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: <sta...@vger.kernel.org> # v6.15+
Signed-off-by: Aleksa Sarai <cyp...@cyphar.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
