed, so be careful ;-)

(Someone please correct me if I got it wrong - this is what I gathered
so far from my experience.)

Good luck in your future work!

> Fixes: 99dbbb593fe6 ("selinux: rewrite selinux_sb_eat_lsm_opts()")
> ---
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 3ec702c..f329fc0 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2616,6 +2616,7 @@ static int selinux_sb_eat_lsm_opts(char *options, void **mnt_opts)
>         char *from = options;
>         char *to = options;
>         bool first = true;
> +       int ret;
>
>         while (1) {
>                 int len = opt_len(from);
> @@ -2635,15 +2636,16 @@ static int selinux_sb_eat_lsm_opts(char *options, void **mnt_opts)
>                                                 *q++ = c;
>                                 }
>                                 arg = kmemdup_nul(arg, q - arg, GFP_KERNEL);
> +                               if (!arg) {
> +                                       ret = -ENOMEM;
> +                                       goto free_opt;
> +                               }
>                         }
>                         rc = selinux_add_opt(token, arg, mnt_opts);
>                         if (unlikely(rc)) {
> +                               ret = rc;
>                                 kfree(arg);
> -                               if (*mnt_opts) {
> -                                       selinux_free_mnt_opts(*mnt_opts);
> -                                       *mnt_opts = NULL;
> -                               }
> -                               return rc;
> +                               goto free_opt;
>                         }
>                 } else {
>                         if (!first) {   // copy with preceding comma
> @@ -2661,6 +2663,12 @@ static int selinux_sb_eat_lsm_opts(char *options, void **mnt_opts)
>         }
>         *to = '\0';
>         return 0;
> +free_opt:
> +       if (*mnt_opts) {
> +               selinux_free_mnt_opts(*mnt_opts);
> +               *mnt_opts = NULL;
> +       }
> +       return ret;
>  }
>
>  static int selinux_sb_remount(struct super_block *sb, void *mnt_opts)

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Software Engineer, Security Technologies
Red Hat, Inc.