Re: [PATCH 10/10] module loader: enforce symbol import protection

[Petr Pavlu <petr.pavlu@suse.com>]

On 8/29/25 12:54 PM, Siddharth Nayyar wrote:
> The module loader will reject unsigned modules from loading if such a
> module attempts to import a symbol which has the import protection bit
> set in the kflagstab entry for the symbol.
> 
> Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
> ---
> [...]
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 4437c2a451ea..ece074a6ba7b 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -380,6 +380,7 @@ static bool find_exported_symbol_in_section(const struct symsearch *syms,
>  	fsa->crc = symversion(syms->crcs, sym - syms->start);
>  	fsa->sym = sym;
>  	fsa->license = (sym_flags & KSYM_FLAG_GPL_ONLY) ? GPL_ONLY : NOT_GPL_ONLY;
> +	fsa->is_protected = sym_flags & KSYM_FLAG_PROTECTED;
>  
>  	return true;
>  }
> @@ -1273,6 +1274,11 @@ static const struct kernel_symbol *resolve_symbol(struct module *mod,
>  		goto getname;
>  	}
>  
> +	if (fsa.is_protected && !mod->sig_ok) {
> +		fsa.sym = ERR_PTR(-EACCES);
> +		goto getname;
> +	}
> +
>  getname:
>  	/* We must make copy under the lock if we failed to get ref. */
>  	strscpy(ownername, module_name(fsa.owner), MODULE_NAME_LEN);

The is_protected check should be moved before the ref_module() call.
Adding a reference to another module should be always the last step,
after all symbol checks have been performed.

> @@ -1550,8 +1556,12 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
>  				break;
>  
>  			ret = PTR_ERR(ksym) ?: -ENOENT;
> -			pr_warn("%s: Unknown symbol %s (err %d)\n",
> -				mod->name, name, ret);
> +			if (ret == -EACCES)
> +				pr_warn("%s: Protected symbol %s (err %d)\n",
> +					mod->name, name, ret);
> +			else
> +				pr_warn("%s: Unknown symbol %s (err %d)\n",
> +					mod->name, name, ret);
>  			break;
>  
>  		default:

I suggest moving the error message about the symbol being protected down
into resolve_symbol(), at the point where this issue is detected. This
approach is generally used for other checks, such as the CRC or
namespace check. Additionally, I think it would make sense to change the
current "Unknown symbol" warning here to "Unresolved symbol" to be more
accurate.

-- 
Thanks,
Petr