linux-kernel - Re: [PATCH v7 14/14] module: Prevent module removal racing with text

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Date:   Thu, 6 Dec 2018 11:01:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux_dti@...oud.com, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v7 14/14] module: Prevent module removal racing with
 text_poke()

On Tue, Dec 04, 2018 at 05:34:08PM -0800, Nadav Amit wrote:
> It seems dangerous to allow code modifications to take place
> concurrently with module unloading. So take the text_mutex while the
> memory of the module is freed.

Fun detail, only x86 seems to actually take text_mutex while poking
text.

> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
>  kernel/module.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 57c5b23746e7..b45754961143 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -64,6 +64,7 @@
>  #include <linux/bsearch.h>
>  #include <linux/dynamic_debug.h>
>  #include <linux/audit.h>
> +#include <linux/memory.h>
>  #include <uapi/linux/module.h>
>  #include "module-internal.h"
>  
> @@ -2181,6 +2182,9 @@ static void free_module(struct module *mod)
>  	synchronize_sched();
>  	mutex_unlock(&module_mutex);
>  
> +	/* Protect against patching of the module while it is being removed */
> +	mutex_lock(&text_mutex);
> +
>  	/* This may be empty, but that's OK */
>  	module_restore_mappings(&mod->init_layout);
>  	module_arch_freeing_init(mod);
> @@ -2194,6 +2198,7 @@ static void free_module(struct module *mod)
>  	/* Finally, free the core (containing the module structure) */
>  	module_restore_mappings(&mod->core_layout);
>  	module_memfree(mod->core_layout.base);
> +	mutex_unlock(&text_mutex);
>  }
>  
>  void *__symbol_get(const char *symbol)
> -- 
> 2.17.1
>