Re: [PATCH v7 13/14] module: Do not set nx for module memory before freeing

[Andrea Parri]

On Tue, Dec 04, 2018 at 05:34:07PM -0800, Nadav Amit wrote:
> When module memory is about to be freed, there is no apparent reason to
> make it (and its data) executable, but that's exactly what is done
> today. This is not efficient and not secure.

Looks to me like you forgot to Cc the maintainer of this file: doing it
now.  The same consideration would hold for 14/14.

  Andrea


> 
> There are various theories why it was done, but none of them seem as
> something that really require it today. nios2 uses kmalloc for module
> memory, but anyhow it does not change the PTEs of the module memory.  In
> x86, changing vmalloc'd memory mappings also modifies the direct mapping
> alias, but the NX-bit is not modified in such way.
> 
> So let's remove it. Andy suggested that the changes of the PTEs can be
> avoided (excluding the direct-mapping alias), which is true. However,
> in x86 it requires some cleanup of the contiguous page allocator, which
> is outside of the scope of this patch-set.
> 
> Cc: Rick P Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> ---
>  kernel/module.c | 35 ++++++++++++++++++++++-------------
>  1 file changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 7cb207249437..57c5b23746e7 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2027,20 +2027,29 @@ void set_all_modules_text_ro(void)
>  	mutex_unlock(&module_mutex);
>  }
>  
> -static void disable_ro_nx(const struct module_layout *layout)
> +static void module_restore_mappings(const struct module_layout *layout)
>  {
> -	if (rodata_enabled) {
> -		frob_text(layout, set_memory_rw);
> -		frob_rodata(layout, set_memory_rw);
> -		frob_ro_after_init(layout, set_memory_rw);
> -	}
> -	frob_rodata(layout, set_memory_x);
> -	frob_ro_after_init(layout, set_memory_x);
> -	frob_writable_data(layout, set_memory_x);
> +	/*
> +	 * First, make the mappings of the code non-executable to prevent
> +	 * transient W+X mappings from being set when the text is set as RW.
> +	 */
> +	frob_text(layout, set_memory_nx);
> +
> +	if (!rodata_enabled)
> +		return;
> +
> +	/*
> +	 * Second, set the memory as writable. Although the module memory is
> +	 * about to be freed, these calls are required (at least on x86) to
> +	 * restore the direct map to its "correct" state.
> +	 */
> +	frob_text(layout, set_memory_rw);
> +	frob_rodata(layout, set_memory_rw);
> +	frob_ro_after_init(layout, set_memory_rw);
>  }
>  
>  #else
> -static void disable_ro_nx(const struct module_layout *layout) { }
> +static void module_restore_mappings(const struct module_layout *layout) { }
>  static void module_enable_nx(const struct module *mod) { }
>  static void module_disable_nx(const struct module *mod) { }
>  #endif
> @@ -2173,7 +2182,7 @@ static void free_module(struct module *mod)
>  	mutex_unlock(&module_mutex);
>  
>  	/* This may be empty, but that's OK */
> -	disable_ro_nx(&mod->init_layout);
> +	module_restore_mappings(&mod->init_layout);
>  	module_arch_freeing_init(mod);
>  	module_memfree(mod->init_layout.base);
>  	kfree(mod->args);
> @@ -2183,7 +2192,7 @@ static void free_module(struct module *mod)
>  	lockdep_free_key_range(mod->core_layout.base, mod->core_layout.size);
>  
>  	/* Finally, free the core (containing the module structure) */
> -	disable_ro_nx(&mod->core_layout);
> +	module_restore_mappings(&mod->core_layout);
>  	module_memfree(mod->core_layout.base);
>  }
>  
> @@ -3507,7 +3516,7 @@ static noinline int do_init_module(struct module *mod)
>  #endif
>  	module_enable_ro(mod, true);
>  	mod_tree_remove_init(mod);
> -	disable_ro_nx(&mod->init_layout);
> +	module_restore_mappings(&mod->init_layout);
>  	module_arch_freeing_init(mod);
>  	mod->init_layout.base = NULL;
>  	mod->init_layout.size = 0;
> -- 
> 2.17.1
>