Re: [PATCH 1/2] module: Correctly truncate sysfs sections output

[Jessica Yu]

+++ Kees Cook [06/08/20 23:35 -0700]:
> The only-root-readable /sys/module/$module/sections/$section files
> did not truncate their output to the available buffer size. While most
> paths into the kernfs read handlers end up using PAGE_SIZE buffers,
> it's possible to get there through other paths (e.g. splice, sendfile).
> Actually limit the output to the "count" passed into the read function,
> and report it back correctly. *sigh*
>
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Link: https://lore.kernel.org/lkml/20200805002015.GE23458@shao2-debian
> Fixes: ed66f991bb19 ("module: Refactor section attr into bin attribute")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Oof, thanks for fixing this!

Acked-by: Jessica Yu <jeyu@xxxxxxxxxx>

> ---
> kernel/module.c | 22 +++++++++++++++++++---
> 1 file changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/module.c b/kernel/module.c
> index aa183c9ac0a2..08c46084d8cc 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1520,18 +1520,34 @@ struct module_sect_attrs {
> 	struct module_sect_attr attrs[];
> };
>
> +#define MODULE_SECT_READ_SIZE (3 /* "0x", "\n" */ + (BITS_PER_LONG / 4))
> 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(battr, struct module_sect_attr, battr);
> +	char bounce[MODULE_SECT_READ_SIZE + 1];
> +	size_t wrote;
>
> 	if (pos != 0)
> 		return -EINVAL;
>
> -	return sprintf(buf, "0x%px\n",
> -		       kallsyms_show_value(file->f_cred) ? (void *)sattr->address : NULL);
> +	/*
> +	 * Since we're a binary read handler, we must account for the
> +	 * trailing NUL byte that sprintf will write: if "buf" is
> +	 * too small to hold the NUL, or the NUL is exactly the last
> +	 * byte, the read will look like it got truncated by one byte.
> +	 * Since there is no way to ask sprintf nicely to not write
> +	 * the NUL, we have to use a bounce buffer.
> +	 */
> +	wrote = scnprintf(bounce, sizeof(bounce), "0x%px\n",
> +			 kallsyms_show_value(file->f_cred)
> +				? (void *)sattr->address : NULL);
> +	count = min(count, wrote);
> +	memcpy(buf, bounce, count);
> +
> +	return count;
> }
>
> static void free_sect_attrs(struct module_sect_attrs *sect_attrs)
> @@ -1580,7 +1596,7 @@ static void add_sect_attrs(struct module *mod, const struct load_info *info)
> 			goto out;
> 		sect_attrs->nsections++;
> 		sattr->battr.read = module_sect_read;
> -		sattr->battr.size = 3 /* "0x", "\n" */ + (BITS_PER_LONG / 4);
> +		sattr->battr.size = MODULE_SECT_READ_SIZE;
> 		sattr->battr.attr.mode = 0400;
> 		*(gattr++) = &(sattr++)->battr;
> 	}
> --
> 2.25.1