linux-kernel - Re: [PATCH] exec: load_script: Allow interpreter argument truncation

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Date:   Thu, 14 Feb 2019 17:08:21 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Samuel Dionne-Riel <samuel@...nne-riel.com>,
        Richard Weinberger <richard.weinberger@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Graham Christensen <graham@...hamc.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] exec: load_script: Allow interpreter argument truncation

On 02/13, Kees Cook wrote:
>
> While we want to make sure the kernel doesn't attempt to execute a
> truncated interpreter path, we must allow the interpreter arguments to
> be truncated. Perl, for example, will re-read the script itself to parse
> arguments correctly.

Heh. I still think that 8099b047ecc4 does the right thing.

But I can't argue with the fact that it caused the regression, so it should
be reverted.

> This documents the parsing steps, and will fail to exec if the string was
> truncated with neither an end-of-line nor any trailing whitespace.

You know, I have already spent 3 hours trying to write something simple and
clear, but failed. Still trying...

Nor I can really understand your fix ;) Will try to read it again, just one
question for now,

>  	for (cp = bprm->buf+2;; cp++) {
> -		if (cp >= bprm->buf + BINPRM_BUF_SIZE)
> -			return -ENOEXEC;
> -		if (!*cp || (*cp == '\n'))
> +		if (cp == bprm->buf + BINPRM_BUF_SIZE - 1) {
> +			truncated = true;

Off-by-one, no? "bprm->buf + BINPRM_BUF_SIZE - 1" is the very last char, it can
be '\n' or '\0', this should set end_of_interp.

>  			break;
> +		}
> +		if (!*cp || (*cp == '\n')) {
> +			end_of_interp = true;
> +			break;
> +		}

so unless I am totally confused you should move this block up before the
"bprm->buf + BINPRM_BUF_SIZE - 1" check or that check should use
"bprm->buf + BINPRM_BUF_SIZE".

No?

Oleg.