[PATCH v2] lockd: pin next file across nlm_inspect_file lock-drop

[Michael Bommarito <michael.bommarito@gmail.com>]

nlm_traverse_files() pins the current file with f_count++ across
a mutex_unlock for nlm_inspect_file(), but nothing pins the saved
next pointer.  A concurrent nlm_release_file() can kfree the next
file during the unlock window, and the iterator dereferences freed
memory on the next loop step.

Pin both current and next before the lock-drop.  Advance by
swapping the pinned cursors at the end of each iteration so next
is always held alive across the unlock.

Only call nlm_file_release() for files that matched the predicate
and were inspected.  Skipped files just get f_count-- to undo the
iteration pin; their f_locks is stale and must not drive cleanup.

Cc: stable@vger.kernel.org
Fixes: 01df9c5e918a ("LOCKD: Fix a deadlock in nlm_traverse_files()")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 fs/lockd/svcsubs.c | 64 +++++++++++++++++++++++++++++++---------------
 1 file changed, 44 insertions(+), 20 deletions(-)


Changes since v1:
 - Fixed premature kfree of non-matching files: nlm_file_release()
   is now called only for files that matched the predicate and were
   inspected.  Non-matching files just get f_count-- to undo the
   iteration pin.  (Spotted by sashiko.dev automated review.)

Reproduced under UML + KASAN with 768 concurrent POSIX holders and
parallel /proc/fs/nfsd/unlock_filesystem writes.

Stock kernel:

  BUG: KASAN: slab-use-after-free in nlm_traverse_files+0x71d/0x9d0

  Allocated by: nlm_lookup_file via nlm4svc_proc_lock
  Freed by:     another nlm_traverse_files instance

Patched v2 UML kernel ran the same harness silently.

diff --git a/fs/lockd/svcsubs.c b/fs/lockd/svcsubs.c
index dd0214dcb6950..0b38125cf86ab 100644
--- a/fs/lockd/svcsubs.c
+++ b/fs/lockd/svcsubs.c
@@ -295,36 +295,60 @@ static void nlm_close_files(struct nlm_file *file)
 /*
  * Loop over all files in the file table.
  */
+static void nlm_file_release(struct nlm_file *file)
+{
+	if (list_empty(&file->f_blocks) && !file->f_locks
+	    && !file->f_shares && !file->f_count) {
+		hlist_del(&file->f_list);
+		nlm_close_files(file);
+		kfree(file);
+	}
+}
+
 static int
 nlm_traverse_files(void *data, nlm_host_match_fn_t match,
 		int (*is_failover_file)(void *data, struct nlm_file *file))
 {
-	struct hlist_node *next;
-	struct nlm_file	*file;
+	struct nlm_file *file, *next;
 	int i, ret = 0;
 
 	mutex_lock(&nlm_file_mutex);
 	for (i = 0; i < FILE_NRHASH; i++) {
-		hlist_for_each_entry_safe(file, next, &nlm_files[i], f_list) {
-			if (is_failover_file && !is_failover_file(data, file))
-				continue;
+		file = hlist_entry_safe(nlm_files[i].first,
+					struct nlm_file, f_list);
+		if (file)
 			file->f_count++;
-			mutex_unlock(&nlm_file_mutex);
-
-			/* Traverse locks, blocks and shares of this file
-			 * and update file->f_locks count */
-			if (nlm_inspect_file(data, file, match))
-				ret = 1;
-
-			mutex_lock(&nlm_file_mutex);
-			file->f_count--;
-			/* No more references to this file. Let go of it. */
-			if (list_empty(&file->f_blocks) && !file->f_locks
-			 && !file->f_shares && !file->f_count) {
-				hlist_del(&file->f_list);
-				nlm_close_files(file);
-				kfree(file);
+		while (file) {
+			/*
+			 * Pin the next neighbour before we drop the mutex
+			 * for nlm_inspect_file(); a concurrent
+			 * nlm_release_file() under the same mutex would
+			 * otherwise be free to unlink and kfree it during
+			 * the unlock window, leaving us to dereference a
+			 * freed slab when we walked to next afterwards.
+			 */
+			next = hlist_entry_safe(file->f_list.next,
+						struct nlm_file, f_list);
+			if (next)
+				next->f_count++;
+
+			if (!is_failover_file || is_failover_file(data, file)) {
+				mutex_unlock(&nlm_file_mutex);
+
+				/*
+				 * Traverse locks, blocks and shares of this
+				 * file and update file->f_locks count.
+				 */
+				if (nlm_inspect_file(data, file, match))
+					ret = 1;
+
+				mutex_lock(&nlm_file_mutex);
+				file->f_count--;
+				nlm_file_release(file);
+			} else {
+				file->f_count--;
 			}
+			file = next;
 		}
 	}
 	mutex_unlock(&nlm_file_mutex);
-- 
2.53.0