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

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Message-ID: <20250530.oh5pahH9Nui9@digikod.net>
Date: Fri, 30 May 2025 16:20:39 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Song Liu <song@...nel.org>
Cc: 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 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.

> 
> Thanks,
> Song
>