[PATCH v4 0/3] nvmet: avoid recursive configfs open

[PATCH v4 0/3] nvmet: avoid recursive configfs open

[Runyu Xiao]

The nvmet configfs store callbacks hold the item's frag_sem while they
enable file-backed namespaces or passthru controllers. Both paths open a
user-configured pathname. If it resolves into configfs, the open path can
re-enter __configfs_open_file() and try to acquire the same frag_sem again.

Add configfs helpers that resolve configured paths, reject configfs-backed
paths, and open the resolved path with file_open_root(). This keeps the
normal open-time permission and security checks while preventing the
recursive configfs open.

Changes since v3:
- Move path lookup, configfs rejection, and file_open_root() into the
  configfs helper, as requested by Christoph Hellwig.
- Use the helper from both nvmet backend open paths.
- Provide a root-relative helper for callers that retain a resolved root,
  including callers that create files below a pinned root.
- Drop the previous Reviewed-by tags because the helper interface changed.

Runyu Xiao (3):
  fs: configfs: add helpers for opening non-configfs paths
  nvmet: avoid recursive configfs open for file-backed namespaces
  nvmet: avoid recursive configfs open for passthru

 drivers/nvme/target/io-cmd-file.c |  3 ++-
 drivers/nvme/target/passthru.c    |  3 ++-
 fs/configfs/mount.c               | 33 +++++++++++++++++++++++++++++++
 include/linux/configfs.h          |  6 ++++++
 4 files changed, 43 insertions(+), 2 deletions(-)


base-commit: df2908090cda368b01ff43709f51890076c56157
-- 
2.34.1

[PATCH v4 1/3] fs: configfs: add helpers for opening non-configfs paths

[Runyu Xiao]

Configfs store callbacks may open user-configured paths while holding an
item frag_sem. Opening a configfs path from such a callback can re-enter
configfs and try to acquire the same semaphore.

Add helpers that reject configfs paths by comparing the actual filesystem
type and open the resolved path with file_open_root(), preserving normal
open-time permission and security checks. Provide a root-relative form for
callers that retain a resolved root and may create a file below it.

Assisted-by: LLM Codex
Signed-off-by: Runyu Xiao <runyu.xiao@seu.edu.cn>
---
 fs/configfs/mount.c      | 33 +++++++++++++++++++++++++++++++++
 include/linux/configfs.h |  6 ++++++
 2 files changed, 39 insertions(+)

diff --git a/fs/configfs/mount.c b/fs/configfs/mount.c
index d8cac1cbf3bd5..77e1788ce2372 100644
--- a/fs/configfs/mount.c
+++ b/fs/configfs/mount.c
@@ -13,6 +13,7 @@
 #include <linux/module.h>
 #include <linux/mount.h>
 #include <linux/fs_context.h>
+#include <linux/namei.h>
 #include <linux/pagemap.h>
 #include <linux/init.h>
 #include <linux/slab.h>
@@ -118,6 +119,38 @@ static struct file_system_type configfs_fs_type = {
 };
 MODULE_ALIAS_FS("configfs");
 
+bool configfs_path_is_configfs(const struct path *path)
+{
+	return path->dentry->d_sb->s_type == &configfs_fs_type;
+}
+EXPORT_SYMBOL_GPL(configfs_path_is_configfs);
+
+struct file *configfs_open_root(const struct path *root, const char *name,
+				int flags, umode_t mode)
+{
+	if (configfs_path_is_configfs(root))
+		return ERR_PTR(-EINVAL);
+
+	return file_open_root(root, name, flags, mode);
+}
+EXPORT_SYMBOL_GPL(configfs_open_root);
+
+struct file *configfs_file_open(const char *filename, int flags, umode_t mode)
+{
+	struct file *file;
+	struct path path;
+	int ret;
+
+	ret = kern_path(filename, LOOKUP_FOLLOW, &path);
+	if (ret)
+		return ERR_PTR(ret);
+
+	file = configfs_open_root(&path, "", flags, mode);
+	path_put(&path);
+	return file;
+}
+EXPORT_SYMBOL_GPL(configfs_file_open);
+
 struct dentry *configfs_pin_fs(void)
 {
 	int err = simple_pin_fs(&configfs_fs_type, &configfs_mount,
diff --git a/include/linux/configfs.h b/include/linux/configfs.h
index ef65c75beeaad..768132b2be1ae 100644
--- a/include/linux/configfs.h
+++ b/include/linux/configfs.h
@@ -34,6 +34,8 @@ struct configfs_group_operations;
 struct configfs_attribute;
 struct configfs_bin_attribute;
 struct configfs_subsystem;
+struct file;
+struct path;
 
