Re: [PATCH v3] mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC

On Fri, Dec 02, 2022 at 01:34:00AM +0000, jeffxu@chromium.org wrote:

From: Jeff Xu jeffxu@chromium.org

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.

^ nit: missing space

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.

Co-developed-by: Daniel Verkamp dverkamp@chromium.org Signed-off-by: Daniel Verkamp dverkamp@chromium.org Signed-off-by: Jeff Xu jeffxu@chromium.org

---

include/linux/pid_namespace.h | 19 ++++++++++++++ include/uapi/linux/memfd.h | 4 +++ kernel/pid_namespace.c | 47 +++++++++++++++++++++++++++++++++++ mm/memfd.c | 44 ++++++++++++++++++++++++++++++-- 4 files changed, 112 insertions(+), 2 deletions(-)

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..71dd9b0a0f62 100644 --- a/kernel/pid_namespace.c +++ b/kernel/pid_namespace.c @@ -110,6 +110,10 @@ 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 = MEMFD_NOEXEC_SCOPE_EXEC;

+#endif

I think this should be inherited from the parent pid namespace instead?

• return ns;

out_free_idr: @@ -255,6 +259,45 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns) return; } +#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE) +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;
  

Yeah, I like this kind of enforcement.

• 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 +498,10 @@ 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..69e897dea6d5 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 +264,13 @@ 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) {

• struct pid_namespace *ns; unsigned int *file_seals; struct file *file; int fd, error;

@@ -285,6 +287,36 @@ 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;
  

• if (sysctl == MEMFD_NOEXEC_SCOPE_EXEC) {
  

• 	flags |= MFD_EXEC;
  

• } else if (sysctl == MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL) {
  

• 	flags |= MFD_NOEXEC_SEAL;
  

• } else {
  

• 	pr_warn_ratelimited(
  

• 		"memfd_create(): MFD_NOEXEC_SEAL is enforced, pid=%d\n",
  

• 		task_pid_nr(current));
  

• 	return -EINVAL;
  

• }
  

Not a huge deal, but the above could be a switch statement to improve readability.

+#else

• flags |= MFD_EXEC;
  

+#endif

• pr_warn_ratelimited(
  

• 	"memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL, pid=%d\n",
  

• 	task_pid_nr(current));
  

Perhaps include process name as well -- makes admin lives easier. :)

pr_warn_ratelimited( "memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL: %s[%d]\n", current->comm, task_pid_nr(current));

• }
• /* length includes terminating zero */ len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1); if (len <= 0)

@@ -328,7 +360,15 @@ 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.rc0.267.gcb52ba06e7-goog

Otherwise, looks good to me.

Reviewed-by: Kees Cook keescook@chromium.org