seccomp: Report number of loaded filters in /proc/$pid/status

A common question asked when debugging seccomp filters is "how many
filters are attached to your process?" Provide a way to easily answer
this question through /proc/$pid/status with a "Seccomp_filters" line.
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/proc/array.c

@@ -341,6 +341,8 @@ static inline void task_seccomp(struct seq_file *m, struct task_struct *p)
 	seq_put_decimal_ull(m, "NoNewPrivs:\t", task_no_new_privs(p));
 #ifdef CONFIG_SECCOMP
 	seq_put_decimal_ull(m, "\nSeccomp:\t", p->seccomp.mode);
+	seq_put_decimal_ull(m, "\nSeccomp_filters:\t",
+			    atomic_read(&p->seccomp.filter_count));
 #endif
 	seq_puts(m, "\nSpeculation_Store_Bypass:\t");
 	switch (arch_prctl_spec_ctrl_get(p, PR_SPEC_STORE_BYPASS)) {

include/linux/seccomp.h

@@ -13,6 +13,7 @@
 #ifdef CONFIG_SECCOMP
 
 #include <linux/thread_info.h>
+#include <linux/atomic.h>
 #include <asm/seccomp.h>
 
 struct seccomp_filter;
@@ -29,6 +30,7 @@ struct seccomp_filter;
  */
 struct seccomp {
 	int mode;
+	atomic_t filter_count;
 	struct seccomp_filter *filter;
 };
 

init/init_task.c

@@ -204,6 +204,9 @@ struct task_struct init_task
 #ifdef CONFIG_SECURITY
 	.security	= NULL,
 #endif
+#ifdef CONFIG_SECCOMP
+	.seccomp	= { .filter_count = ATOMIC_INIT(0) },
+#endif
 };
 EXPORT_SYMBOL(init_task);
 

kernel/seccomp.c

@@ -398,6 +398,8 @@ static inline void seccomp_sync_threads(unsigned long flags)
 		put_seccomp_filter(thread);
 		smp_store_release(&thread->seccomp.filter,
 				  caller->seccomp.filter);
+		atomic_set(&thread->seccomp.filter_count,
+			   atomic_read(&thread->seccomp.filter_count));
 
 		/*
 		 * Don't let an unprivileged task work around
@@ -544,6 +546,7 @@ static long seccomp_attach_filter(unsigned int flags,
 	 */
 	filter->prev = current->seccomp.filter;
 	current->seccomp.filter = filter;
+	atomic_inc(&current->seccomp.filter_count);
 
 	/* Now that the new filter is in place, synchronize to all threads. */
 	if (flags & SECCOMP_FILTER_FLAG_TSYNC)