linux-hardening - Re: [PATCH v9 03/15] kallsyms: Hide layout

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Date:   Wed, 5 Jan 2022 19:46:01 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     linux-hardening@...r.kernel.org, x86@...nel.org,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Ard Biesheuvel <ardb@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Bruce Schlobohm <bruce.schlobohm@...el.com>,
        Jessica Yu <jeyu@...nel.org>,
        kernel test robot <lkp@...el.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Marios Pomonis <pomonis@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Nicolas Pitre <nico@...xnic.net>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arch@...r.kernel.org, live-patching@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v9 03/15] kallsyms: Hide layout

On Thu, Dec 23, 2021 at 01:21:57AM +0100, Alexander Lobakin wrote:
> @@ -687,11 +697,12 @@ static void reset_iter(struct kallsym_iter *iter, loff_t new_pos)
>  	iter->name[0] = '\0';
>  	iter->nameoff = get_symbol_offset(new_pos);
>  	iter->pos = new_pos;
> -	if (new_pos == 0) {

	if (!iter->show_layout)
		return;

> +	if (iter->show_layout && new_pos == 0) {
>  		iter->pos_arch_end = 0;
>  		iter->pos_mod_end = 0;
>  		iter->pos_ftrace_mod_end = 0;
>  		iter->pos_bpf_end = 0;
> +		iter->pos_end = 0;
>  	}
>  }

...

> @@ -838,16 +860,54 @@ static int kallsyms_open(struct inode *inode, struct file *file)
>  	 * using get_symbol_offset for every symbol.
>  	 */
>  	struct kallsym_iter *iter;
> -	iter = __seq_open_private(file, &kallsyms_op, sizeof(*iter));
> -	if (!iter)
> -		return -ENOMEM;
> -	reset_iter(iter, 0);
> +	/*
> +	 * This fake iter is needed for the cases with unprivileged
> +	 * access. We need to know the exact number of symbols to
> +	 * randomize the display layout.
> +	 */
> +	struct kallsym_iter fake;
> +	size_t size = sizeof(*iter);
> +	loff_t pos;
> +
> +	fake.show_layout = true;
> +	reset_iter(&fake, 0);
>  
>  	/*
>  	 * Instead of checking this on every s_show() call, cache
>  	 * the result here at open time.
>  	 */
> -	iter->show_value = kallsyms_show_value(file->f_cred);
> +	fake.show_layout = kallsyms_show_value(file->f_cred);
> +	if (fake.show_layout)
> +		goto open;

There are those silly labels again:

	if (!fake.show_layout) {
		for (... )
			;
		size = ...
	}

	iter = __seq_open_private(...

> +
> +	for (pos = kallsyms_num_syms; update_iter_mod(&fake, pos); pos++)
> +		;
> +
> +	size = struct_size(iter, shuffled_pos, fake.pos_end + 1);
> +
> +open:
> +	iter = __seq_open_private(file, &kallsyms_op, size);
> +	if (!iter)
> +		return -ENOMEM;
> +
> +	iter->show_layout = fake.show_layout;
> +	reset_iter(iter, 0);
> +
> +	if (iter->show_layout)
> +		return 0;
> +
> +	/* Copy the bounds since they were already discovered above */
> +	iter->pos_arch_end = fake.pos_arch_end;
> +	iter->pos_mod_end = fake.pos_mod_end;
> +	iter->pos_ftrace_mod_end = fake.pos_ftrace_mod_end;
> +	iter->pos_bpf_end = fake.pos_bpf_end;
> +	iter->pos_end = fake.pos_end;
> +
> +	for (pos = 0; pos <= iter->pos_end; pos++)
> +		iter->shuffled_pos[pos] = pos;
> +
> +	shuffle_array(iter->shuffled_pos, iter->pos_end + 1);
> +
>  	return 0;
>  }

Thx.

-- 
Regards/Gruss,
    Boris.

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