linux-kernel - Re: [PATCH] arm64/module: use mod->klp

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Message-ID: <20181025080816.525dppcfrrevf6jc@pathway.suse.cz>
Date:   Thu, 25 Oct 2018 10:08:16 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Torsten Duwe <duwe@....de>, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Julien Thierry <julien.thierry@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH] arm64/module: use mod->klp_info section header
 information

On Tue 2018-10-23 19:55:54, Jessica Yu wrote:
> The arm64 module loader keeps a pointer into info->sechdrs to keep track
> of section header information for .plt section(s). A pointer to the
> relevent section header (struct elf64_shdr) in info->sechdrs is stored
> in mod->arch.{init,core}.plt. This pointer may be accessed while
> applying relocations in apply_relocate_add() for example. And unlike
> normal modules, livepatch modules can call apply_relocate_add() after
> module load. But the info struct (and therefore info->sechdrs) gets
> freed at the end of load_module() and so mod->arch.{init,core}.plt
> becomes an invalid pointer after the module is done loading.
> 
> Luckily, livepatch modules already keep a copy of Elf section header
> information in mod->klp_info. So make sure livepatch modules on arm64
> have access to the section headers in klp_info and set
> mod->arch.{init,core}.plt to the appropriate section header in
> mod->klp_info so that they can call apply_relocate_add() even after
> module load.
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index f475f30eed8c..f3ac04cc9fc3 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3367,6 +3367,8 @@ int __weak module_finalize(const Elf_Ehdr *hdr,
> 
> static int post_relocation(struct module *mod, const struct load_info *info)
> {
> +	int err;
> +
> 	/* Sort exception table now relocations are done. */
> 	sort_extable(mod->extable, mod->extable + mod->num_exentries);
> 
> @@ -3377,8 +3379,18 @@ static int post_relocation(struct module *mod, const struct load_info *info)
> 	/* Setup kallsyms-specific fields. */
> 	add_kallsyms(mod, info);
> 
> +	if (is_livepatch_module(mod)) {
> +		err = copy_module_elf(mod, info);
> +		if (err < 0)
> +			return err;
> +	}
> +
> 	/* Arch-specific module finalizing. */
> -	return module_finalize(info->hdr, info->sechdrs, mod);
> +	err = module_finalize(info->hdr, info->sechdrs, mod);
> +	if (err < 0)

	if (err < 0 && is_livepatch_module(mod))

> +		free_module_elf(mod);
> +
> +	return err;
> }

Also we need to free the copied stuff in load_module() when
anything called after post_relocation() fails. I think
that the following would work:

--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3823,6 +3823,8 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	kfree(mod->args);
  free_arch_cleanup:
 	module_arch_cleanup(mod);
+	if (is_livepatch_module(mod))
+		free_module_elf(mod);
  free_modinfo:
 	free_modinfo(mod);
  free_unload:


But I suggest to just move copy_module_elf() up and keep
calling it from load_module() directly. It would make
the error handling more clear.

Best Regards,
Petr