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
>
