seccomp: allow mode setting across threads

This changes the mode setting helper to allow threads to change the
seccomp mode from another thread. We must maintain barriers to keep
TIF_SECCOMP synchronized with the rest of the seccomp state.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Andy Lutomirski <luto@amacapital.net>

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index d5543e787e4e84fb0643d670113bd1b94f6148cb..9065d2c79c569c671b5d62ea14c835ba0c33c83f 100644 (file)
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -173,21 +173,24 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
  */
 static u32 seccomp_run_filters(int syscall)
 {
-       struct seccomp_filter *f;
+       struct seccomp_filter *f = ACCESS_ONCE(current->seccomp.filter);
        struct seccomp_data sd;
        u32 ret = SECCOMP_RET_ALLOW;
 
        /* Ensure unexpected behavior doesn't result in failing open. */
-       if (WARN_ON(current->seccomp.filter == NULL))
+       if (unlikely(WARN_ON(f == NULL)))
                return SECCOMP_RET_KILL;
 
+       /* Make sure cross-thread synced filter points somewhere sane. */
+       smp_read_barrier_depends();
+
        populate_seccomp_data(&sd);
 
        /*
         * All filters in the list are evaluated and the lowest BPF return
         * value always takes priority (ignoring the DATA).
         */
-       for (f = current->seccomp.filter; f; f = f->prev) {
+       for (; f; f = f->prev) {
                u32 cur_ret = SK_RUN_FILTER(f->prog, (void *)&sd);
 
                if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
@@ -207,12 +210,18 @@ static inline bool seccomp_may_assign_mode(unsigned long seccomp_mode)
        return true;
 }
 
-static inline void seccomp_assign_mode(unsigned long seccomp_mode)
+static inline void seccomp_assign_mode(struct task_struct *task,
+                                      unsigned long seccomp_mode)
 {
-       BUG_ON(!spin_is_locked(&current->sighand->siglock));
+       BUG_ON(!spin_is_locked(&task->sighand->siglock));
 
-       current->seccomp.mode = seccomp_mode;
-       set_tsk_thread_flag(current, TIF_SECCOMP);
+       task->seccomp.mode = seccomp_mode;
+       /*
+        * Make sure TIF_SECCOMP cannot be set before the mode (and
+        * filter) is set.
+        */
+       smp_mb__before_atomic();
+       set_tsk_thread_flag(task, TIF_SECCOMP);
 }
 
 #ifdef CONFIG_SECCOMP_FILTER
@@ -435,12 +444,17 @@ static int mode1_syscalls_32[] = {
 
 int __secure_computing(int this_syscall)
 {
-       int mode = current->seccomp.mode;
        int exit_sig = 0;
        int *syscall;
        u32 ret;
 
-       switch (mode) {
+       /*
+        * Make sure that any changes to mode from another thread have
+        * been seen after TIF_SECCOMP was seen.
+        */
+       rmb();
+
+       switch (current->seccomp.mode) {
        case SECCOMP_MODE_STRICT:
                syscall = mode1_syscalls;
 #ifdef CONFIG_COMPAT
@@ -545,7 +559,7 @@ static long seccomp_set_mode_strict(void)
 #ifdef TIF_NOTSC
        disable_TSC();
 #endif
-       seccomp_assign_mode(seccomp_mode);
+       seccomp_assign_mode(current, seccomp_mode);
        ret = 0;
 
 out:
@@ -595,7 +609,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
        /* Do not free the successfully attached filter. */
        prepared = NULL;
 
-       seccomp_assign_mode(seccomp_mode);
+       seccomp_assign_mode(current, seccomp_mode);
 out:
        spin_unlock_irq(&current->sighand->siglock);
        seccomp_filter_free(prepared);