arm64: Add kernel return probes support (kretprobes)

The pre-handler of this special 'trampoline' kprobe executes the return
probe handler functions and restores original return address in ELR_EL1.
This way the saved pt_regs still hold the original register context to be
carried back to the probed kernel function.

Signed-off-by: Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com>
Signed-off-by: David A. Long <dave.long@linaro.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

2 files changed, 90 insertions, 1 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1f7d644f7f36..6af0e2e5c710 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -89,6 +89,7 @@ config ARM64
         select HAVE_RCU_TABLE_FREE
         select HAVE_SYSCALL_TRACEPOINTS
         select HAVE_KPROBES
+        select HAVE_KRETPROBES if HAVE_KPROBES
         select IOMMU_DMA if IOMMU_SUPPORT
         select IRQ_DOMAIN
         select IRQ_FORCED_THREADING
diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index be1f074b5736..9c70e8812ea9 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -578,7 +578,95 @@ bool arch_within_kprobe_blacklist(unsigned long addr)
 
 void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
 {
-        return NULL;
+        struct kretprobe_instance *ri = NULL;
+        struct hlist_head *head, empty_rp;
+        struct hlist_node *tmp;
+        unsigned long flags, orig_ret_address = 0;
+        unsigned long trampoline_address =
+                (unsigned long)&kretprobe_trampoline;
+        kprobe_opcode_t *correct_ret_addr = NULL;
+
+        INIT_HLIST_HEAD(&empty_rp);
+        kretprobe_hash_lock(current, &head, &flags);
+
+        /*
+         * It is possible to have multiple instances associated with a given
+         * task either because multiple functions in the call path have
+         * return probes installed on them, and/or more than one
+         * return probe was registered for a target function.
+         *
+         * We can handle this because:
+         *     - instances are always pushed into the head of the list
+         *     - when multiple return probes are registered for the same
+         *       function, the (chronologically) first instance's ret_addr
+         *       will be the real return address, and all the rest will
+         *       point to kretprobe_trampoline.
+         */
+        hlist_for_each_entry_safe(ri, tmp, head, hlist) {
+                if (ri->task != current)
+                        /* another task is sharing our hash bucket */
+                        continue;
+
+                orig_ret_address = (unsigned long)ri->ret_addr;
+
+                if (orig_ret_address != trampoline_address)
+                        /*
+                         * This is the real return address. Any other
+                         * instances associated with this task are for
+                         * other calls deeper on the call stack
+                         */
+                        break;
+        }
+
+        kretprobe_assert(ri, orig_ret_address, trampoline_address);
+
+        correct_ret_addr = ri->ret_addr;
+        hlist_for_each_entry_safe(ri, tmp, head, hlist) {
+                if (ri->task != current)
+                        /* another task is sharing our hash bucket */
+                        continue;
+
+                orig_ret_address = (unsigned long)ri->ret_addr;
+                if (ri->rp && ri->rp->handler) {
+                        __this_cpu_write(current_kprobe, &ri->rp->kp);
+                        get_kprobe_ctlblk()->kprobe_status = KPROBE_HIT_ACTIVE;
+                        ri->ret_addr = correct_ret_addr;
+                        ri->rp->handler(ri, regs);
+                        __this_cpu_write(current_kprobe, NULL);
+                }
+
+                recycle_rp_inst(ri, &empty_rp);
+
+                if (orig_ret_address != trampoline_address)
+                        /*
+                         * This is the real return address. Any other
+                         * instances associated with this task are for
+                         * other calls deeper on the call stack
+                         */
+                        break;
+        }
+
+        kretprobe_hash_unlock(current, &flags);
+
+        hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
+                hlist_del(&ri->hlist);
+                kfree(ri);
+        }
+        return (void *)orig_ret_address;
+}
+
+void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
+                                      struct pt_regs *regs)
+{
+        ri->ret_addr = (kprobe_opcode_t *)regs->regs[30];
+
+        /* replace return addr (x30) with trampoline */
+        regs->regs[30] = (long)&kretprobe_trampoline;
+}
+
+int __kprobes arch_trampoline_kprobe(struct kprobe *p)
+{
+        return 0;
 }
 
 int __init arch_init_kprobes(void)