[PATCH v9 10/11] seccomp: allow mode setting across threads

[Kees Cook]

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 <keesc...@chromium.org>
---
 kernel/seccomp.c |   27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index e1ff2c193190..7bbcb9ed16df 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -207,12 +207,18 @@ static inline bool seccomp_check_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
@@ -433,12 +439,17 @@ 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
@@ -543,7 +554,7 @@ 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:
@@ -593,7 +604,7 @@ 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);
-- 
1.7.9.5

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