linux-kernel - Safety of resolving untrusted paths with detached mount dirfd

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Message-ID: <87cy5eqgn8.fsf@alyssa.is>
Date: Wed, 19 Nov 2025 14:46:35 +0100
From: Alyssa Ross <hi@...ssa.is>
To: linux-fsdevel@...r.kernel.org
Cc: Demi Marie Obenour <demiobenour@...il.com>, Aleksa Sarai
 <cyphar@...har.com>, Jann Horn <jannh@...gle.com>, "Eric W. Biederman"
 <ebiederm@...ssion.com>, jlayton@...nel.org, Bruce Fields
 <bfields@...ldses.org>, Al Viro <viro@...iv.linux.org.uk>, Arnd Bergmann
 <arnd@...db.de>, shuah@...nel.org, David Howells <dhowells@...hat.com>,
 Andy Lutomirski <luto@...nel.org>, Christian Brauner <brauner@...nel.org>,
 Tycho Andersen <tycho@...ho.pizza>, linux-kernel@...r.kernel.org,
 linux-api@...r.kernel.org
Subject: Safety of resolving untrusted paths with detached mount dirfd

Hello,

As we know, it's not safe to use chroot() for resolving untrusted paths
within some root, as a subdirectory could be moved outside of the
process root while walking the path[1].  On the other hand,
LOOKUP_BENEATH is supposed to be robust against this, and going by [2],
it sounds like resolving with the mount namespace root as dirfd should
also be.

My question is: would resolving an untrusted path against a detached
mount root dirfd opened with OPEN_TREE_CLONE (not necessarily a
filesystem root) also be expected to be robust against traversal issues?
i.e. can I rely on an untrusted path never resolving to a path that
isn't under the mount root?

[1]: https://lore.kernel.org/lkml/CAG48ez30WJhbsro2HOc_DR7V91M+hNFzBP5ogRMZaxbAORvqzg@mail.gmail.com/
[2]: https://lore.kernel.org/lkml/C89D720F-3CC4-4FA9-9CBB-E41A67360A6B@amacapital.net/

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