Re: [PATCH 1/1] modules: add ro_after_init support

From: Kees Cook
Date: Tue Jun 14 2016 - 17:33:25 EST

On Mon, Jun 13, 2016 at 5:13 PM, Jessica Yu <jeyu@xxxxxxxxxx> wrote:
> Add ro_after_init support for modules by adding a new page-aligned section
> in the module layout (after rodata) for ro_after_init data and enabling RO
> protection for that section after module init runs.
>
> Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>

This looks awesome! Thanks for working on it.

> ---
> include/linux/module.h | 2 ++
> include/uapi/linux/elf.h | 1 +
> kernel/module.c | 73 +++++++++++++++++++++++++++++++++++++++++-------
> 3 files changed, 66 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index f777164..a698d23 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -311,6 +311,8 @@ struct module_layout {
> unsigned int text_size;
> /* Size of RO section of the module (text+rodata) */
> unsigned int ro_size;
> + /* Size of RO after init section */
> + unsigned int ro_after_init_size;
>
> #ifdef CONFIG_MODULES_TREE_LOOKUP
> struct mod_tree_node mtn;
> diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
> index cb4a72f..70b172ba 100644
> --- a/include/uapi/linux/elf.h
> +++ b/include/uapi/linux/elf.h
> @@ -286,6 +286,7 @@ typedef struct elf64_phdr {
> #define SHF_ALLOC 0x2
> #define SHF_EXECINSTR 0x4
> #define SHF_RELA_LIVEPATCH 0x00100000
> +#define SHF_RO_AFTER_INIT 0x00200000

This gives us some flexibility in the ELF flags, but it seems like
overkill since nothing will actually be setting it externally. I defer
to you and Rusty on this one. :)

