From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 042A223BCE3; Mon, 23 Jun 2025 10:16:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750673797; cv=none; b=lrl/HOcoOI3Rpq9s6A742gyDPZoOiMGDUoZwz7N6IAW0xN4Sa+zGkRyay3+BT+0vIl9EPj4tll71BDsjy+KHaAJv5Y+xdGltCfbX1fCa60K2I4bDcACa/Y6LOeljs+MdHxoDHtUkbAALcQAOhbYhiK3k6VnN0q6wqo+9TZqdVd4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750673797; c=relaxed/simple; bh=uANWeMNyTEGUxGx9R6g80F8R/lxIBbDL+x6N92SBdcA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=JUnpS19orifCDmbUMPSk0rQ6C37fil2xLFr8MImo2MEnTUcOKw0ngffs/507nCbVXtT4AJvuOX7uca4qTo6pdODGKlulZz3h9T16w+Xrhq3K8PQqSgID0d+S2h3C2RE5IhWSRYjckIgqLiW9f8Y3C1qex5hS0yXsieTXo5Th9D0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kUjqUI2t; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kUjqUI2t" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BCE6EC4CEF0; Mon, 23 Jun 2025 10:16:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1750673796; bh=uANWeMNyTEGUxGx9R6g80F8R/lxIBbDL+x6N92SBdcA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=kUjqUI2tWmvPUPoMTkwJDfBa0vFFJLqGjdn+pzExsg2adnrlJ9aQi6TlF/4QMR/lW 2QXmBWrc7i8N7RFeJu39wki9n3pVEpGTCOi0ZpPJtUv6LNbbJ3BNjzNmJO0vexSjhH Y/b36coxu7XGxmkU8Hyefqemrb15U6fOREfQ++8wzL6B1EJj0ne3IyZRLBicjbgPIn Rg3FXxu2I5263anXonUzxyhQWRxCkfM54smFSWhOL/Zq5kfLzUE5wqU7ZaA5OFQV7r t+k/siDb5gQAv+lmeoLZT038qa06KHlD2yAS2P0IiIPxL0vJJoyvjkbKf7C9V9kHwD VMr35Yv8SS0Sw== Date: Mon, 23 Jun 2025 12:16:27 +0200 From: Christian Brauner To: Sean Christopherson Cc: Mike Rapoport , Vlastimil Babka , Shivank Garg , david@redhat.com, akpm@linux-foundation.org, paul@paul-moore.com, viro@zeniv.linux.org.uk, willy@infradead.org, pbonzini@redhat.com, tabba@google.com, afranji@google.com, ackerleytng@google.com, jack@suse.cz, hch@infradead.org, cgzones@googlemail.com, ira.weiny@intel.com, roypat@amazon.co.uk, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org Subject: Re: [PATCH] fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass Message-ID: <20250623-warmwasser-giftig-ff656fce89ad@brauner> References: <20250619073136.506022-2-shivankg@amd.com> <20250619-fixpunkt-querfeldein-53eb22d0135f@brauner> <20250619-ablichten-korpulent-0efe2ddd0ee6@brauner> Precedence: bulk X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On Fri, Jun 20, 2025 at 08:02:18AM -0700, Sean Christopherson wrote: > On Thu, Jun 19, 2025, Mike Rapoport wrote: > > On Thu, Jun 19, 2025 at 02:06:17PM +0200, Christian Brauner wrote: > > > On Thu, Jun 19, 2025 at 02:01:22PM +0300, Mike Rapoport wrote: > > > > On Thu, Jun 19, 2025 at 12:38:25PM +0200, Christian Brauner wrote: > > > > > On Thu, Jun 19, 2025 at 11:13:49AM +0200, Vlastimil Babka wrote: > > > > > > On 6/19/25 09:31, Shivank Garg wrote: > > > > > > > Export anon_inode_make_secure_inode() to allow KVM guest_memfd to create > > > > > > > anonymous inodes with proper security context. This replaces the current > > > > > > > pattern of calling alloc_anon_inode() followed by > > > > > > > inode_init_security_anon() for creating security context manually. > > > > > > > > > > > > > > This change also fixes a security regression in secretmem where the > > > > > > > S_PRIVATE flag was not cleared after alloc_anon_inode(), causing > > > > > > > LSM/SELinux checks to be bypassed for secretmem file descriptors. > > > > > > > > > > > > > > As guest_memfd currently resides in the KVM module, we need to export this > > > > > > > > > > > > Could we use the new EXPORT_SYMBOL_GPL_FOR_MODULES() thingy to make this > > > > > > explicit for KVM? > > > > > > > > > > Oh? Enlighten me about that, if you have a second, please. > > > > > > > > From Documentation/core-api/symbol-namespaces.rst: > > > > > > > > The macro takes a comma separated list of module names, allowing only those > > > > modules to access this symbol. Simple tail-globs are supported. > > > > > > > > For example:: > > > > > > > > EXPORT_SYMBOL_GPL_FOR_MODULES(preempt_notifier_inc, "kvm,kvm-*") > > > > > > > > will limit usage of this symbol to modules whoes name matches the given > > > > patterns. > > > > > > Is that still mostly advisory and can still be easily circumenvented? > > Yes and no. For out-of-tree modules, it's mostly advisory. Though I can imagine > if someone tries to report a bug because their module is masquerading as e.g. kvm, > then they will be told to go away (in far less polite words :-D). > > For in-tree modules, the restriction is much more enforceable. Renaming a module > to circumvent a restricted export will raise major red flags, and getting "proper" > access to a symbol would require an ack from the relevant maintainers. E.g. for > many KVM-induced exports, it's not that other module writers are trying to misbehave, > there simply aren't any guardrails to deter them from using a "dangerous" export. > > The other big benefit I see is documentation, e.g. both for readers/developers to > understand the intent, and for auditing purposes (I would be shocked if there > aren't exports that were KVM-induced, but that are no longer necessary). > > And we can utilize the framework to do additional hardening. E.g. for exports > that exist solely for KVM, I plan on adding wrappers so that the symbols are > exproted if and only if KVM is enabled in the kernel .config[*]. Again, that's > far from perfect, e.g. AFAIK every distro enables KVM, but it should help keep > everyone honest. > > [*] https://lore.kernel.org/all/ZzJOoFFPjrzYzKir@google.com > > > The commit message says > > > > will limit the use of said function to kvm.ko, any other module trying > > to use this symbol will refure to load (and get modpost build > > failures). > > To Christian's point, the restrictions are trivial to circumvent by out-of-tree > modules. E.g. to get access to the above, simply name your module kvm-lol.ko or > whatever. Thanks for all the details! I'm more than happy to switch a bunch of our exports so that we only allow them for specific modules. But for that we also need EXPOR_SYMBOL_FOR_MODULES() so we can switch our non-gpl versions.