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Subject[PATCH v8 3/5] mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC
Date
From: Jeff Xu <jeffxu@google.com>

The new MFD_NOEXEC_SEAL and MFD_EXEC flags allows application to
set executable bit at creation time (memfd_create).

When MFD_NOEXEC_SEAL is set, memfd is created without executable bit
(mode:0666), and sealed with F_SEAL_EXEC, so it can't be chmod to
be executable (mode: 0777) after creation.

when MFD_EXEC flag is set, memfd is created with executable bit
(mode:0777), this is the same as the old behavior of memfd_create.

The new pid namespaced sysctl vm.memfd_noexec has 3 values:
0: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
MFD_EXEC was set.
1: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
MFD_NOEXEC_SEAL was set.
2: memfd_create() without MFD_NOEXEC_SEAL will be rejected.

The sysctl allows finer control of memfd_create for old-software
that doesn't set the executable bit, for example, a container with
vm.memfd_noexec=1 means the old-software will create non-executable
memfd by default. Also, the value of memfd_noexec is passed to child
namespace at creation time. For example, if the init namespace has
vm.memfd_noexec=2, all its children namespaces will be created with 2.

Signed-off-by: Jeff Xu <jeffxu@google.com>
Co-developed-by: Daniel Verkamp <dverkamp@chromium.org>
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
include/linux/pid_namespace.h | 19 +++++++++++
include/uapi/linux/memfd.h | 4 +++
kernel/pid_namespace.c | 5 +++
kernel/pid_sysctl.h | 59 +++++++++++++++++++++++++++++++++++
mm/memfd.c | 48 ++++++++++++++++++++++++++--
5 files changed, 133 insertions(+), 2 deletions(-)
create mode 100644 kernel/pid_sysctl.h

diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 07481bb87d4e..c758809d5bcf 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -16,6 +16,21 @@

struct fs_pin;

+#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
+/*
+ * sysctl for vm.memfd_noexec
+ * 0: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL
+ * acts like MFD_EXEC was set.
+ * 1: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL
+ * acts like MFD_NOEXEC_SEAL was set.
+ * 2: memfd_create() without MFD_NOEXEC_SEAL will be
+ * rejected.
+ */
+#define MEMFD_NOEXEC_SCOPE_EXEC 0
+#define MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL 1
+#define MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED 2
+#endif
+
struct pid_namespace {
struct idr idr;
struct rcu_head rcu;
@@ -31,6 +46,10 @@ struct pid_namespace {
struct ucounts *ucounts;
int reboot; /* group exit code if this pidns was rebooted */
struct ns_common ns;
+#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
+ /* sysctl for vm.memfd_noexec */
+ int memfd_noexec_scope;
+#endif
} __randomize_layout;

extern struct pid_namespace init_pid_ns;
diff --git a/include/uapi/linux/memfd.h b/include/uapi/linux/memfd.h
index 7a8a26751c23..273a4e15dfcf 100644
--- a/include/uapi/linux/memfd.h
+++ b/include/uapi/linux/memfd.h
@@ -8,6 +8,10 @@
#define MFD_CLOEXEC 0x0001U
#define MFD_ALLOW_SEALING 0x0002U
#define MFD_HUGETLB 0x0004U
+/* not executable and sealed to prevent changing to executable. */
+#define MFD_NOEXEC_SEAL 0x0008U
+/* executable */
+#define MFD_EXEC 0x0010U

/*
* Huge page size encoding when MFD_HUGETLB is specified, and a huge page
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index f4f8cb0435b4..8a98b1af9376 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -23,6 +23,7 @@
#include <linux/sched/task.h>
#include <linux/sched/signal.h>
#include <linux/idr.h>
+#include "pid_sysctl.h"

static DEFINE_MUTEX(pid_caches_mutex);
static struct kmem_cache *pid_ns_cachep;
@@ -110,6 +111,8 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
ns->ucounts = ucounts;
ns->pid_allocated = PIDNS_ADDING;

+ initialize_memfd_noexec_scope(ns);
+
return ns;

out_free_idr:
@@ -455,6 +458,8 @@ static __init int pid_namespaces_init(void)
#ifdef CONFIG_CHECKPOINT_RESTORE
register_sysctl_paths(kern_path, pid_ns_ctl_table);
#endif
+
+ register_pid_ns_sysctl_table_vm();
return 0;
}

