[v4,3/6] mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC

Commit Message

Jeff Xu Dec. 6, 2022, 3:02 p.m. UTC

From: Jeff Xu <[e-mail]>

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.

Signed-off-by: Jeff Xu <[e-mail]>
Co-developed-by: Daniel Verkamp <[e-mail]>
Signed-off-by: Daniel Verkamp <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
---
 include/linux/pid_namespace.h | 19 ++++++++++++++
 include/uapi/linux/memfd.h    |  4 +++
 kernel/pid_namespace.c        | 48 +++++++++++++++++++++++++++++++++++
 mm/memfd.c                    | 48 +++++++++++++++++++++++++++++++++--
 4 files changed, 117 insertions(+), 2 deletions(-)

Patch

diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 07481bb87d4e..a4789a7b34a9 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..2b7563ddd22c 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -110,6 +110,11 @@  static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 	ns->ucounts = ucounts;
 	ns->pid_allocated = PIDNS_ADDING;
 
+#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
+	ns->memfd_noexec_scope =
+		task_active_pid_ns(current)->memfd_noexec_scope;
+#endif
+
 	return ns;
 
 out_free_idr:
@@ -255,6 +260,45 @@  void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	return;
 }
 
+#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
+static int pid_mfd_noexec_dointvec_minmax(struct ctl_table *table, int write,
+	void *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct pid_namespace *ns = task_active_pid_ns(current);
+	struct ctl_table table_copy;
+
+	if (write && !capable(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, buffer, 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", }, { } };
+#endif
+
 #ifdef CONFIG_CHECKPOINT_RESTORE
 static int pid_ns_ctl_handler(struct ctl_table *table, int write,
 		void *buffer, size_t *lenp, loff_t *ppos)
@@ -455,6 +499,10 @@  static __init int pid_namespaces_init(void)
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	register_sysctl_paths(kern_path, pid_ns_ctl_table);
 #endif
+
+#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
+	register_sysctl_paths(vm_path, pid_ns_ctl_table_vm);
+#endif
 	return 0;
 }
 
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;
 	}