 struct config_item {
 	char			*ci_name;
@@ -243,6 +245,10 @@ void configfs_unregister_subsystem(struct configfs_subsystem *subsys);
 int configfs_register_group(struct config_group *parent_group,
 			    struct config_group *group);
 void configfs_unregister_group(struct config_group *group);
+bool configfs_path_is_configfs(const struct path *path);
+struct file *configfs_open_root(const struct path *root, const char *name,
+				int flags, umode_t mode);
+struct file *configfs_file_open(const char *filename, int flags, umode_t mode);
 
 void configfs_remove_default_groups(struct config_group *group);
 
-- 
2.34.1

Re: [PATCH v4 1/3] fs: configfs: add helpers for opening non-configfs paths

[Christoph Hellwig]

On Mon, Sep 21, 2026 at 05:05:57PM +0800, Runyu Xiao wrote:
> +bool configfs_path_is_configfs(const struct path *path)
> +{
> +	return path->dentry->d_sb->s_type == &configfs_fs_type;
> +}
> +EXPORT_SYMBOL_GPL(configfs_path_is_configfs);
> +
> +struct file *configfs_open_root(const struct path *root, const char *name,
> +				int flags, umode_t mode)
> +{
> +	if (configfs_path_is_configfs(root))
> +		return ERR_PTR(-EINVAL);
> +
> +	return file_open_root(root, name, flags, mode);
> +}
> +EXPORT_SYMBOL_GPL(configfs_open_root);

These two aren't used outside this file, so they could be static,
or in case of configfs_path_is_configfs even be folded into the
only caller.  Or do you plan to have other users?

> +struct file *configfs_file_open(const char *filename, int flags, umode_t mode)

Can you add a kerneldoc comment explaining how/why this should be used?

Re: [PATCH v4 1/3] fs: configfs: add helpers for opening non-configfs paths

[Runyu Xiao]

Thanks for the reviews.

> > +bool configfs_path_is_configfs(const struct path *path)
> > +{
> > +	return path->dentry->d_sb->s_type == &configfs_fs_type;
> > +}
> > +EXPORT_SYMBOL_GPL(configfs_path_is_configfs);
> > +
> > +struct file *configfs_open_root(const struct path *root, const char *name,
> > +			int flags, umode_t mode)
> > +{
> > +	if (configfs_path_is_configfs(root))
> > +		return ERR_PTR(-EINVAL);
> > +
> > +	return file_open_root(root, name, flags, mode);
> > +}
> > +EXPORT_SYMBOL_GPL(configfs_open_root);
>
> These two aren't used outside this file, so they could be static,
> or in case of configfs_path_is_configfs even be folded into the
> only caller.  Or do you plan to have other users?

I folded the filesystem-type check into configfs_open_root() and dropped
the predicate entirely.  configfs_file_open() is for callers that have a
pathname, while configfs_open_root() is for callers that keep a resolved
root and open files below it.

> > +struct file *configfs_file_open(const char *filename, int flags, umode_t mode)
>
> Can you add a kerneldoc comment explaining how/why this should be used?

I will add kerneldoc comments for both helpers in v5.

> Didn't you also have patches for drivers/target/ that should use
> the new helper?

Yes.  The target-core patch titled "scsi: target: pin db_root for metadata
writes" is another consumer of configfs_open_root().  I will rebase that
patch on this series and drop its duplicate configfs changes.

