6.16-stable review patch. If anyone has any objections, please let me know.
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From: Aleksa Sarai cyphar@cyphar.com
commit 9308366f062129d52e0ee3f7a019f7dd41db33df upstream.
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-v1-1-0ec7bc0564... Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/namespace.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
--- a/fs/namespace.c +++ b/fs/namespace.c @@ -5302,7 +5302,8 @@ SYSCALL_DEFINE5(open_tree_attr, int, dfd 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;