linux-kernel - [PATCH 1/2] open_tree_attr: do not allow id-mapping changes without OPEN_TREE

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Message-Id: <20250808-open_tree_attr-bugfix-idmap-v1-1-0ec7bc05646c@cyphar.com>
Date: Fri, 08 Aug 2025 03:55:05 +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 1/2] 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.

Fixes: 2462651ffa76 ("fs: allow changing idmappings")
Cc: <stable@...r.kernel.org> # v6.15+
Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
 fs/namespace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 55f28cebbe7d..196a7873897b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5302,7 +5302,8 @@ SYSCALL_DEFINE5(open_tree_attr, int, dfd, const char __user *, filename,
 		int ret;
 		struct mount_kattr kattr = {};

-		kattr.kflags = MOUNT_KATTR_IDMAP_REPLACE;
+		if (flags & OPEN_TREE_CLONE)
+			kattr.kflags = MOUNT_KATTR_IDMAP_REPLACE;
 		if (flags & AT_RECURSIVE)
 			kattr.kflags |= MOUNT_KATTR_RECURSE;

-- 
2.50.1