[PATCH] nvme: Do not drop the cdev reference twice when cdev_device_add() fails

[Yuqiang Sun <izaiahsun@gmail.com>]

Since commit 9952f3882ecb ("nvme: fix cdev lifetime"),
nvme_add_ns_cdev() and nvme_add_ns_head_cdev() take a reference to the
namespace (or namespace head) before calling nvme_cdev_add(), to be
dropped by the char device's release, nvme_cdev_rel(), and drop it
themselves when nvme_cdev_add() fails.  But when cdev_device_add()
fails, nvme_cdev_add() has already put the device, and nvme_cdev_rel()
has already dropped that reference: the caller drops it a second time.

The namespace (or head) is then freed while it is still in use.  With
one allocation failure injected into cdev_device_add() for a namespace
on an NVMe over loop controller, deleting the controller gives, with
native multipath:

  BUG: KASAN: slab-use-after-free in nvme_put_ns_head+0x21/0x140
  Write of size 4 at addr ff1100000a4ea078 by task init/1
   nvme_put_ns_head
   nvme_put_ns
   nvme_remove_namespaces
   nvme_do_delete_ctrl
   nvme_delete_ctrl_sync
   nvme_sysfs_delete
  Freed by task 1:
   kfree
   nvme_ns_remove
   nvme_remove_namespaces
  refcount_t: underflow; use-after-free.

and without it, a use-after-free of the namespace in
nvme_mpath_clear_ctrl_paths(), the namespace having been freed by the
last close of its block device, followed by a general protection fault.

Take the reference in nvme_cdev_add() instead, right before
cdev_device_add(), so that it is held exactly while the device can be
released, and let the callers neither take nor drop it.  The earlier
failures in nvme_cdev_add(), ida_alloc() and dev_set_name(), return
before the device is initialized and so before the reference is taken.

The problem was found with kbound, a static analysis prototype that
checks whether callers rely on an object after a callee returns a value
for which the callee may have released it, on v7.2.12; it is present
in mainline.

Build-tested with x86_64 and arm64 allmodconfig (W=1, sparse): no new
warnings.  Run-tested on QEMU (x86_64, KVM) with KASAN: a loop target
exporting a null_blk device, connected through /dev/nvme-fabrics, with
failslab failing each allocation under nvme_cdev_add() in turn, with
nvme_core.multipath=N and Y.  Before this patch, every failure in
cdev_device_add() (7 of 7 with each setting) gives the reports above;
with it nothing is reported, also with lockdep and kmemleak, which
finds no leak either.  A failure in dev_set_name() gives no report,
before and after.

Fixes: 9952f3882ecb ("nvme: fix cdev lifetime")
Cc: stable@vger.kernel.org
Signed-off-by: Yuqiang Sun <izaiahsun@gmail.com>
---

Notes:
    The bug was found with kbound, a research static analysis prototype.
    The patch, the changelog and the test setup were written with the help
    of an AI coding assistant (Claude); I have reviewed and tested them as
    described above.
    
    The runs were on next-20261030; the code this patch touches is the same
    there and in mainline, which it is based on.

 drivers/nvme/host/core.c      | 13 +++++++++++--
 drivers/nvme/host/multipath.c |  2 --
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index af71d02b8476..f9d23e9d35ec 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -3978,6 +3978,17 @@ int nvme_cdev_add(struct cdev *cdev, struct device *cdev_device,
 	device_initialize(cdev_device);
 	cdev_init(cdev, fops);
 	cdev->owner = owner;
+	/*
+	 * Held for the lifetime of the cdev and dropped in nvme_cdev_rel(),
+	 * which also runs when cdev_device_add() fails.
+	 */
+	if (cdev_device->parent->class == &nvme_class)
+		nvme_get_ns(container_of(cdev_device, struct nvme_ns,
+					 cdev_device));
+	else
+		nvme_get_ns_head(container_of(cdev_device,
+					      struct nvme_ns_head,
+					      cdev_device));
 	ret = cdev_device_add(cdev, cdev_device);
 	if (ret)
 		put_device(cdev_device);
@@ -4010,13 +4021,11 @@ static void nvme_add_ns_cdev(struct nvme_ns *ns)
 {
 	ns->cdev_device.parent = ns->ctrl->device;
 
-	nvme_get_ns(ns); /* Undone in nvme_cdev_rel() */
 	if (nvme_cdev_add(&ns->cdev, &ns->cdev_device,
 			&nvme_ns_chr_fops, ns->ctrl->ops->module,
 			ns->ctrl->instance, ns->head->instance)) {
 		dev_err(ns->ctrl->device, "Unable to create the ng%dn%d device\n",
 			ns->ctrl->instance, ns->head->instance);
-		nvme_put_ns(ns);
 		return;
 	}
 	set_bit(NVME_NS_CDEV_LIVE, &ns->flags);
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 9e0a0fd77c56..ae8059366b57 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -681,14 +681,12 @@ static void nvme_add_ns_head_cdev(struct nvme_ns_head *head)
 {
 	head->cdev_device.parent = &head->subsys->dev;
 
-	nvme_get_ns_head(head); /* Undone in nvme_cdev_rel() */
 	if (nvme_cdev_add(&head->cdev, &head->cdev_device,
 			&nvme_ns_head_chr_fops, THIS_MODULE,
 			head->subsys->instance, head->instance)) {
 		dev_err(disk_to_dev(head->disk),
 			"Unable to create the ng%dn%d device\n",
 			head->subsys->instance, head->instance);
-		nvme_put_ns_head(head);
 		return;
 	}
 	set_bit(NVME_NSHEAD_CDEV_LIVE, &head->flags);

base-commit: d135ab453a7400a34930bdda1ef4bdc8acfac80e
-- 
2.53.0