FAILED: patch "[PATCH] dm: avoid leaking the caller's thread keyring via the table" 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 981ccd97f7153d310dfa92a534525bbaf46752c2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072016-oppressed-overstuff-75e8@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From 981ccd97f7153d310dfa92a534525bbaf46752c2 Mon Sep 17 00:00:00 2001
From: Ingo Blechschmidt <[email protected]>
Date: Wed, 17 Jun 2026 16:33:42 +0200
Subject: [PATCH] dm: avoid leaking the caller's thread keyring via the table
device file

The refactoring in commit a28d893eb327 ("md: port block device access to file") accidentally causes the caller's thread keyring to be kept alive long beyond the caller's lifetime.

As a result, "cryptsetup luksSuspend" silently fails to wipe the LUKS volume key from memory.

In detail: "cryptsetup luksOpen" uses its supposedly ephemeral thread keyring to pass the volume key to the kernel. dm-crypt's crypt_set_keyring_key() copies the key material into its own crypt_config structure and then drops its own reference to the key in the keyring with key_put().

With this fix, restoring pre-v6.9 behavior, the copy in the thread keyring is then promptly garbage collected, such that exactly one copy of the volume key remains. This single copy is correctly wiped from memory on "cryptsetup luksSuspend".

Without this fix, the thread keyring and the volume key in it remains. This second copy is only freed on "luksClose". "luksSuspend" neither knows about this copy nor has any way to remove it, so the key remains recoverable from RAM after a suspend that is documented to have wiped it.

This fix should not introduce new security problems, as the code is anyway gated by CAP_SYS_ADMIN. The device-mapper core, not the calling task, is the legitimate owner of this long-lived file.

index 7287bed6eb64..d413bfaf3527 100644
@@ -735,7 +735,16 @@ static struct table_device *open_table_device(struct mapped_device *md,
 		return ERR_PTR(-ENOMEM);
 	refcount_set(&td->count, 1);
 
-	bdev_file = bdev_file_open_by_dev(dev, mode, _dm_claim_ptr, NULL);
+	/*
+	 * Open the backing device with kernel rather than caller
+	 * credentials. Otherwise the caller's credentials would be
+	 * pinned in bdev_file->f_cred until the table device is closed.
+	 * That would keep the caller's thread keyring alive long beyond the
+	 * lifetime of the caller, breaking userspace expectation (e.g.
+	 * cryptsetup(8) leaking the LUKS volume key).
+	 */
+	scoped_with_kernel_creds()
+		bdev_file = bdev_file_open_by_dev(dev, mode, _dm_claim_ptr, NULL);
 	if (IS_ERR(bdev_file)) {
 		r = PTR_ERR(bdev_file);
 		goto out_free_td;

From: Christian Brauner <[email protected]>

[ Upstream commit 4c7ceeb62d3330b6fb2b549ae833a92c0f481f3e ]

Access kernel creds based off of init_task. This will let us avoid any direct access to init_cred.

Stable-dep-of: 981ccd97f715 ("dm: avoid leaking the caller's thread keyring via the table device file")

---
 include/linux/cred.h | 7 +++++++
 1 file changed, 7 insertions(+)

index 89ae50ad2acea9..8ab3718184adf8 100644
@@ -20,6 +20,8 @@
 struct cred;
 struct inode;
 
+extern struct task_struct init_task;
+
 /*
  * COW Supplementary groups list
  */

@@ -156,6 +158,11 @@ extern struct cred *prepare_exec_creds(void);
 extern int commit_creds(struct cred *);
 extern void abort_creds(struct cred *);
 extern struct cred *prepare_kernel_cred(struct task_struct *);
+static inline const struct cred *kernel_cred(void)
+{
+	/* shut up sparse */
+	return rcu_dereference_raw(init_task.cred);
+}
 extern int set_security_override(struct cred *, u32);
 extern int set_security_override_from_ctx(struct cred *, const char *);
 extern int set_create_files_as(struct cred *, struct inode *);

2.53.0

From: Christian Brauner <[email protected]>

[ Upstream commit ae40e6c65791f47c76cc14d0cce2707fe6053f72 ]

Add a new cleanup class for override creds. We can make use of this in a bunch of places going forward.

Based on this scoped_with_kernel_creds() that can be used to temporarily assume kernel credentials for specific tasks such as firmware loading, or coredump socket connections. At no point will the caller interact with the kernel credentials directly.

Stable-dep-of: 981ccd97f715 ("dm: avoid leaking the caller's thread keyring via the table device file")

---
 include/linux/cred.h | 8 ++++++++
 1 file changed, 8 insertions(+)

index 8ab3718184adf8..be2cd07b174c5e 100644
@@ -187,6 +187,14 @@ static inline const struct cred *revert_creds(const struct cred *revert_cred)
 	return rcu_replace_pointer(current->cred, revert_cred, 1);
 }
 
+DEFINE_CLASS(override_creds,
+	     const struct cred *,
+	     revert_creds(_T),
+	     override_creds(override_cred), const struct cred *override_cred)
+
+#define scoped_with_kernel_creds() \
+	scoped_class(override_creds, __UNIQUE_ID(cred), kernel_cred())
+
 /**
  * get_cred_many - Get references on a set of credentials
  * @cred: The credentials to reference

2.53.0

From: Ingo Blechschmidt <[email protected]>

[ Upstream commit 981ccd97f7153d310dfa92a534525bbaf46752c2 ]

The refactoring in commit a28d893eb327 ("md: port block device access to file") accidentally causes the caller's thread keyring to be kept alive long beyond the caller's lifetime.

As a result, "cryptsetup luksSuspend" silently fails to wipe the LUKS volume key from memory.

In detail: "cryptsetup luksOpen" uses its supposedly ephemeral thread keyring to pass the volume key to the kernel. dm-crypt's crypt_set_keyring_key() copies the key material into its own crypt_config structure and then drops its own reference to the key in the keyring with key_put().

With this fix, restoring pre-v6.9 behavior, the copy in the thread keyring is then promptly garbage collected, such that exactly one copy of the volume key remains. This single copy is correctly wiped from memory on "cryptsetup luksSuspend".

Without this fix, the thread keyring and the volume key in it remains. This second copy is only freed on "luksClose". "luksSuspend" neither knows about this copy nor has any way to remove it, so the key remains recoverable from RAM after a suspend that is documented to have wiped it.

This fix should not introduce new security problems, as the code is anyway gated by CAP_SYS_ADMIN. The device-mapper core, not the calling task, is the legitimate owner of this long-lived file.

---
 drivers/md/dm.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

index fb6bcd598c5b85..28d825cc69c537 100644
@@ -735,7 +735,16 @@ static struct table_device *open_table_device(struct mapped_device *md,
 		return ERR_PTR(-ENOMEM);
 	refcount_set(&td->count, 1);
 
-	bdev_file = bdev_file_open_by_dev(dev, mode, _dm_claim_ptr, NULL);
+	/*
+	 * Open the backing device with kernel rather than caller
+	 * credentials. Otherwise the caller's credentials would be
+	 * pinned in bdev_file->f_cred until the table device is closed.
+	 * That would keep the caller's thread keyring alive long beyond the
+	 * lifetime of the caller, breaking userspace expectation (e.g.
+	 * cryptsetup(8) leaking the LUKS volume key).
+	 */
+	scoped_with_kernel_creds()
+		bdev_file = bdev_file_open_by_dev(dev, mode, _dm_claim_ptr, NULL);
 	if (IS_ERR(bdev_file)) {
 		r = PTR_ERR(bdev_file);
 		goto out_free_td;

2.53.0