[Seccomp] Switch to refcount logic for kernels >= 5.9

Starting from kernel 5.9+ function 'put_seccomp_filter' has been inlined
and unavailble for hooking. However, internal not-inline function was used
to mitigate the problem. Unfortunately, there is no equivalent counter-part
function for new hook and the old one looks incompatible which we overlooked.
This patch is switching the logic for kernels 5.9+ to custom implementation
of refcount logic and it should address the issue reported as #338

src/modules/exploit_detection/p_exploit_detection.c

@@ -424,7 +424,7 @@ static notrace void p_dump_creds(struct p_cred *p_where, const struct cred *p_fr
 #if defined(CONFIG_SECCOMP)
 static notrace void p_dump_seccomp(struct p_seccomp *p_sec, struct task_struct *p_task, char p_force) {
 
-   P_SYM(p_get_seccomp_filter)(p_task);
+   p_lkrg_seccomp_filter_get(p_task);
    p_sec->sec.mode    = p_task->seccomp.mode;   // Mode
    p_sec->sec.filter  = p_task->seccomp.filter; // Filter
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(5,11,0)
@@ -437,12 +437,7 @@ static notrace void p_dump_seccomp(struct p_seccomp *p_sec, struct task_struct *
       p_sec->flag = 0;
    if (p_force)
       p_sec->flag_sync_thread = 0;
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,9,0)
-   P_SYM(p_put_seccomp_filter)(p_task->seccomp.filter);
-#else
-   P_SYM(p_put_seccomp_filter)(p_task);
-#endif
-
+   p_lkrg_seccomp_filter_put(p_task);
 }
 #endif
 
@@ -1373,8 +1368,12 @@ static int p_cmp_tasks(struct p_ed_process *p_orig, struct task_struct *p_curren
 
 #if defined(CONFIG_SECCOMP)
    /* Seccomp */
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5,9,0)
    if (p_orig->p_ed_task.p_sec.flag) { // SECCOMP was enabled so it make sense to compare...
-      P_SYM(p_get_seccomp_filter)(p_current);
+#else
+   if (p_orig->p_ed_task.p_sec.flag && current == p_current) { // SECCOMP was enabled so it make sense to compare...
+#endif
+      p_lkrg_seccomp_filter_get(p_current);
 
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(5,11,0)
       if (test_task_syscall_work(p_current,SECCOMP) != p_orig->p_ed_task.p_sec.flag) {
@@ -1408,11 +1407,7 @@ static int p_cmp_tasks(struct p_ed_process *p_orig, struct task_struct *p_curren
 
       P_CMP_PTR(p_orig->p_ed_task.p_sec.sec.filter, p_current->seccomp.filter, "seccomp filter")
 
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,9,0)
-      P_SYM(p_put_seccomp_filter)(p_current->seccomp.filter);
-#else
-      P_SYM(p_put_seccomp_filter)(p_current);
-#endif
+      p_lkrg_seccomp_filter_put(p_current);
    }
 #endif
 
@@ -1980,13 +1975,11 @@ int p_exploit_detection_init(void) {
    P_SYM_INIT(__kernel_text_address)
    P_SYM_INIT(mm_find_pmd)
 #if defined(CONFIG_SECCOMP)
-   P_SYM_INIT(get_seccomp_filter)
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,9,0)
-#define p___put_seccomp_filter p_put_seccomp_filter
-   P_SYM_INIT(__put_seccomp_filter)
-#else
-   P_SYM_INIT(put_seccomp_filter)
-#endif
+   if (P_LKRG_SUCCESS != p_lkrg_seccomp_init()) {
+      p_print_log(P_LOG_FATAL, "Can't initialize seccomp() logic");
+      p_ret = P_LKRG_GENERAL_ERROR;
+      goto p_exploit_detection_init_out;
+   }
 #endif
 
 #ifdef CONFIG_SECURITY_SELINUX

src/modules/exploit_detection/syscalls/p_seccomp/p_seccomp.c

@@ -34,6 +34,43 @@ static struct kretprobe p_seccomp_kretprobe = {
     .data_size = sizeof(struct p_seccomp_data),
 };
 
+int p_lkrg_seccomp_init(void) {
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5,9,0)
+   P_SYM_INIT(get_seccomp_filter)
+   P_SYM_INIT(put_seccomp_filter)
+#endif
+
+   return P_LKRG_SUCCESS;
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5,9,0)
+p_sym_error:
+   return P_LKRG_GENERAL_ERROR;
+#endif
+}
+
+void p_lkrg_seccomp_filter_get(struct task_struct *p_task) {
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5,9,0)
+   P_SYM(p_get_seccomp_filter)(p_task);
+#else
+   struct p_fake_seccomp_filter *p_filter = (struct p_fake_seccomp_filter *)p_task->seccomp.filter;
+
+   if (p_filter)
+      refcount_inc(&p_filter->refs);
+#endif
+}
+
+void p_lkrg_seccomp_filter_put(struct task_struct *p_task) {
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5,9,0)
+   P_SYM(p_put_seccomp_filter)(p_task);
+#else
+   struct p_fake_seccomp_filter *p_filter = (struct p_fake_seccomp_filter *)p_task->seccomp.filter;
+
+   if (p_filter)
+      refcount_dec(&p_filter->refs);
+#endif
+}
+
 /*
  * x86-64 syscall ABI:
  *  *rax - syscall_number

src/modules/exploit_detection/syscalls/p_seccomp/p_seccomp.h

@@ -28,6 +28,15 @@ struct p_seccomp_data {
     ktime_t entry_stamp;
 };
 
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,9,0)
+struct p_fake_seccomp_filter {
+    refcount_t refs;
+};
+#endif
+
+int p_lkrg_seccomp_init(void);
+void p_lkrg_seccomp_filter_get(struct task_struct *p_task);
+void p_lkrg_seccomp_filter_put(struct task_struct *p_task);
 
 int p_seccomp_ret(struct kretprobe_instance *p_ri, struct pt_regs *p_regs);
 int p_seccomp_entry(struct kretprobe_instance *p_ri, struct pt_regs *p_regs);