Subject: Re: [PATCH 2/5] module: Refactor section attr into bin attribute

From: Greg Kroah-Hartman <[email-masked]>
Date: Fri, 3 Jul 2020 08:02:07 +0200

On Thu, Jul 02, 2020 at 04:26:35PM -0700, Kees Cook wrote:
> In order to gain access to the open file's f_cred for kallsym visibility
> permission checks, refactor the module section attributes to use the
> bin_attribute instead of attribute interface. Additionally removes the
> redundant "name" struct member.
> 
> Cc: [email-masked]
> Signed-off-by: Kees Cook <[email-masked]>
> ---
>  kernel/module.c | 45 ++++++++++++++++++++++++---------------------
>  1 file changed, 24 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index a5022ae84e50..9e2954519259 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1510,8 +1510,7 @@ static inline bool sect_empty(const Elf_Shdr *sect)
>  }
>  
>  struct module_sect_attr {
> -	struct module_attribute mattr;
> -	char *name;
> +	struct bin_attribute battr;
>  	unsigned long address;
>  };
>  
> @@ -1521,11 +1520,16 @@ struct module_sect_attrs {
>  	struct module_sect_attr attrs[];
>  };
>  
> -static ssize_t module_sect_show(struct module_attribute *mattr,
> -				struct module_kobject *mk, char *buf)
> +static ssize_t module_sect_read(struct file *file, struct kobject *kobj,
> +				struct bin_attribute *battr,
> +				char *buf, loff_t pos, size_t count)
>  {
>  	struct module_sect_attr *sattr =
> -		container_of(mattr, struct module_sect_attr, mattr);
> +		container_of(battr, struct module_sect_attr, battr);
> +
> +	if (pos != 0)
> +		return -EINVAL;
> +
>  	return sprintf(buf, "0x%px\n", kptr_restrict < 2 ?
>  		       (void *)sattr->address : NULL);
>  }
> @@ -1535,7 +1539,7 @@ static void free_sect_attrs(struct module_sect_attrs *sect_attrs)
>  	unsigned int section;
>  
>  	for (section = 0; section < sect_attrs->nsections; section++)
> -		kfree(sect_attrs->attrs[section].name);
> +		kfree(sect_attrs->attrs[section].battr.attr.name);
>  	kfree(sect_attrs);
>  }
>  
> @@ -1544,42 +1548,41 @@ static void add_sect_attrs(struct module *mod, const struct load_info *info)
>  	unsigned int nloaded = 0, i, size[2];
>  	struct module_sect_attrs *sect_attrs;
>  	struct module_sect_attr *sattr;
> -	struct attribute **gattr;
> +	struct bin_attribute **gattr;
>  
>  	/* Count loaded sections and allocate structures */
>  	for (i = 0; i < info->hdr->e_shnum; i++)
>  		if (!sect_empty(&info->sechdrs[i]))
>  			nloaded++;
>  	size[0] = ALIGN(struct_size(sect_attrs, attrs, nloaded),
> -			sizeof(sect_attrs->grp.attrs[0]));
> -	size[1] = (nloaded + 1) * sizeof(sect_attrs->grp.attrs[0]);
> +			sizeof(sect_attrs->grp.bin_attrs[0]));
> +	size[1] = (nloaded + 1) * sizeof(sect_attrs->grp.bin_attrs[0]);
>  	sect_attrs = kzalloc(size[0] + size[1], GFP_KERNEL);
>  	if (sect_attrs == NULL)
>  		return;
>  
>  	/* Setup section attributes. */
>  	sect_attrs->grp.name = "sections";
> -	sect_attrs->grp.attrs = (void *)sect_attrs + size[0];
> +	sect_attrs->grp.bin_attrs = (void *)sect_attrs + size[0];
>  
>  	sect_attrs->nsections = 0;
>  	sattr = &sect_attrs->attrs[0];
> -	gattr = &sect_attrs->grp.attrs[0];
> +	gattr = &sect_attrs->grp.bin_attrs[0];
>  	for (i = 0; i < info->hdr->e_shnum; i++) {
>  		Elf_Shdr *sec = &info->sechdrs[i];
>  		if (sect_empty(sec))
>  			continue;
> +		sysfs_bin_attr_init(&sattr->battr);
>  		sattr->address = sec->sh_addr;
> -		sattr->name = kstrdup(info->secstrings + sec->sh_name,
> -					GFP_KERNEL);
> -		if (sattr->name == NULL)
> +		sattr->battr.attr.name =
> +			kstrdup(info->secstrings + sec->sh_name, GFP_KERNEL);
> +		if (sattr->battr.attr.name == NULL)
>  			goto out;
>  		sect_attrs->nsections++;
> -		sysfs_attr_init(&sattr->mattr.attr);
> -		sattr->mattr.show = module_sect_show;
> -		sattr->mattr.store = NULL;
> -		sattr->mattr.attr.name = sattr->name;
> -		sattr->mattr.attr.mode = S_IRUSR;
> -		*(gattr++) = &(sattr++)->mattr.attr;
> +		sattr->battr.read = module_sect_read;
> +		sattr->battr.size = 3 /* "0x", "\n" */ + (BITS_PER_LONG / 4);
> +		sattr->battr.attr.mode = 0400;
> +		*(gattr++) = &(sattr++)->battr;
>  	}
>  	*gattr = NULL;
>  
> @@ -1669,7 +1672,7 @@ static void add_notes_attrs(struct module *mod, const struct load_info *info)
>  			continue;
>  		if (info->sechdrs[i].sh_type == SHT_NOTE) {
>  			sysfs_bin_attr_init(nattr);
> -			nattr->attr.name = mod->sect_attrs->attrs[loaded].name;
> +			nattr->attr.name = mod->sect_attrs->attrs[loaded].battr.attr.name;
>  			nattr->attr.mode = S_IRUGO;
>  			nattr->size = info->sechdrs[i].sh_size;
>  			nattr->private = (void *) info->sechdrs[i].sh_addr;
> -- 
> 2.25.1
> 

They get a correct "size" value now, nice!

Reviewed-by: Greg Kroah-Hartman <[email-masked]>