[PATCH v3 2/3] nvmet: avoid recursive configfs open for file-backed namespaces

[Runyu Xiao]

nvmet_ns_enable_store() runs as a configfs store callback while configfs
holds the item's frag_sem. A file-backed namespace then opens the
configured device_path with filp_open(). If the path resolves into
configfs, the open path re-enters __configfs_open_file() and attempts to
acquire the same frag_sem again.

Resolve the configured path first, reject paths resolved on configfs, and
open the resolved path with file_open_root(). This preserves the standard
open-time permission checks without performing a second pathname walk. Use
the configfs helper for the filesystem type check so this caller shares the
classification with other configfs users.

Fixes: d5eff33ee6f8 ("nvmet: add simple file backed ns support")
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Sagi Grimberg <sagi@xxxxxxxxxxx>
Assisted-by: LLM Codex
Signed-off-by: Runyu Xiao <runyu.xiao@xxxxxxxxxx>
---
 drivers/nvme/target/io-cmd-file.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/target/io-cmd-file.c b/drivers/nvme/target/io-cmd-file.c
index 0b22d183f..fbe58aa4a 100644
--- a/drivers/nvme/target/io-cmd-file.c
+++ b/drivers/nvme/target/io-cmd-file.c
@@ -8,7 +8,9 @@
 #include <linux/uio.h>
 #include <linux/falloc.h>
 #include <linux/file.h>
+#include <linux/configfs.h>
 #include <linux/fs.h>
+#include <linux/namei.h>
 #include "nvmet.h"
 
 #define NVMET_MIN_MPOOL_OBJ		16
@@ -33,12 +35,28 @@ void nvmet_file_ns_disable(struct nvmet_ns *ns)
 int nvmet_file_ns_enable(struct nvmet_ns *ns)
 {
 	int flags = O_RDWR | O_LARGEFILE;
+	struct path path;
 	int ret = 0;
 
 	if (!ns->buffered_io)
 		flags |= O_DIRECT;
 
-	ns->file = filp_open(ns->device_path, flags, 0);
+	ret = kern_path(ns->device_path, LOOKUP_FOLLOW, &path);
+	if (ret) {
+		pr_err("failed to open file %s: (%d)\n",
+		       ns->device_path, ret);
+		return ret;
+	}
+
+	if (configfs_path_is_configfs(&path)) {
+		pr_err("configfs paths cannot back namespace %s\n",
+		       ns->device_path);
+		path_put(&path);
+		return -EINVAL;
+	}
+
+	ns->file = file_open_root(&path, "", flags, 0);
+	path_put(&path);
 	if (IS_ERR(ns->file)) {
 		ret = PTR_ERR(ns->file);
 		pr_err("failed to open file %s: (%d)\n",
-- 
2.34.1