diff --git a/kernel/pid_sysctl.h b/kernel/pid_sysctl.h
new file mode 100644
index 000000000000..90a93161a122
--- /dev/null
+++ b/kernel/pid_sysctl.h
@@ -0,0 +1,59 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef LINUX_PID_SYSCTL_H
+#define LINUX_PID_SYSCTL_H
+
+#include <linux/pid_namespace.h>
+
+#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
+static inline void initialize_memfd_noexec_scope(struct pid_namespace *ns)
+{
+ ns->memfd_noexec_scope =
+ task_active_pid_ns(current)->memfd_noexec_scope;
+}
+
+static int pid_mfd_noexec_dointvec_minmax(struct ctl_table *table,
+ int write, void *buf, size_t *lenp, loff_t *ppos)
+{
+ struct pid_namespace *ns = task_active_pid_ns(current);
+ struct ctl_table table_copy;
+
+ if (write && !ns_capable(ns->user_ns, CAP_SYS_ADMIN))
+ return -EPERM;
+
+ table_copy = *table;
+ if (ns != &init_pid_ns)
+ table_copy.data = &ns->memfd_noexec_scope;
+
+ /*
+ * set minimum to current value, the effect is only bigger
+ * value is accepted.
+ */
+ if (*(int *)table_copy.data > *(int *)table_copy.extra1)
+ table_copy.extra1 = table_copy.data;
+
+ return proc_dointvec_minmax(&table_copy, write, buf, lenp, ppos);
+}
+
+static struct ctl_table pid_ns_ctl_table_vm[] = {
+ {
+ .procname = "memfd_noexec",
+ .data = &init_pid_ns.memfd_noexec_scope,
+ .maxlen = sizeof(init_pid_ns.memfd_noexec_scope),
+ .mode = 0644,
+ .proc_handler = pid_mfd_noexec_dointvec_minmax,
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = SYSCTL_TWO,
+ },
+ { }
+};
+static struct ctl_path vm_path[] = { { .procname = "vm", }, { } };
+static inline void register_pid_ns_sysctl_table_vm(void)
+{
+ register_sysctl_paths(vm_path, pid_ns_ctl_table_vm);
+}
+#else
+static inline void set_memfd_noexec_scope(struct pid_namespace *ns) {}
+static inline void register_pid_ns_ctl_table_vm(void) {}
+#endif
+
+#endif /* LINUX_PID_SYSCTL_H */
diff --git a/mm/memfd.c b/mm/memfd.c
index 4ebeab94aa74..ec70675a7069 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -18,6 +18,7 @@
#include <linux/hugetlb.h>
#include <linux/shmem_fs.h>
#include <linux/memfd.h>
+#include <linux/pid_namespace.h>
#include <uapi/linux/memfd.h>

/*
@@ -263,12 +264,14 @@ long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
#define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
#define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)

-#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
+#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC)

SYSCALL_DEFINE2(memfd_create,
const char __user *, uname,
unsigned int, flags)
{
+ char comm[TASK_COMM_LEN];
+ struct pid_namespace *ns;
unsigned int *file_seals;
struct file *file;
int fd, error;
@@ -285,6 +288,39 @@ SYSCALL_DEFINE2(memfd_create,
return -EINVAL;
}

+ /* Invalid if both EXEC and NOEXEC_SEAL are set.*/
+ if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL))
+ return -EINVAL;
+
+ if (!(flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {
+#ifdef CONFIG_SYSCTL
+ int sysctl = MEMFD_NOEXEC_SCOPE_EXEC;
+
+ ns = task_active_pid_ns(current);
+ if (ns)
+ sysctl = ns->memfd_noexec_scope;
+
+ switch (sysctl) {
+ case MEMFD_NOEXEC_SCOPE_EXEC:
+ flags |= MFD_EXEC;
+ break;
+ case MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL:
+ flags |= MFD_NOEXEC_SEAL;
+ break;
+ default:
+ pr_warn_ratelimited(
+ "memfd_create(): MFD_NOEXEC_SEAL is enforced, pid=%d '%s'\n",
+ task_pid_nr(current), get_task_comm(comm, current));
+ return -EINVAL;
+ }
+#else
+ flags |= MFD_EXEC;
+#endif
+ pr_warn_ratelimited(
+ "memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL, pid=%d '%s'\n",
+ task_pid_nr(current), get_task_comm(comm, current));
+ }
+
/* length includes terminating zero */
len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
if (len <= 0)
@@ -328,7 +364,15 @@ SYSCALL_DEFINE2(memfd_create,
file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
file->f_flags |= O_LARGEFILE;

- if (flags & MFD_ALLOW_SEALING) {
+ if (flags & MFD_NOEXEC_SEAL) {
+ struct inode *inode = file_inode(file);
+
+ inode->i_mode &= ~0111;
+ file_seals = memfd_file_seals_ptr(file);
+ *file_seals &= ~F_SEAL_SEAL;
+ *file_seals |= F_SEAL_EXEC;
+ } else if (flags & MFD_ALLOW_SEALING) {
+ /* MFD_EXEC and MFD_ALLOW_SEALING are set */
file_seals = memfd_file_seals_ptr(file);
*file_seals &= ~F_SEAL_SEAL;
}
--
2.39.0.rc1.256.g54fd8350bd-goog
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