Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

[Jessica Yu]

+++ Mauro Carvalho Chehab [08/08/20 10:12 +0200]:
[snip]
> Right now, what happens is:
>
> 	# modprobe wlcore
> 	modprobe: ERROR: could not insert 'wlcore': Exec format error
>
> This seems to be failing for all modules, as doesn't show anything
> probed.
>
> Btw, IMO, it would be useful to have some pr_debug() infra in order to
> explain why insmod is failing, or to have more error codes used there,
> as nothing was printed at dmesg. That makes harder to debug issues
> there. I ended losing a lot of time yesterday rebuilding the Kernel
> and checking the FS, before deciding to add some printks inside the
> Kernel ;-)
>
> In order for modprobe to start working again, I had to apply this
> dirty hack:
>
> <hack>
> diff --git a/kernel/module.c b/kernel/module.c
> index 910a57640818..10d590dc48ad 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2051,11 +2051,12 @@ static int module_enforce_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
> 	const unsigned long shf_wx = SHF_WRITE|SHF_EXECINSTR;
> 	int i;
>
> +#if 0
> 	for (i = 0; i < hdr->e_shnum; i++) {
> 		if ((sechdrs[i].sh_flags & shf_wx) == shf_wx)
> 			return -ENOEXEC;
> 	}
> -
> +#endif
> 	return 0;
> }
> </hack>

All this hunk does it reject loading modules that have any sections
that have both the writable and executable flags. You're saying it's
happening for all modules on your setup - I am curious as to which
sections have both these flags - what does readelf -S tell you?

Thanks,

Jessica