[PATCH] landlock: Fix use-after-free of the source's parent directory

current_check_refer_path() reads old_dentry->d_parent without holding a
reference nor a lock on it, and then dereferences it in
collect_domain_accesses() and in the audit record.

A reference on a child does not pin its parent: __d_move() reassigns
dentry->d_parent and drops the reference the child held on its former
parent.  hook_path_rename() is not affected because the rename path calls
lock_rename() before the hook, so the source cannot be reparented under
it.  hook_path_link() has no such protection: do_linkat() holds a
reference on the source dentry but neither locks nor references its
parent, so a concurrent rename(2) can reparent the source while
security_path_link() runs, and the former parent can then be removed and
freed while the hook walks it.

Any process able to sandbox itself with LANDLOCK_ACCESS_FS_REFER can
trigger this with a linkat(2) loop racing rename(2) and rmdir(2):

  BUG: KASAN: slab-use-after-free in collect_domain_accesses+0x278/0x290
  Read of size 4 at addr ffff888160bd53f4 by task llrepro2/549
   collect_domain_accesses+0x278/0x290
   current_check_refer_path+0x952/0x1120
   security_path_link+0x1be/0x320
   filename_linkat+0x342/0x6d0
   __x64_sys_linkat+0xfa/0x150
  Freed by task 562:
   kmem_cache_free+0x139/0x4c0
   i_callback+0x4b/0x80
   rcu_core+0x7dc/0x10a0

Take a reference on the parent with dget_parent(), and release it once
the hierarchy walk and the audit record are done.

Cc: [email protected]
Fixes: b91c3e4ea756 ("landlock: Add support for file reparenting with LANDLOCK_ACCESS_FS_REFER")
Signed-off-by: Norbert Szetei <[email protected]>
---
 security/landlock/fs.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 30aa6ce13590..200c83372bbe 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -1298,11 +1298,12 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 	/*
 	 * old_dentry may be the root of the common mount point and
 	 * !IS_ROOT(old_dentry) at the same time (e.g. with open_tree() and
-	 * OPEN_TREE_CLONE).  We do not need to call dget(old_parent) because
-	 * we keep a reference to old_dentry.
+	 * OPEN_TREE_CLONE).  Pins the parent in both cases: a reference on
+	 * old_dentry does not pin its parent, which may then be freed after a
+	 * concurrent rename(2).
 	 */
-	old_parent = (old_dentry == mnt_dir.dentry) ? old_dentry :
-						      old_dentry->d_parent;
+	old_parent = (old_dentry == mnt_dir.dentry) ? dget(old_dentry) :
+						      dget_parent(old_dentry);
 
 	/* new_dir->dentry is equal to new_dentry->d_parent */
 	allow_parent1 = collect_domain_accesses(subject->domain, mnt_dir.dentry,
@@ -1311,8 +1312,10 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 	allow_parent2 = collect_domain_accesses(subject->domain, mnt_dir.dentry,
 						new_dir->dentry,
 						&layer_masks_parent2);
-	if (allow_parent1 && allow_parent2)
+	if (allow_parent1 && allow_parent2) {
+		dput(old_parent);
 		return 0;
+	}
 
 	/*
 	 * To be able to compare source and destination domain access rights,
@@ -1324,8 +1327,10 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 		    subject->domain, &mnt_dir, access_request_parent1,
 		    &layer_masks_parent1, &request1, old_dentry,
 		    access_request_parent2, &layer_masks_parent2, &request2,
-		    exchange ? new_dentry : NULL))
+		    exchange ? new_dentry : NULL)) {
+		dput(old_parent);
 		return 0;
+	}
 
 	if (request1.access) {
 		request1.audit.u.path.dentry = old_parent;
@@ -1335,6 +1340,7 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 		request2.audit.u.path.dentry = new_dir->dentry;
 		landlock_log_denial(subject, &request2);
 	}
+	dput(old_parent);
 
 	/*
 	 * This prioritizes EACCES over EXDEV for all actions, including
-- 
2.55.0