> #define SHF_MASKPROC 0xf0000000
>
> /* special section indexes */
> diff --git a/kernel/module.c b/kernel/module.c
> index 7f21ab2..055bf6f 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1857,10 +1857,11 @@ static void mod_sysfs_teardown(struct module *mod)
> * from modification and any data from execution.
> *
> * General layout of module is:
> - * [text] [read-only-data] [writable data]
> - * text_size -----^ ^ ^
> - * ro_size ------------------------| |
> - * size -------------------------------------------|
> + * [text] [read-only-data] [ro-after-init] [writable data]
> + * text_size -----^ ^ ^ ^
> + * ro_size ------------------------| | |
> + * ro_after_init_size -----------------------------| |
> + * size -----------------------------------------------------------|
> *
> * These values are always page-aligned (as is base)
> */
> @@ -1883,14 +1884,24 @@ static void frob_rodata(const struct module_layout *layout,
> (layout->ro_size - layout->text_size) >> PAGE_SHIFT);
> }
>
> +static void frob_ro_after_init(const struct module_layout *layout,
> + int (*set_memory)(unsigned long start, int num_pages))
> +{
> + BUG_ON((unsigned long)layout->base & (PAGE_SIZE-1));
> + BUG_ON((unsigned long)layout->ro_size & (PAGE_SIZE-1));
> + BUG_ON((unsigned long)layout->ro_after_init_size & (PAGE_SIZE-1));
> + set_memory((unsigned long)layout->base + layout->ro_size,
> + (layout->ro_after_init_size - layout->ro_size) >> PAGE_SHIFT);
> +}
> +
> static void frob_writable_data(const struct module_layout *layout,
> int (*set_memory)(unsigned long start, int num_pages))
> {
> BUG_ON((unsigned long)layout->base & (PAGE_SIZE-1));
> - BUG_ON((unsigned long)layout->ro_size & (PAGE_SIZE-1));
> + BUG_ON((unsigned long)layout->ro_after_init_size & (PAGE_SIZE-1));
> BUG_ON((unsigned long)layout->size & (PAGE_SIZE-1));
> - set_memory((unsigned long)layout->base + layout->ro_size,
> - (layout->size - layout->ro_size) >> PAGE_SHIFT);
> + set_memory((unsigned long)layout->base + layout->ro_after_init_size,
> + (layout->size - layout->ro_after_init_size) >> PAGE_SHIFT);
> }
>
> /* livepatching wants to disable read-only so it can frob module. */
> @@ -1898,6 +1909,7 @@ void module_disable_ro(const struct module *mod)
> {
> frob_text(&mod->core_layout, set_memory_rw);
> frob_rodata(&mod->core_layout, set_memory_rw);
> + frob_ro_after_init(&mod->core_layout, set_memory_rw);
> frob_text(&mod->init_layout, set_memory_rw);
> frob_rodata(&mod->init_layout, set_memory_rw);
> }
> @@ -1908,11 +1920,16 @@ void module_enable_ro(const struct module *mod)
> frob_rodata(&mod->core_layout, set_memory_ro);
> frob_text(&mod->init_layout, set_memory_ro);
> frob_rodata(&mod->init_layout, set_memory_ro);
> +
> + /* after init */
> + if (mod->state == MODULE_STATE_LIVE)
> + frob_ro_after_init(&mod->core_layout, set_memory_ro);
> }
>
> static void module_enable_nx(const struct module *mod)
> {
> frob_rodata(&mod->core_layout, set_memory_nx);
> + frob_ro_after_init(&mod->core_layout, set_memory_nx);
> frob_writable_data(&mod->core_layout, set_memory_nx);
> frob_rodata(&mod->init_layout, set_memory_nx);
> frob_writable_data(&mod->init_layout, set_memory_nx);
> @@ -1921,6 +1938,7 @@ static void module_enable_nx(const struct module *mod)
> static void module_disable_nx(const struct module *mod)
> {
> frob_rodata(&mod->core_layout, set_memory_x);
> + frob_ro_after_init(&mod->core_layout, set_memory_x);
> frob_writable_data(&mod->core_layout, set_memory_x);
> frob_rodata(&mod->init_layout, set_memory_x);
> frob_writable_data(&mod->init_layout, set_memory_x);
> @@ -1963,6 +1981,8 @@ static void disable_ro_nx(const struct module_layout *layout)
> frob_text(layout, set_memory_rw);
> frob_rodata(layout, set_memory_rw);
> frob_rodata(layout, set_memory_x);
> + frob_ro_after_init(layout, set_memory_rw);
> + frob_ro_after_init(layout, set_memory_x);
> frob_writable_data(layout, set_memory_x);
> }
>
> @@ -2305,6 +2325,7 @@ static void layout_sections(struct module *mod, struct load_info *info)
> * finder in the two loops below */
> { SHF_EXECINSTR | SHF_ALLOC, ARCH_SHF_SMALL },
> { SHF_ALLOC, SHF_WRITE | ARCH_SHF_SMALL },
> + { SHF_RO_AFTER_INIT | SHF_ALLOC, ARCH_SHF_SMALL },
> { SHF_WRITE | SHF_ALLOC, ARCH_SHF_SMALL },
> { ARCH_SHF_SMALL | SHF_ALLOC, 0 }
> };
> @@ -2336,7 +2357,11 @@ static void layout_sections(struct module *mod, struct load_info *info)
> mod->core_layout.size = debug_align(mod->core_layout.size);
> mod->core_layout.ro_size = mod->core_layout.size;
> break;
> - case 3: /* whole core */
> + case 2: /* RO after init */
> + mod->core_layout.size = debug_align(mod->core_layout.size);
> + mod->core_layout.ro_after_init_size = mod->core_layout.size;
> + break;
> + case 4: /* whole core */
> mod->core_layout.size = debug_align(mod->core_layout.size);
> break;
> }
> @@ -2366,7 +2391,14 @@ static void layout_sections(struct module *mod, struct load_info *info)
> mod->init_layout.size = debug_align(mod->init_layout.size);
> mod->init_layout.ro_size = mod->init_layout.size;
> break;
> - case 3: /* whole init */
> + case 2:
> + /*
> + * RO after init doesn't apply to init_layout (only
> + * core_layout), so it just takes the value of ro_size.
> + */
> + mod->init_layout.ro_after_init_size = mod->init_layout.ro_size;
> + break;
> + case 4: /* whole init */
> mod->init_layout.size = debug_align(mod->init_layout.size);
> break;
> }
> @@ -3172,6 +3204,7 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
> {
> /* Module within temporary copy. */
> struct module *mod;
> + unsigned int ndx;
> int err;
>
> mod = setup_load_info(info, flags);
> @@ -3191,6 +3224,15 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
> /* We will do a special allocation for per-cpu sections later. */
> info->sechdrs[info->index.pcpu].sh_flags &= ~(unsigned long)SHF_ALLOC;
>
> + /*
> + * Mark ro_after_init section with SHF_RO_AFTER_INIT so that
> + * layout_sections() can put it in the right place.
> + * Note: ro_after_init sections also have SHF_{WRITE,ALLOC} set.
> + */
> + ndx = find_sec(info, ".data..ro_after_init");
> + if (ndx)
> + info->sechdrs[ndx].sh_flags |= SHF_RO_AFTER_INIT;
> +
> /* Determine total sizes, and put offsets in sh_entsize. For now
> this is done generically; there doesn't appear to be any
> special cases for the architectures. */
> @@ -3357,12 +3399,19 @@ static noinline int do_init_module(struct module *mod)
> /* Switch to core kallsyms now init is done: kallsyms may be walking! */
> rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
> #endif
> + /*
> + * RO and NX regions should already be protected at this point,
> + * so the below module_enable_ro() call enables additional RO
> + * protection for the ro_after_init section only.
> + */
> + module_enable_ro(mod);

My only thought here is that this seems like needless setting of all
the already-set regions. I'm fine with this personally, but I wonder
if maybe it would be better to just call the frob instead?

frob_ro_after_init(&mod->core_layout, set_memory_ro);


> mod_tree_remove_init(mod);
> disable_ro_nx(&mod->init_layout);
> module_arch_freeing_init(mod);
> mod->init_layout.base = NULL;
> mod->init_layout.size = 0;
> mod->init_layout.ro_size = 0;
> + mod->init_layout.ro_after_init_size = 0;
> mod->init_layout.text_size = 0;
> /*
> * We want to free module_init, but be aware that kallsyms may be
> @@ -3454,7 +3503,11 @@ static int complete_formation(struct module *mod, struct load_info *info)
> /* This relies on module_mutex for list integrity. */
> module_bug_finalize(info->hdr, info->sechdrs, mod);
>
> - /* Set RO and NX regions */
> + /*
> + * Set RO and NX regions. Since module is not LIVE yet,
> + * the ro_after_init section will remain RW until after
> + * do_one_initcall().
> + */
> module_enable_ro(mod);
> module_enable_nx(mod);
>
> --
> 2.4.3
>

Regardless, either way:

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

--
Kees Cook
Chrome OS & Brillo Security