linux-kernel - [PATCH 0/2] open_tree_attr: do not allow id-mapping changes without OPEN_TREE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20250808-open_tree_attr-bugfix-idmap-v1-0-0ec7bc05646c@cyphar.com>
Date: Fri, 08 Aug 2025 03:55:04 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Alexander Viro <viro@...iv.linux.org.uk>, 
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
 "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>, 
 Shuah Khan <shuah@...nel.org>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>, 
 stable@...r.kernel.org
Subject: [PATCH 0/2] open_tree_attr: do not allow id-mapping changes
 without OPEN_TREE_CLONE

As described in commit 7a54947e727b ('Merge patch series "fs: allow
changing idmappings"'), open_tree_attr(2) was necessary in order to
allow for a detached mount to be created and have its idmappings changed
without the risk of any racing threads operating on it. For this reason,
mount_setattr(2) still does not allow for id-mappings to be changed.

However, there was a bug in commit 2462651ffa76 ("fs: allow changing
idmappings") which allowed users to bypass this restriction by calling
open_tree_attr(2) *without* OPEN_TREE_CLONE.

can_idmap_mount() prevented this bug from allowing an attached
mountpoint's id-mapping from being modified (thanks to an is_anon_ns()
check), but this still allows for detached (but visible) mounts to have
their be id-mapping changed. This risks the same UAF and locking issues
as described in the merge commit, and was likely unintentional.

For what it's worth, I found this while working on the open_tree_attr(2)
man page, and was trying to figure out what open_tree_attr(2)'s
behaviour was in the (slightly fruity) ~OPEN_TREE_CLONE case.

Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
Aleksa Sarai (2):
      open_tree_attr: do not allow id-mapping changes without OPEN_TREE_CLONE
      selftests/mount_setattr: add smoke tests for open_tree_attr(2) bug

 fs/namespace.c                                     |  3 +-
 .../selftests/mount_setattr/mount_setattr_test.c   | 77 ++++++++++++++++++----
 2 files changed, 66 insertions(+), 14 deletions(-)
---
base-commit: 66639db858112bf6b0f76677f7517643d586e575
change-id: 20250808-open_tree_attr-bugfix-idmap-bb741166dc04

Best regards,
-- 
Aleksa Sarai <cyphar@...har.com>