modules: inherit TAINT_PROPRIETARY_MODULE

If a TAINT_PROPRIETARY_MODULE exports symbol, inherit the taint flag
for all modules importing these symbols, and don't allow loading
symbols from TAINT_PROPRIETARY_MODULE modules if the module previously
imported gplonly symbols.  Add a anti-circumvention devices so people
don't accidentally get themselves into trouble this way.

Comment from Greg:
  "Ah, the proven-to-be-illegal "GPL Condom" defense :)"

[jeyu: pr_info -> pr_err and pr_warn as per discussion]
Link: http://lore.kernel.org/r/20200730162957.GA22469@lst.de
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jessica Yu <jeyu@kernel.org>

Author: Christoph Hellwig

include/linux/module.h

@@ -389,6 +389,7 @@ struct module {
 	unsigned int num_gpl_syms;
 	const struct kernel_symbol *gpl_syms;
 	const s32 *gpl_crcs;
+	bool using_gplonly_symbols;
 
 #ifdef CONFIG_UNUSED_SYMBOLS
 	/* unused exported symbols. */

kernel/module.c

@@ -1431,6 +1431,24 @@ static int verify_namespace_is_imported(const struct load_info *info,
 	return 0;
 }
 
+static bool inherit_taint(struct module *mod, struct module *owner)
+{
+	if (!owner || !test_bit(TAINT_PROPRIETARY_MODULE, &owner->taints))
+		return true;
+
+	if (mod->using_gplonly_symbols) {
+		pr_err("%s: module using GPL-only symbols uses symbols from proprietary module %s.\n",
+			mod->name, owner->name);
+		return false;
+	}
+
+	if (!test_bit(TAINT_PROPRIETARY_MODULE, &mod->taints)) {
+		pr_warn("%s: module uses symbols from proprietary module %s, inheriting taint.\n",
+			mod->name, owner->name);
+		set_bit(TAINT_PROPRIETARY_MODULE, &mod->taints);
+	}
+	return true;
+}
 
 /* Resolve a symbol for this module.  I.e. if we find one, record usage. */
 static const struct kernel_symbol *resolve_symbol(struct module *mod,
@@ -1456,6 +1474,14 @@ static const struct kernel_symbol *resolve_symbol(struct module *mod,
 	if (!sym)
 		goto unlock;
 
+	if (license == GPL_ONLY)
+		mod->using_gplonly_symbols = true;
+
+	if (!inherit_taint(mod, owner)) {
+		sym = NULL;
+		goto getname;
+	}
+
 	if (!check_version(info, name, mod, crc)) {
 		sym = ERR_PTR(-EINVAL);
 		goto getname;