[PATCH 00/12] credential guards: credential preparation

[PATCH 00/12] credential guards: credential preparation

[Christian Brauner]

This converts most users combining

* prepare_creds()
* modify new creds
* override_creds()
* revert_creds()
* put_cred()

to rely on credentials guards.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Christian Brauner (12):
      cred: add prepare credential guard
      sev-dev: use guard for path
      sev-dev: use prepare credential guard
      sev-dev: use override credential guards
      coredump: move revert_cred() before coredump_cleanup()
      coredump: pass struct linux_binfmt as const
      coredump: mark struct mm_struct as const
      coredump: split out do_coredump() from vfs_coredump()
      coredump: use prepare credential guard
      coredump: use override credential guard
      trace: use prepare credential guard
      trace: use override credential guard

 drivers/crypto/ccp/sev-dev.c     |  15 ++---
 fs/coredump.c                    | 142 +++++++++++++++++++--------------------
 include/linux/cred.h             |   5 ++
 include/linux/sched/coredump.h   |   2 +-
 kernel/trace/trace_events_user.c |  15 ++---
 5 files changed, 86 insertions(+), 93 deletions(-)
---
base-commit: bcbcea89c608394efecb35237fa9fc1bf5f349d1
change-id: 20251103-work-creds-guards-prepare_creds-101e75226f70

[PATCH 01/12] cred: add prepare credential guard

[Christian Brauner]

A lot of code uses the following pattern:

* prepare new credentials
* modify them for their use-case
* drop them

Support that easier with the new guard infrastructure.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/cred.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 1778c0535b90..a1e33227e0c2 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -285,6 +285,11 @@ static inline void put_cred(const struct cred *cred)
 	put_cred_many(cred, 1);
 }
 
+DEFINE_CLASS(prepare_creds,
+	      struct cred *,
+	      if (_T) put_cred(_T),
+	      prepare_creds(), void)
+
 DEFINE_FREE(put_cred, struct cred *, if (!IS_ERR_OR_NULL(_T)) put_cred(_T))
 
 /**

-- 
2.47.3

[PATCH 02/12] sev-dev: use guard for path

[Christian Brauner]

Just use a guard and also move the path_put() out of the credential
change's scope. There's no need to do this with the overridden
credentials.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 drivers/crypto/ccp/sev-dev.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
index 0d13d47c164b..c5e22af04abb 100644
--- a/drivers/crypto/ccp/sev-dev.c
+++ b/drivers/crypto/ccp/sev-dev.c
@@ -259,8 +259,8 @@ static int sev_cmd_buffer_len(int cmd)
 
 static struct file *open_file_as_root(const char *filename, int flags, umode_t mode)
 {
+	struct path root __free(path_put) = {};
 	struct file *fp;
-	struct path root;
 	struct cred *cred;
 	const struct cred *old_cred;
 
@@ -275,7 +275,6 @@ static struct file *open_file_as_root(const char *filename, int flags, umode_t m
 	old_cred = override_creds(cred);
 
 	fp = file_open_root(&root, filename, flags, mode);
-	path_put(&root);
 
 	put_cred(revert_creds(old_cred));
 

-- 
2.47.3

[PATCH 03/12] sev-dev: use prepare credential guard

[Christian Brauner]

Use the prepare credential guard for allocating a new set of
credentials.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 drivers/crypto/ccp/sev-dev.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
index c5e22af04abb..09e4c9490d58 100644
--- a/drivers/crypto/ccp/sev-dev.c
+++ b/drivers/crypto/ccp/sev-dev.c
@@ -268,15 +268,16 @@ static struct file *open_file_as_root(const char *filename, int flags, umode_t m
 	get_fs_root(init_task.fs, &root);
 	task_unlock(&init_task);
 
-	cred = prepare_creds();
+	CLASS(prepare_creds, cred)();
 	if (!cred)
 		return ERR_PTR(-ENOMEM);
+
 	cred->fsuid = GLOBAL_ROOT_UID;
 	old_cred = override_creds(cred);
 
 	fp = file_open_root(&root, filename, flags, mode);
 
-	put_cred(revert_creds(old_cred));
+	revert_creds(old_cred);
 
 	return fp;
 }

-- 
2.47.3

[PATCH 04/12] sev-dev: use override credential guards

[Christian Brauner]

