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From:   Kees Cook <keescook@chromium.org>
Date:   Tue, 13 Nov 2018 14:16:29 -0600
Message-ID: <CAGXu5jLCdWZ1qdTvo_VK0VT8AbGQ44wsKPypRPRzNVed7f1xeg@mail.gmail.com>
Subject: Re: [PATCH 1/2] exec: load_script: don't blindly truncate shebang string
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Ben Woodard <woodard@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Michal Hocko <mhocko@suse.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Nov 12, 2018 at 10:09 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> load_script() simply truncates bprm->buf and this is very wrong if the
> length of shebang string exceeds BINPRM_BUF_SIZE-2. This can silently
> truncate i_arg or (worse) we can execute the wrong binary if buf[2:126]
> happens to be the valid executable path.
>
> Change load_script() to return ENOEXEC if it can't find '\n' or zero in
> bprm->buf. Note that '\0' can come from either prepare_binprm()->memset()
> or from kernel_read(), we do not care.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  fs/binfmt_script.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
> index 7cde3f4..d0078cb 100644
> --- a/fs/binfmt_script.c
> +++ b/fs/binfmt_script.c
> @@ -42,10 +42,14 @@ static int load_script(struct linux_binprm *bprm)
>         fput(bprm->file);
>         bprm->file = NULL;
>
> -       bprm->buf[BINPRM_BUF_SIZE - 1] = '\0';
> -       if ((cp = strchr(bprm->buf, '\n')) == NULL)
> -               cp = bprm->buf+BINPRM_BUF_SIZE-1;
> +       for (cp = bprm->buf+2;; cp++) {
> +               if (cp >= bprm->buf + BINPRM_BUF_SIZE)
> +                       return -ENOEXEC;
> +               if (!*cp || (*cp == '\n'))
> +                       break;
> +       }
>         *cp = '\0';
> +
>         while (cp > bprm->buf) {
>                 cp--;
>                 if ((*cp == ' ') || (*cp == '\t'))
> --
> 2.5.0
>
>


-- 
Kees Cook