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@vger.kernel.org # v6.15+
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/20250808-open_tree_attr-bugfix-idmap-v1-1-0ec7bc05646c@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/namespace.c b/fs/namespace.c
index ddfd4457d338372a6ee88f2fc54075eab25be3a8..ceb6b57e6a57939b7ec632d591a6e5707297b01f 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5176,7 +5176,8 @@ 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;