linux-hardening - Re: [PATCH v9 02/15] livepatch: use `-z unique-symbol` if available to nuke pos-based search

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Date:   Mon, 3 Jan 2022 14:44:09 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
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>,
        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>,
        Borislav Petkov <bp@...en8.de>,
        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 02/15] livepatch: use `-z unique-symbol` if available
 to nuke pos-based search

> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -143,11 +143,13 @@ static int klp_find_callback(void *data, const char *name,
>  	args->count++;
>  
>  	/*
> -	 * Finish the search when the symbol is found for the desired position
> -	 * or the position is not defined for a non-unique symbol.
> +	 * Finish the search when unique symbol names are enabled
> +	 * or the symbol is found for the desired position or the
> +	 * position is not defined for a non-unique symbol.
>  	 */
> -	if ((args->pos && (args->count == args->pos)) ||
> -	    (!args->pos && (args->count > 1)))
> +	if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL) ||
> +	    (args->pos && args->count == args->pos) ||
> +	    (!args->pos && args->count > 1))
>  		return 1;
>  
>  	return 0;
> @@ -171,17 +173,21 @@ static int klp_find_object_symbol(const char *objname, const char *name,
>  
>  	/*
>  	 * Ensure an address was found. If sympos is 0, ensure symbol is unique;
> -	 * otherwise ensure the symbol position count matches sympos.
> +	 * otherwise ensure the symbol position count matches sympos. If the LD
> +	 * `-z unique` flag is enabled, sympos checks are not relevant.
>  	 */
> -	if (args.addr == 0)
> +	if (args.addr == 0) {
>  		pr_err("symbol '%s' not found in symbol table\n", name);
> -	else if (args.count > 1 && sympos == 0) {
> +	} else if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL)) {
> +		goto out_ok;
> +	} else if (args.count > 1 && sympos == 0) {
>  		pr_err("unresolvable ambiguity for symbol '%s' in object '%s'\n",
>  		       name, objname);
>  	} else if (sympos != args.count && sympos > 0) {
>  		pr_err("symbol position %lu for symbol '%s' in object '%s' not found\n",
>  		       sympos, name, objname ? objname : "vmlinux");
>  	} else {
> +out_ok:
>  		*addr = args.addr;
>  		return 0;
>  	}

I think it would be better to return to something like

if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL))
        sympos = 0;

in klp_find_object_symbol() just after kallsyms search is performed.

You would not need the above changes at all. I did not like it before when 
sympos clearing was proposed, but the situation was different back then. 
'-z unique-symbol' was not available. It has changed and we have a 
guarantee that symbols are unique now. There would not be any impact on 
the user.

What do you think?

Miroslav