Re: [PATCH] module: Harden STRICT_MODULE_RWX

Josh Poimboeuf
2020-04-03T16:56:43Z

On Fri, Apr 03, 2020 at 06:37:16PM +0200, Peter Zijlstra wrote:
>
> We're very close to enforcing W^X memory, refuse to load modules that
> violate this principle per construction.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Jessica Yu <jeyu@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> ---
> kernel/module.c | 24 ++++++++++++++++++++++++
> 1 file changed, 24 insertions(+)
>
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2044,9 +2044,28 @@ static void module_enable_x(const struct
> frob_text(&mod->core_layout, set_memory_x);
> frob_text(&mod->init_layout, set_memory_x);
> }
> +
> +static int module_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
> + char *secstrings, struct module *mod)

A verb would be nice: "module_enforce_rwx_sections"?

Shouldn't this be under STRICT_MODULE_RWX instead of
ARCH_HAS_STRICT_MODULE_RWX?

> +{
> + int i;
> +
> + for (i = 0; i < hdr->e_shnum; i++) {
> + if (sechdrs[i].sh_flags & (SHF_EXECINSTR|SHF_WRITE))
> + return -ENOEXEC;

I think you only want the error when both are set?

if (sechdrs[i].sh_flags & (SHF_EXECINSTR|SHF_WRITE) == (SHF_EXECINSTR|SHF_WRITE))

--
Josh