Use override credential guards for scoped credential override with
automatic restoration on scope exit.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 drivers/crypto/ccp/sev-dev.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
index 09e4c9490d58..19422f422a59 100644
--- a/drivers/crypto/ccp/sev-dev.c
+++ b/drivers/crypto/ccp/sev-dev.c
@@ -260,7 +260,6 @@ static int sev_cmd_buffer_len(int cmd)
 static struct file *open_file_as_root(const char *filename, int flags, umode_t mode)
 {
 	struct path root __free(path_put) = {};
-	struct file *fp;
 	struct cred *cred;
 	const struct cred *old_cred;
 
@@ -273,13 +272,9 @@ static struct file *open_file_as_root(const char *filename, int flags, umode_t m
 		return ERR_PTR(-ENOMEM);
 
 	cred->fsuid = GLOBAL_ROOT_UID;
-	old_cred = override_creds(cred);
-
-	fp = file_open_root(&root, filename, flags, mode);
-
-	revert_creds(old_cred);
 
-	return fp;
+	with_creds(cred);
+	return file_open_root(&root, filename, flags, mode);
 }
 
 static int sev_read_init_ex_file(void)

-- 
2.47.3

[PATCH 05/12] coredump: move revert_cred() before coredump_cleanup()

[Christian Brauner]

There's no need to pin the credentials across the coredump_cleanup()
call. Nothing in there depends on elevated credentials.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/coredump.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 5c1c381ee380..4fce2a2f279c 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -1197,8 +1197,8 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 	}
 
 close_fail:
-	coredump_cleanup(&cn, &cprm);
 	revert_creds(old_cred);
+	coredump_cleanup(&cn, &cprm);
 	return;
 }
 

-- 
2.47.3

[PATCH 06/12] coredump: pass struct linux_binfmt as const

[Christian Brauner]

We don't actually modify it.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/coredump.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 4fce2a2f279c..590360ba0a28 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -1036,7 +1036,7 @@ static bool coredump_pipe(struct core_name *cn, struct coredump_params *cprm,
 
 static bool coredump_write(struct core_name *cn,
 			  struct coredump_params *cprm,
-			  struct linux_binfmt *binfmt)
+			  const struct linux_binfmt *binfmt)
 {
 
 	if (dump_interrupted())
@@ -1093,7 +1093,7 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 	struct core_state core_state;
 	struct core_name cn;
 	struct mm_struct *mm = current->mm;
-	struct linux_binfmt *binfmt = mm->binfmt;
+	const struct linux_binfmt *binfmt = mm->binfmt;
 	const struct cred *old_cred;
 	int argc = 0;
 	struct coredump_params cprm = {

-- 
2.47.3

[PATCH 07/12] coredump: mark struct mm_struct as const

[Christian Brauner]

We don't actually modify it.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/coredump.c                  | 2 +-
 include/linux/sched/coredump.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 590360ba0a28..8253b28bc728 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -1092,7 +1092,7 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 	size_t *argv __free(kfree) = NULL;
 	struct core_state core_state;
 	struct core_name cn;
-	struct mm_struct *mm = current->mm;
+	const struct mm_struct *mm = current->mm;
 	const struct linux_binfmt *binfmt = mm->binfmt;
 	const struct cred *old_cred;
 	int argc = 0;
diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index b7fafe999073..624fda17a785 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -8,7 +8,7 @@
 #define SUID_DUMP_USER		1	/* Dump as user of process */
 #define SUID_DUMP_ROOT		2	/* Dump as root */
 
-static inline unsigned long __mm_flags_get_dumpable(struct mm_struct *mm)
+static inline unsigned long __mm_flags_get_dumpable(const struct mm_struct *mm)
 {
 	/*
 	 * By convention, dumpable bits are contained in first 32 bits of the

-- 
2.47.3

[PATCH 08/12] coredump: split out do_coredump() from vfs_coredump()

[Christian Brauner]

Make the function easier to follow and prepare for some of the following
changes.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/coredump.c | 131 ++++++++++++++++++++++++++++++----------------------------
 1 file changed, 68 insertions(+), 63 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 8253b28bc728..79c681f1d647 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -1086,6 +1086,73 @@ static inline bool coredump_skip(const struct coredump_params *cprm,
 	return false;
 }
 