The separate patch titled "scsi: target: file: avoid recursive configfs open
in fd_init_prot()" will use configfs_file_open(), not configfs_open_root().

I will send v5 as a new thread.  Patches 2/3 and 3/3 are unchanged and
retain your Reviewed-by.

Thanks,
Runyu

[PATCH v4 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 frag_sem. File-backed namespace enable used filp_open() on
the configured device_path, so a path into configfs could re-enter
__configfs_open_file() and try to acquire the same semaphore again.

Use configfs_file_open() so the path is resolved before opening,
configfs-backed paths are rejected, and the resolved path is opened with
file_open_root() while retaining the normal open-time permission and
security checks.

Fixes: d5eff33ee6f8 ("nvmet: add simple file backed ns support")
Cc: stable@vger.kernel.org
Assisted-by: LLM Codex
Signed-off-by: Runyu Xiao <runyu.xiao@seu.edu.cn>
---
 drivers/nvme/target/io-cmd-file.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/target/io-cmd-file.c b/drivers/nvme/target/io-cmd-file.c
index 0b22d183f9279..2a4f25de94ba1 100644
--- a/drivers/nvme/target/io-cmd-file.c
+++ b/drivers/nvme/target/io-cmd-file.c
@@ -8,6 +8,7 @@
 #include <linux/uio.h>
 #include <linux/falloc.h>
 #include <linux/file.h>
+#include <linux/configfs.h>
 #include <linux/fs.h>
 #include "nvmet.h"
 
@@ -38,7 +39,7 @@ int nvmet_file_ns_enable(struct nvmet_ns *ns)
 	if (!ns->buffered_io)
 		flags |= O_DIRECT;
 
-	ns->file = filp_open(ns->device_path, flags, 0);
+	ns->file = configfs_file_open(ns->device_path, flags, 0);
 	if (IS_ERR(ns->file)) {
 		ret = PTR_ERR(ns->file);
 		pr_err("failed to open file %s: (%d)\n",
-- 
2.34.1

Re: [PATCH v4 2/3] nvmet: avoid recursive configfs open for file-backed namespaces

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCH v4 3/3] nvmet: avoid recursive configfs open for passthru

[Runyu Xiao]

nvmet_passthru_enable_store() runs as a configfs store callback while
configfs holds the item frag_sem. Passthru enable used filp_open() on the
configured controller path, so a path into configfs could re-enter
__configfs_open_file() and try to acquire the same semaphore again.

Use configfs_file_open() so the path is resolved before opening,
configfs-backed paths are rejected, and the resolved path is opened with
file_open_root() while retaining the normal open-time permission and
security checks.

Fixes: cae5b01a2afc ("nvmet: introduce the passthru configfs interface")
Cc: stable@vger.kernel.org
Assisted-by: LLM Codex
Signed-off-by: Runyu Xiao <runyu.xiao@seu.edu.cn>
---
 drivers/nvme/target/passthru.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/target/passthru.c b/drivers/nvme/target/passthru.c
index fa6527c537e26..d60256004e6cf 100644
--- a/drivers/nvme/target/passthru.c
+++ b/drivers/nvme/target/passthru.c
@@ -9,6 +9,7 @@
  */
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 #include <linux/module.h>
+#include <linux/configfs.h>
 
 #include "../host/nvme.h"
 #include "nvmet.h"
@@ -602,7 +603,7 @@ int nvmet_passthru_ctrl_enable(struct nvmet_subsys *subsys)
 		goto out_unlock;
 	}
 
-	file = filp_open(subsys->passthru_ctrl_path, O_RDWR, 0);
+	file = configfs_file_open(subsys->passthru_ctrl_path, O_RDWR, 0);
 	if (IS_ERR(file)) {
 		ret = PTR_ERR(file);
 		goto out_unlock;
-- 
2.34.1

Re: [PATCH v4 3/3] nvmet: avoid recursive configfs open for passthru

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Didn't you also have patches for drivers/target/ that should use
the new helper?