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

[Rusty Russell]

Jessica Yu <jeyu@xxxxxxxxxx> writes:
> 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>

I would prefer a "bool after_init" flag to module_enable_ro().  It's
more explicit.

Exposing the flags via uapi looks like a wart, but it's kind of a
feature, since we don't *unset* it in any section; userspace may want to
know about it.

If you could re-spin, that would be great.

Thanks!
Rusty.

> ---
>  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
>  #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);
>  	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