linux-kernel - Re: [PATCH v3 5/6] x86/ftrace: Use text

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Date:   Tue, 22 Oct 2019 10:45:23 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

On Wed, 16 Oct 2019, Peter Zijlstra wrote:

> On Wed, Oct 16, 2019 at 08:51:27AM +0200, Miroslav Benes wrote:
> > On Tue, 15 Oct 2019, Joe Lawrence wrote:
> > 
> > > On 10/15/19 10:13 AM, Miroslav Benes wrote:
> > > > Yes, it does. klp_module_coming() calls module_disable_ro() on all
> > > > patching modules which patch the coming module in order to call
> > > > apply_relocate_add(). New (patching) code for a module can be relocated
> > > > only when the relevant module is loaded.
> > > 
> > > FWIW, would the LPC blue-sky2 model (ie, Steve's suggestion @ plumber's where
> > > livepatches only patch a single object and updates are kept on disk to handle
> > > coming module updates as they are loaded) eliminate those outstanding
> > > relocations and the need to perform this late permission flipping?
> > 
> > Yes, it should, but we don't have to wait for it. PeterZ proposed a 
> > different solution to this specific issue in 
> > https://lore.kernel.org/lkml/20191015141111.GP2359@hirez.programming.kicks-ass.net/
> > 
> > It should not be a problem to create a live patch module like that and the 
> > code in kernel/livepatch/ is almost ready. Something like 
> > module_section_disable_ro(mod, section) (and similar for X protection) 
> > should be enough. Module reloads would still require juggling with the 
> > protections, but I think it is all feasible.
> 
> Something a little like so.. completely fresh of the keyboard.

Yes, but I noticed you found different and better way through text_poke() 
(I was not aware that text_poke() works around the protections).

Miroslav
 
> ---
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -853,6 +853,18 @@ static inline void module_enable_ro(cons
>  static inline void module_disable_ro(const struct module *mod) { }
>  #endif
>  
> +#if defined(CONFIG_STRICT_MODULE_RWX) && defined(CONFIG_LIVEPATCH)
> +extern void module_section_disable_ro(struct module *mod, const char *sec);
> +extern void module_section_enable_ro(struct module *mod, const char *sec);
> +extern void module_section_disable_x(struct module *mod, const char *sec);
> +extern void module_section_enable_x(struct module *mod, const char *sec);
> +#else
> +static inline void module_section_disable_ro(struct module *mod, const char *sec) { }
> +static inline void module_section_enable_ro(struct module *mod, const char *sec) { }
> +static inline void module_section_disable_x(struct module *mod, const char *sec) { }
> +static inline void module_section_enable_x(struct module *mod, const char *sec) { }
> +#endif
> +
>  #ifdef CONFIG_GENERIC_BUG
>  void module_bug_finalize(const Elf_Ehdr *, const Elf_Shdr *,
>  			 struct module *);
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2107,6 +2107,54 @@ static void free_module_elf(struct modul
>  	kfree(mod->klp_info->secstrings);
>  	kfree(mod->klp_info);
>  }
> +
> +#ifdef CONFIG_STRICT_MODULE_RWX
> +
> +static void __frob_section(struct Elf_Shdr *sec, int (*set_memory)(unsigned long start, int num_pages))
> +{
> +	BUG_ON((unsigned long)sec->sh_addr & (PAGE_SIZE-1));
> +	BUG_ON((unsigned long)sec->sh_size & (PAGE_SIZE-1));
> +	set_memory((unsigned long)sec->sh_addr, sec->sh_size >> PAGE_SHIFT);
> +}
> +
> +static void frob_section(struct module *mod, const char *section,
> +			 int (*set_memory)(unsigned long start, int num_pages))
> +{
> +	struct klp_modinfo *info = mod->klp_info;
> +	const char *secname;
> +	Elf_Shdr *s;
> +
> +	for (s = info->sechdrs; s < info->sechdrs + info->hdr.e_shnum; s++) {
> +		secname = mod->klp_info->secstrings + s->sh_name;
> +		if (strcmp(secname, section))
> +			continue;
> +
> +		__frob_section(s, set_memory);
> +	}
> +}
> +
> +void module_section_disable_ro(struct module *mod, const char *section)
> +{
> +	frob_section(mod, section, set_memory_rw);
> +}
> +
> +void module_section_enable_ro(struct module *mod, const char *section)
> +{
> +	frob_section(mod, section, set_memory_ro);
> +}
> +
> +void module_section_disable_x(struct module *mod, const char *section)
> +{
> +	frob_section(mod, section, set_memory_nx);
> +}
> +
> +void module_section_enable_x(struct module *mod, const char *section)
> +{
> +	frob_section(mod, section, set_memory_x);
> +}
> +
> +#endif /* ONFIG_STRICT_MODULE_RWX */
> +
>  #else /* !CONFIG_LIVEPATCH */
>  static int copy_module_elf(struct module *mod, struct load_info *info)
>  {
>