[PATCH v4 19/21] afs: Fix premature cell exposure through /afs

[David Howells <dhowells@redhat.com>]

AFS cell records are prematurely exposured through the /afs dynamic root by
virtue of adding them immediately to the net->cells_dyn_ino IDR when the
cell is allocated rather than when it is added to the lookup tree.  This
allows a candidate record to be accessed, even if it's actually a duplicate
or not published yet.

Fix this by not adding the cell to cells_dyn_ino until it's confirmed
non-duplicate and is being published.  A flag is then used to record
whether it is added to the IDR to make removal from the IDR conditional.

Closes: https://sashiko.dev/#/patchset/20260618155141.2513212-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
---
 fs/afs/cell.c     | 27 +++++++++++++++++----------
 fs/afs/internal.h |  1 +
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/fs/afs/cell.c b/fs/afs/cell.c
index 9d8937ae24e2..47a2645768d7 100644
--- a/fs/afs/cell.c
+++ b/fs/afs/cell.c
@@ -205,14 +205,7 @@ static struct afs_cell *afs_alloc_cell(struct afs_net *net,
 	cell->dns_source = vllist->source;
 	cell->dns_status = vllist->status;
 	smp_store_release(&cell->dns_lookup_count, 1); /* vs source/status */
-	down_write(&net->cells_lock);
-	ret = idr_alloc_cyclic(&net->cells_dyn_ino, cell,
-			       2, INT_MAX / 2, GFP_KERNEL);
-	up_write(&net->cells_lock);
-	if (ret < 0)
-		goto error;
 	atomic_inc(&net->cells_outstanding);
-	cell->dynroot_ino = ret;
 	cell->debug_id = atomic_inc_return(&cell_debug_id);
 
 	trace_afs_cell(cell->debug_id, 1, 0, afs_cell_trace_alloc);
@@ -306,6 +299,13 @@ struct afs_cell *afs_lookup_cell(struct afs_net *net,
 			goto cell_already_exists;
 	}
 
+	ret = idr_alloc_cyclic(&net->cells_dyn_ino, candidate,
+			       2, INT_MAX / 2, GFP_KERNEL);
+	if (ret < 0)
+		goto cant_alloc_ino;
+	candidate->dynroot_ino = ret;
+	set_bit(AFS_CELL_FL_HAVE_INO, &candidate->flags);
+
 	cell = candidate;
 	candidate = NULL;
 	afs_use_cell(cell, trace);
@@ -380,6 +380,11 @@ struct afs_cell *afs_lookup_cell(struct afs_net *net,
 	_leave(" = %p [cell]", cell);
 	return cell;
 
+cant_alloc_ino:
+	up_write(&net->cells_lock);
+	afs_put_cell(candidate, afs_cell_trace_put_candidate);
+	goto error_noput;
+
 cell_already_exists:
 	_debug("cell exists");
 	cell = cursor;
@@ -596,9 +601,11 @@ static void afs_destroy_cell_work(struct work_struct *work)
 	timer_delete_sync(&cell->management_timer);
 	cancel_work_sync(&cell->manager);
 
-	down_write(&cell->net->cells_lock);
-	idr_remove(&cell->net->cells_dyn_ino, cell->dynroot_ino);
-	up_write(&cell->net->cells_lock);
+	if (test_bit(AFS_CELL_FL_HAVE_INO, &cell->flags)) {
+		down_write(&cell->net->cells_lock);
+		idr_remove(&cell->net->cells_dyn_ino, cell->dynroot_ino);
+		up_write(&cell->net->cells_lock);
+	}
 
 	call_rcu(&cell->rcu, afs_cell_destroy);
 }
diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index 785c646856d7..601f01e5c15f 100644
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -388,6 +388,7 @@ struct afs_cell {
 #define AFS_CELL_FL_NO_GC	0		/* The cell was added manually, don't auto-gc */
 #define AFS_CELL_FL_DO_LOOKUP	1		/* DNS lookup requested */
 #define AFS_CELL_FL_CHECK_ALIAS	2		/* Need to check for aliases */
+#define AFS_CELL_FL_HAVE_INO	3		/* Have dynroot_ino */
 	enum afs_cell_state	state;
 	short			error;
 	enum dns_record_source	dns_source:8;	/* Latest source of data from lookup */