FAILED: patch "[PATCH] ublk: fix NULL pointer dereference in ublk_ctrl_set_size()" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 25966fc097691e5c925ad080f64a2f19c5fd940a
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026031728-bunch-limelight-a7e5@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 25966fc097691e5c925ad080f64a2f19c5fd940a Mon Sep 17 00:00:00 2001
From: Mehul Rao <[email protected]>
Date: Thu, 5 Mar 2026 14:31:46 -0500
Subject: [PATCH] ublk: fix NULL pointer dereference in ublk_ctrl_set_size()

ublk_ctrl_set_size() unconditionally dereferences ub->ub_disk via set_capacity_and_notify() without checking if it is NULL.

ub->ub_disk is NULL before UBLK_CMD_START_DEV completes (it is only assigned in ublk_ctrl_start_dev()) and after UBLK_CMD_STOP_DEV runs (ublk_detach_disk() sets it to NULL). Since the UBLK_CMD_UPDATE_SIZE handler performs no state validation, a user can trigger a NULL pointer dereference by sending UPDATE_SIZE to a device that has been added but not yet started, or one that has been stopped.

Fix this by checking ub->ub_disk under ub->mutex before dereferencing it, and returning -ENODEV if the disk is not available.

index c13cda58a7c6..03edabdf8977 100644
@@ -5003,15 +5003,22 @@ static int ublk_ctrl_get_features(const struct ublksrv_ctrl_cmd *header)
 	return 0;
 }
 
-static void ublk_ctrl_set_size(struct ublk_device *ub, const struct ublksrv_ctrl_cmd *header)
+static int ublk_ctrl_set_size(struct ublk_device *ub, const struct ublksrv_ctrl_cmd *header)
 {
 	struct ublk_param_basic *p = &ub->params.basic;
 	u64 new_size = header->data[0];
+	int ret = 0;
 
 	mutex_lock(&ub->mutex);
+	if (!ub->ub_disk) {
+		ret = -ENODEV;
+		goto out;
+	}
 	p->dev_sectors = new_size;
 	set_capacity_and_notify(ub->ub_disk, p->dev_sectors);
+out:
 	mutex_unlock(&ub->mutex);
+	return ret;
 }
 
 struct count_busy {

@@ -5331,8 +5338,7 @@ static int ublk_ctrl_uring_cmd(struct io_uring_cmd *cmd,
 		ret = ublk_ctrl_end_recovery(ub, &header);
 		break;
 	case UBLK_CMD_UPDATE_SIZE:
-		ublk_ctrl_set_size(ub, &header);
-		ret = 0;
+		ret = ublk_ctrl_set_size(ub, &header);
 		break;
 	case UBLK_CMD_QUIESCE_DEV:
 		ret = ublk_ctrl_quiesce_dev(ub, &header);

From: Mehul Rao <[email protected]>

[ Upstream commit 25966fc097691e5c925ad080f64a2f19c5fd940a ]

ublk_ctrl_set_size() unconditionally dereferences ub->ub_disk via set_capacity_and_notify() without checking if it is NULL.

ub->ub_disk is NULL before UBLK_CMD_START_DEV completes (it is only assigned in ublk_ctrl_start_dev()) and after UBLK_CMD_STOP_DEV runs (ublk_detach_disk() sets it to NULL). Since the UBLK_CMD_UPDATE_SIZE handler performs no state validation, a user can trigger a NULL pointer dereference by sending UPDATE_SIZE to a device that has been added but not yet started, or one that has been stopped.

Fix this by checking ub->ub_disk under ub->mutex before dereferencing it, and returning -ENODEV if the disk is not available.

---
 drivers/block/ublk_drv.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

index 965460d4fc76e..2729b1556e810 100644
@@ -3604,15 +3604,22 @@ static int ublk_ctrl_get_features(const struct ublksrv_ctrl_cmd *header)
 	return 0;
 }
 
-static void ublk_ctrl_set_size(struct ublk_device *ub, const struct ublksrv_ctrl_cmd *header)
+static int ublk_ctrl_set_size(struct ublk_device *ub, const struct ublksrv_ctrl_cmd *header)
 {
 	struct ublk_param_basic *p = &ub->params.basic;
 	u64 new_size = header->data[0];
+	int ret = 0;
 
 	mutex_lock(&ub->mutex);
+	if (!ub->ub_disk) {
+		ret = -ENODEV;
+		goto out;
+	}
 	p->dev_sectors = new_size;
 	set_capacity_and_notify(ub->ub_disk, p->dev_sectors);
+out:
 	mutex_unlock(&ub->mutex);
+	return ret;
 }
 
 struct count_busy {

@@ -3902,8 +3909,7 @@ static int ublk_ctrl_uring_cmd(struct io_uring_cmd *cmd,
 		ret = ublk_ctrl_end_recovery(ub, header);
 		break;
 	case UBLK_CMD_UPDATE_SIZE:
-		ublk_ctrl_set_size(ub, header);
-		ret = 0;
+		ret = ublk_ctrl_set_size(ub, header);
 		break;
 	case UBLK_CMD_QUIESCE_DEV:
 		ret = ublk_ctrl_quiesce_dev(ub, header);

2.51.0