linux-kernel - Re: [PATCH bpf-next 3/4] bpf: Introduce path iterator

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Message-ID: <vumjuw5ha6jtxtadsr5vwjtuneeqfg3vpydciczsn75qdg2ekv@464a4dxtxx27>
Date: Tue, 3 Jun 2025 11:46:22 +0200
From: Jan Kara <jack@...e.cz>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Song Liu <song@...nel.org>, 
	Alexei Starovoitov <alexei.starovoitov@...il.com>, Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>, 
	bpf <bpf@...r.kernel.org>, Linux-Fsdevel <linux-fsdevel@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, LSM List <linux-security-module@...r.kernel.org>, 
	Kernel Team <kernel-team@...a.com>, Andrii Nakryiko <andrii@...nel.org>, Eduard <eddyz87@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Christian Brauner <brauner@...nel.org>, 
	KP Singh <kpsingh@...nel.org>, Matt Bobrowski <mattbobrowski@...gle.com>, 
	Amir Goldstein <amir73il@...il.com>, repnop@...gle.com, Jeff Layton <jlayton@...nel.org>, 
	Josef Bacik <josef@...icpanda.com>, Günther Noack <gnoack@...gle.com>
Subject: Re: [PATCH bpf-next 3/4] bpf: Introduce path iterator

On Fri 30-05-25 16:20:39, Mickaël Salaün wrote:
> On Thu, May 29, 2025 at 10:05:59AM -0700, Song Liu wrote:
> > On Thu, May 29, 2025 at 9:57 AM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > [...]
> > > >
> > > > How about we describe this as:
> > > >
> > > > Introduce a path iterator, which safely (no crash) walks a struct path.
> > > > Without malicious parallel modifications, the walk is guaranteed to
> > > > terminate. The sequence of dentries maybe surprising in presence
> > > > of parallel directory or mount tree modifications and the iteration may
> > > > not ever finish in face of parallel malicious directory tree manipulations.
> > >
> > > Hold on. If it's really the case then is the landlock susceptible
> > > to this type of attack already ?
> > > landlock may infinitely loop in the kernel ?
> > 
> > I think this only happens if the attacker can modify the mount or
> > directory tree as fast as the walk, which is probably impossible
> > in reality.
> 
> Yes, so this is not an infinite loop but an infinite race between the
> kernel and a very fast malicious user space process with an infinite
> number of available nested writable directories, that would also require
> a filesystem (and a kernel) supporting infinite pathname length.

Well, you definitely don't need infinite pathname length. Example:

Have a dir hierarchy like:

  A
 / \
B   C
|
D

Start iterating from A/B/D, you climb up to A/B. In parallel atacker does:

mv A/B/ A/C/; mkdir A/B

Now by following parent you get to A/C. In parallel attaker does:

mv A/C/ A/B/; mkdir A/C

And now you are essentially where you've started so this can repeat
forever.

As others wrote this particular timing might be hard enough to hit for it
to not be a practical attack but I would not bet much on somebody not being
able to invent some variant that works, in particular with BPF iterator.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR