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 828FE1FBF4F; Tue, 12 Nov 2024 13:57:22 +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=1731419842; cv=none; b=GTK6bklUUY5Omvq53b1X/iUky0rIbDwse2g+wZrUGV9aIlfz0IkEXZ6oxaUuAP4fqQbwnWm1z2/UHKwttFxfoL0vNkSRksZyCrQvT0Cm0A55Rbu1mFfqaboMOowJj3h0T68b2BvpWMp9jQIzYoCd36Gjclv+RrD2O2v6xbr77UA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731419842; c=relaxed/simple; bh=a/EmdG3xpINbbr99embeoJSOzr6JZsMWfCtX8ybCVm8=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Am2fw/M+oVXpdSdB4iw5LWe78Ff3RFD+sqOzBOX4hDCiLsQeN3wJ0mP6LSGVpQjTKtK/6+o3OWbcAruvqHILsuxg3H/aRWPkc28FzFEDodOCkyVeHp+HECg0dzZsygxvl4eLX50HeraXUVUx/CS/Q+hP25eHOZV+awSfOu7JEVA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=udSp44gA; 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="udSp44gA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D3DBC4CECD; Tue, 12 Nov 2024 13:57:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1731419842; bh=a/EmdG3xpINbbr99embeoJSOzr6JZsMWfCtX8ybCVm8=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=udSp44gAcuLSgTdyf67lvLTk4NdbZtxspMb2/5gYXaPthAfqzAkj77EV8xTGWQ5z2 Feucj3VjMyWjG/GsZYVlVvDYKg7OBLmo44bazfZS0rVozASrtNGJf0WRTvfwj1U4ji VXVn6Tk+uqrFktCo1RdyM06OF3q7nhHkX1EPtvjzM2aB+EiUOtVc76DqKYIV2VCU+b XVfjAZGiqdlSe8ICwCl+gmtDdCHBFR4zOsXjhjSib87NMaVh65c1W6VvQUF77VGVKC bKqNOB8lBlakMEcOMvM/nzoBWgvC3kq0J+WDIFwgkW7WsJ9p2bqjG4GJ04NuRY/RaE b3I2Zi4ssgRxw== Message-ID: <45e2da5392c07cfc139a014fbac512bfe14113a7.camel@kernel.org> Subject: Re: [PATCH 0/4] pidfs: implement file handle support From: Jeff Layton To: Christian Brauner , Erin Shepherd , Amir Goldstein Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, christian@brauner.io, paul@paul-moore.com, bluca@debian.org, Chuck Lever Date: Tue, 12 Nov 2024 08:57:20 -0500 In-Reply-To: <20241112-banknoten-ehebett-211d59cb101e@brauner> References: <20241101135452.19359-1-erin.shepherd@e43.eu> <20241112-banknoten-ehebett-211d59cb101e@brauner> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Tue, 2024-11-12 at 14:10 +0100, Christian Brauner wrote: > On Fri, Nov 01, 2024 at 01:54:48PM +0000, Erin Shepherd wrote: > > Since the introduction of pidfs, we have had 64-bit process identifiers= =20 > > that will not be reused for the entire uptime of the system. This great= ly=20 > > facilitates process tracking in userspace. > >=20 > > There are two limitations at present: > >=20 > > * These identifiers are currently only exposed to processes on 64-bit= =20 > > systems. On 32-bit systems, inode space is also limited to 32 bits a= nd=20 > > therefore is subject to the same reuse issues. We should really just move to storing 64-bit inode numbers internally on 32-bit machines. That would at least make statx() give you all 64 bits on 32-bit host. > > * There is no way to go from one of these unique identifiers to a pid = or=20 > > pidfd. > >=20 > > Patch 1 & 2 in this stack implements fh_export for pidfs. This means= =20 > > userspace can retrieve a unique process identifier even on 32-bit syst= ems=20 > > via name_to_handle_at. > >=20 > > Patch 3 & 4 in this stack implement fh_to_dentry for pidfs. This means= =20 > > userspace can convert back from a file handle to the corresponding pidf= d.=20 > > To support us going from a file handle to a pidfd, we have to store a p= id=20 > > inside the file handle. To ensure file handles are invariant and can mo= ve=20 > > between pid namespaces, we stash a pid from the initial namespace insid= e=20 > > the file handle. > >=20 > > I'm not quite sure if stashing an initial-namespace pid inside the file= =20 > > handle is the right approach here; if not, I think that patch 1 & 2 are= =20 > > useful on their own. Hmm... I guess pid namespaces don't have a convenient 64-bit ID like mount namespaces do? In that case, stashing the pid from init_ns is probably the next best thing. >=20 > Sorry for the delayed reply (I'm recovering from a lengthy illness.). >=20 > I like the idea in general. I think this is really useful. A few of my > thoughts but I need input from Amir and Jeff: >=20 > * In the last patch of the series you already implement decoding of > pidfd file handles by adding a .fh_to_dentry export_operations method. >=20 > There are a few things to consider because of how open_by_handle_at() > works. >=20 > - open_by_handle_at() needs to be restricted so it only creates pidfds > from pidfs file handles that resolve to a struct pid that is > reachable in the caller's pid namespace. In other words, it should > mirror pidfd_open(). >=20 > Put another way, open_by_handle_at() must not be usable to open > arbitrary pids to prevent a container from constructing a pidfd file > handle for a process that lives outside it's pid namespace > hierarchy. >=20 > With this restriction in place open_by_handle_at() can be available > to let unprivileged processes open pidfd file handles. >=20 > Related to that, I don't think we need to make open_by_handle_at() > open arbitrary pidfd file handles via CAP_DAC_READ_SEARCH. Simply > because any process in the initial pid namespace can open any other > process via pidfd_open() anyway because pid namespaces are > hierarchical. >=20 > IOW, CAP_DAC_READ_SEARCH must not override the restriction that the > provided pidfs file handle must be reachable from the caller's pid > namespace. >=20 > - open_by_handle_at() uses may_decode_fh() to determine whether it's > possible to decode a file handle as an unprivileged user. The > current checks don't make sense for pidfs. Conceptually, I think > there don't need to place any restrictions based on global > CAP_DAC_READ_SEARCH, owning user namespace of the superblock or > mount on pidfs file handles. >=20 > The only restriction that matters is that the requested pidfs file > handle is reachable from the caller's pid namespace. > > - A pidfd always has exactly a single inode and a single dentry. > There's no aliases. >=20 > - Generally, in my naive opinion, I think that decoding pidfs file > handles should be a lot simpler than decoding regular path based > file handles. Because there should be no need to verify any > ancestors, or reconnect paths. Pidfs also doesn't have directory > inodes, only regular inodes. In other words, any dentry is > acceptable. >=20 > Essentially, the only thing we need is for exportfs_decode_fh_raw() > to verify that the provided pidfs file handle is resolvable in the > caller's pid namespace. If so we're done. The challenge is how to > nicely plumb this into the code without it sticking out like a sore > thumb. >=20 > - Pidfs should not be exportable via NFS. It doesn't make sense. I haven't looked over the patchset yet, but those restrictions all sound pretty reasonable to me. Special casing the may_decode_fh permission checks may be the tricky bit. I'm not sure what that should look like, tbqh. --=20 Jeff Layton