linux-hardening - Re: [PATCH v10 02/15] livepatch: avoid position-based search if `-z unique-symbol` is available

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Date:   Wed, 16 Feb 2022 16:06:24 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        linux-hardening@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        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>,
        Christoph Hellwig <hch@....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>,
        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 v10 02/15] livepatch: avoid position-based search if `-z
 unique-symbol` is available

> > +++ 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;
> 
> There's no real need to do this.  The code already works as-is, even if
> there are no unique symbols.
> 
> Even if there are no duplicates, there's little harm in going through
> all the symbols anyway, to check for errors just in case something
> unexpected happened with the linking (unexpected duplicate) or the patch
> creation (unexpected sympos).  It's not a hot path, so performance isn't
> really a concern.

Correct.
 
> When the old linker versions eventually age out, we can then go strip
> out all the sympos stuff.

Yes.

> > @@ -169,6 +171,13 @@ static int klp_find_object_symbol(const char *objname, const char *name,
> >  	else
> >  		kallsyms_on_each_symbol(klp_find_callback, &args);
> >  
> > +	/*
> > +	 * If the LD's `-z unique-symbol` flag is available and enabled,
> > +	 * sympos checks are not relevant.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL))
> > +		sympos = 0;
> > +
> 
> Similarly, I don't see a need for this.  If the patch is legit then
> sympos should already be zero.  If not, an error gets reported and the
> patch fails to load.

My concern was that if the patch is not legit (that is, sympos is > 0 for 
some reason), the error would be really cryptic and would not help the 
user at all. So zeroing sympos seems to be a good idea to me. There is no 
harm and the change is very small and compact.

On the other hand, I do not insist on this.

Regards,
Miroslav