+static void do_coredump(struct core_name *cn, struct coredump_params *cprm,
+			size_t **argv, int *argc, const struct linux_binfmt *binfmt)
+{
+	if (!coredump_parse(cn, cprm, argv, argc)) {
+		coredump_report_failure("format_corename failed, aborting core");
+		return;
+	}
+
+	switch (cn->core_type) {
+	case COREDUMP_FILE:
+		if (!coredump_file(cn, cprm, binfmt))
+			return;
+		break;
+	case COREDUMP_PIPE:
+		if (!coredump_pipe(cn, cprm, *argv, *argc))
+			return;
+		break;
+	case COREDUMP_SOCK_REQ:
+		fallthrough;
+	case COREDUMP_SOCK:
+		if (!coredump_socket(cn, cprm))
+			return;
+		break;
+	default:
+		WARN_ON_ONCE(true);
+		return;
+	}
+
+	/* Don't even generate the coredump. */
+	if (cn->mask & COREDUMP_REJECT)
+		return;
+
+	/* get us an unshared descriptor table; almost always a no-op */
+	/* The cell spufs coredump code reads the file descriptor tables */
+	if (unshare_files())
+		return;
+
+	if ((cn->mask & COREDUMP_KERNEL) && !coredump_write(cn, cprm, binfmt))
+		return;
+
+	coredump_sock_shutdown(cprm->file);
+
+	/* Let the parent know that a coredump was generated. */
+	if (cn->mask & COREDUMP_USERSPACE)
+		cn->core_dumped = true;
+
+	/*
+	 * When core_pipe_limit is set we wait for the coredump server
+	 * or usermodehelper to finish before exiting so it can e.g.,
+	 * inspect /proc/<pid>.
+	 */
+	if (cn->mask & COREDUMP_WAIT) {
+		switch (cn->core_type) {
+		case COREDUMP_PIPE:
+			wait_for_dump_helpers(cprm->file);
+			break;
+		case COREDUMP_SOCK_REQ:
+			fallthrough;
+		case COREDUMP_SOCK:
+			coredump_sock_wait(cprm->file);
+			break;
+		default:
+			break;
+		}
+	}
+}
+
 void vfs_coredump(const kernel_siginfo_t *siginfo)
 {
 	struct cred *cred __free(put_cred) = NULL;
@@ -1133,70 +1200,8 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 
 	old_cred = override_creds(cred);
 
-	if (!coredump_parse(&cn, &cprm, &argv, &argc)) {
-		coredump_report_failure("format_corename failed, aborting core");
-		goto close_fail;
-	}
-
-	switch (cn.core_type) {
-	case COREDUMP_FILE:
-		if (!coredump_file(&cn, &cprm, binfmt))
-			goto close_fail;
-		break;
-	case COREDUMP_PIPE:
-		if (!coredump_pipe(&cn, &cprm, argv, argc))
-			goto close_fail;
-		break;
-	case COREDUMP_SOCK_REQ:
-		fallthrough;
-	case COREDUMP_SOCK:
-		if (!coredump_socket(&cn, &cprm))
-			goto close_fail;
-		break;
-	default:
-		WARN_ON_ONCE(true);
-		goto close_fail;
-	}
-
-	/* Don't even generate the coredump. */
-	if (cn.mask & COREDUMP_REJECT)
-		goto close_fail;
-
-	/* get us an unshared descriptor table; almost always a no-op */
-	/* The cell spufs coredump code reads the file descriptor tables */
-	if (unshare_files())
-		goto close_fail;
-
-	if ((cn.mask & COREDUMP_KERNEL) && !coredump_write(&cn, &cprm, binfmt))
-		goto close_fail;
-
-	coredump_sock_shutdown(cprm.file);
-
-	/* Let the parent know that a coredump was generated. */
-	if (cn.mask & COREDUMP_USERSPACE)
-		cn.core_dumped = true;
-
-	/*
-	 * When core_pipe_limit is set we wait for the coredump server
-	 * or usermodehelper to finish before exiting so it can e.g.,
-	 * inspect /proc/<pid>.
-	 */
-	if (cn.mask & COREDUMP_WAIT) {
-		switch (cn.core_type) {
-		case COREDUMP_PIPE:
-			wait_for_dump_helpers(cprm.file);
-			break;
-		case COREDUMP_SOCK_REQ:
-			fallthrough;
-		case COREDUMP_SOCK:
-			coredump_sock_wait(cprm.file);
-			break;
-		default:
-			break;
-		}
-	}
+	do_coredump(&cn, &cprm, &argv, &argc, binfmt);
 
-close_fail:
 	revert_creds(old_cred);
 	coredump_cleanup(&cn, &cprm);
 	return;

-- 
2.47.3

[PATCH 09/12] coredump: use prepare credential guard

[Christian Brauner]

