From: Rusty Russell <rusty@rustcorp.com.au>
To: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Filipe Manana <fdmanana@suse.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        "linux-doc\@vger.kernel.org" <linux-doc@vger.kernel.org>,
        rgoldwyn@suse.com, hare <hare@suse.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Aaron Tomlin <atomlin@redhat.com>, rwright@hpe.com,
        Heinrich Schuchardt <xypron.glpk@gmx.de>,
        Michal Marek <mmarek@suse.com>, martin.wilck@suse.com,
        Jeff Mahoney <jeffm@suse.com>, Ingo Molnar <mingo@redhat.com>,
        Petr Mladek <pmladek@suse.com>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Guenter Roeck <linux@roeck-us.net>,
        "Eric W. Biederman" <ebiederm@xmission.com>, shuah@kernel.org,
        DSterba@suse.com, Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Miroslav Benes <mbenes@suse.cz>, NeilBrown <neilb@suse.com>,
        "linux-kernel\@vger.kernel.o rg" <linux-kernel@vger.kernel.org>,
        David Miller <davem@davemloft.net>, Jessica Yu <jeyu@redhat.com>,
        Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>,
        Julia Lawall <julia.lawall@lip6.fr>
Subject: Re: [RFC 10/10] kmod: add a sanity check on module loading
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References: <20161208184801.1689-1-mcgrof@kernel.org> <20161208194930.2816-1-mcgrof@kernel.org> <87k2b2kpdt.fsf@rustcorp.com.au> <20161216083123.GF13946@wotan.suse.de> <87zijvjjm1.fsf@rustcorp.com.au> <CAB=NE6VvuA9a6hf6yoopGfUxVJQM5HyV5bNzUdsEtUV0UhbG-g@mail.gmail.com> <87fuljjua3.fsf@rustcorp.com.au> <CAB=NE6UgFir2u2UKUSqoxFYyuY9rQyJAvifBGi3QU1u15J=ihQ@mail.gmail.com>
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Date: Wed, 21 Dec 2016 12:51:21 +1030
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"Luis R. Rodriguez" <mcgrof@kernel.org> writes:
> OK -- if userspace messes up again it may be a bit hard to prove
> unless we have a validation debug thing in place, would such a thing
> in debug form be reasonable ?

That makes perfect sense.  Untested hack:

diff --git a/fs/filesystems.c b/fs/filesystems.c
index c5618db110be..e5c90e80c7d3 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -275,9 +275,10 @@ struct file_system_type *get_fs_type(const char *name)
 	int len = dot ? dot - name : strlen(name);
 
 	fs = __get_fs_type(name, len);
-	if (!fs && (request_module("fs-%.*s", len, name) == 0))
+	if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
 		fs = __get_fs_type(name, len);
-
+		WARN_ONCE(!fs, "request_module fs-%.*s succeeded, but still no fs?\n", len, name);
+	}
 	if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {
 		put_filesystem(fs);
 		fs = NULL;

Maybe a similar hack for try_then_request_module(), but many places seem
to open-code request_module() so it's not as trivial...

Cheers,
Rusty.