[PATCH bpf-next 1/2] bpf: Fix memleak due to fentry attach failure

[Yafang Shao <laoar.shao@gmail.com>]

If it fails to attach fentry, the allocated bpf trampoline image will be
left in the system. That can be verified by checking /proc/kallsyms.

This meamleak can be verified by a simple bpf program as follows,

  SEC("fentry/trap_init")
  int fentry_run()
  {
      return 0;
  }

It will fail to attach trap_init because this function is freed after
kernel init, and then we can find the trampoline image is left in the
system by checking /proc/kallsyms.
  $ tail /proc/kallsyms
  ffffffffc0613000 t bpf_trampoline_6442453466_1  [bpf]
  ffffffffc06c3000 t bpf_trampoline_6442453466_1  [bpf]

  $ bpftool btf dump file /sys/kernel/btf/vmlinux | grep "FUNC 'trap_init'"
  [2522] FUNC 'trap_init' type_id=119 linkage=static

  $ echo $((6442453466 & 0x7fffffff))
  2522

Note that there are two left bpf trampoline images, that is because the
libbpf will fallback to raw tracepoint if -EINVAL is returned.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 kernel/bpf/trampoline.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index ac021bc..7067cdf 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -251,6 +251,15 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
 	return tlinks;
 }
 
+static void bpf_tramp_image_free(struct bpf_tramp_image *im)
+{
+	bpf_image_ksym_del(&im->ksym);
+	bpf_jit_free_exec(im->image);
+	bpf_jit_uncharge_modmem(PAGE_SIZE);
+	percpu_ref_exit(&im->pcref);
+	kfree(im);
+}
+
 static void __bpf_tramp_image_put_deferred(struct work_struct *work)
 {
 	struct bpf_tramp_image *im;
@@ -438,7 +447,7 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
 					  &tr->func.model, tr->flags, tlinks,
 					  tr->func.addr);
 	if (err < 0)
-		goto out;
+		goto out_free;
 
 	set_memory_rox((long)im->image, 1);
 
@@ -468,7 +477,7 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
 	}
 #endif
 	if (err)
-		goto out;
+		goto out_free;
 
 	if (tr->cur_image)
 		bpf_tramp_image_put(tr->cur_image);
@@ -480,6 +489,10 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
 		tr->flags = orig_flags;
 	kfree(tlinks);
 	return err;
+
+out_free:
+	bpf_tramp_image_free(im);
+	goto out;
 }
 
 static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(struct bpf_prog *prog)
-- 
1.8.3.1