Use the prepare credential guard for allocating a new set of
credentials.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/coredump.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 79c681f1d647..5424a6c4e360 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -1155,7 +1155,6 @@ static void do_coredump(struct core_name *cn, struct coredump_params *cprm,
 
 void vfs_coredump(const kernel_siginfo_t *siginfo)
 {
-	struct cred *cred __free(put_cred) = NULL;
 	size_t *argv __free(kfree) = NULL;
 	struct core_state core_state;
 	struct core_name cn;
@@ -1183,7 +1182,7 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 	if (coredump_skip(&cprm, binfmt))
 		return;
 
-	cred = prepare_creds();
+	CLASS(prepare_creds, cred)();
 	if (!cred)
 		return;
 	/*

-- 
2.47.3

[PATCH 10/12] coredump: use override credential guard

[Christian Brauner]

Use override credential guards for scoped credential override with
automatic restoration on scope exit.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/coredump.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 5424a6c4e360..fe4099e0530b 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -1160,7 +1160,6 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 	struct core_name cn;
 	const struct mm_struct *mm = current->mm;
 	const struct linux_binfmt *binfmt = mm->binfmt;
-	const struct cred *old_cred;
 	int argc = 0;
 	struct coredump_params cprm = {
 		.siginfo = siginfo,
@@ -1197,11 +1196,8 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 	if (coredump_wait(siginfo->si_signo, &core_state) < 0)
 		return;
 
-	old_cred = override_creds(cred);
-
-	do_coredump(&cn, &cprm, &argv, &argc, binfmt);
-
-	revert_creds(old_cred);
+	scoped_with_creds(cred)
+		do_coredump(&cn, &cprm, &argv, &argc, binfmt);
 	coredump_cleanup(&cn, &cprm);
 	return;
 }

-- 
2.47.3

[PATCH 11/12] trace: use prepare credential guard

[Christian Brauner]

Use the prepare credential guard for allocating a new set of
credentials.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 kernel/trace/trace_events_user.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index c428dafe7496..3461b1d29276 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -1453,8 +1453,7 @@ static int user_event_set_call_visible(struct user_event *user, bool visible)
 	const struct cred *old_cred;
 	struct cred *cred;
 
-	cred = prepare_creds();
-
+	CLASS(prepare_creds, cred)();
 	if (!cred)
 		return -ENOMEM;
 
@@ -1477,7 +1476,6 @@ static int user_event_set_call_visible(struct user_event *user, bool visible)
 		ret = trace_remove_event_call(&user->call);
 
 	revert_creds(old_cred);
-	put_cred(cred);
 
 	return ret;
 }

-- 
2.47.3

Re: [PATCH 11/12] trace: use prepare credential guard

[Steven Rostedt]

On Mon, 03 Nov 2025 15:57:37 +0100
Christian Brauner <brauner@kernel.org> wrote:

> Use the prepare credential guard for allocating a new set of
> credentials.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  kernel/trace/trace_events_user.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

[PATCH 12/12] trace: use override credential guard

[Christian Brauner]

Use override credential guards for scoped credential override with
automatic restoration on scope exit.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 kernel/trace/trace_events_user.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 3461b1d29276..4528c058d7cd 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -1449,8 +1449,6 @@ static struct trace_event_functions user_event_funcs = {
 
 static int user_event_set_call_visible(struct user_event *user, bool visible)
 {
-	int ret;
-	const struct cred *old_cred;
 	struct cred *cred;
 
 	CLASS(prepare_creds, cred)();
@@ -1470,14 +1468,11 @@ static int user_event_set_call_visible(struct user_event *user, bool visible)
 
 	old_cred = override_creds(cred);
 
+	with_creds(cred);
 	if (visible)
-		ret = trace_add_event_call(&user->call);
-	else
-		ret = trace_remove_event_call(&user->call);
+		return trace_add_event_call(&user->call);
 
-	revert_creds(old_cred);
-
-	return ret;
+	return trace_remove_event_call(&user->call);
 }
 
 static int destroy_user_event(struct user_event *user)

-- 
2.47.3

Re: [PATCH 12/12] trace: use override credential guard

[Steven Rostedt]

On Mon, 03 Nov 2025 15:57:38 +0100
Christian Brauner <brauner@kernel.org> wrote:

> Use override credential guards for scoped credential override with
> automatic restoration on scope exit.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  kernel/trace/trace_events_user.c | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve