Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

[Jessica Yu]

+++ Peter Zijlstra [03/04/20 19:13 +0200]:
> 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@xxxxxxxxxxxxx>
> ---
> kernel/module.c |   25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2044,9 +2044,29 @@ 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_enforce_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
> +				       char *secstrings, struct module *mod)
> +{
> +	const unsigned long shf_wx = SHF_WRITE|SHF_EXECINSTR;
> +	int i;
> +
> +	for (i = 0; i < hdr->e_shnum; i++) {
> +		if ((sechdrs[i].sh_flags & shf_wx) == shf_wx)
> +			return -ENOEXEC;
> +	}
> +
> +	return 0;
> +}

Just to clarify, did we want to enforce this only when
CONFIG_STRICT_MODULE_RWX=y?  Because here it's still in the
CONFIG_ARCH_HAS_STRICT_MODULE_RWX block.

Unfortunately, when we add module_enforce_rwx_sections() in the
CONFIG_STRICT_MODULE_RWX block, we'll need two empty stubs, one for
!CONFIG_ARCH_HAS_STRICT_MODULE_RWX and one for !CONFIG_STRICT_MODULE_RWX.

This is because the CONFIG_STRICT_MODULE_RWX block is currently nested
within ARCH_HAS_STRICT_MODULE_RWX :/

That unholy nested ifdef mess needs to be fixed. I will post a patch
shortly that gets rid of this nesting and you could rebase on top of
it if you want to move module_enforce_rwx_sections() under
CONFIG_STRICT_MODULE_RWX. Then you'll only need to provide one empty
stub for !CONFIG_STRICT_MODULE_RWX.

Thanks,

Jessica