Re: [PATCHv2] bpf: Take module reference for trampoline in module

[Daniel Borkmann]

On 3/24/21 6:40 PM, Jiri Olsa wrote:

Currently module can be unloaded even if there's a trampoline
register in it. It's easily reproduced by running in parallel:

   # while :; do ./test_progs -t module_attach; done
   # while :; do rmmod bpf_testmod; sleep 0.5; done

Taking the module reference in case the trampoline's ip is
within the module code. Releasing it when the trampoline's
ip is unregistered.

Signed-off-by: Jiri Olsa <jo...@kernel.org>
---
v2 changes:
   - fixed ip_module_put to do preempt_disable/preempt_enable

  kernel/bpf/trampoline.c | 31 +++++++++++++++++++++++++++++++
  1 file changed, 31 insertions(+)

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 1f3a4be4b175..39e4280f94e4 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -87,6 +87,26 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
        return tr;
  }

+static struct module *ip_module_get(unsigned long ip)

+{
+       struct module *mod;
+       int err = 0;
+
+       preempt_disable();
+       mod = __module_text_address(ip);
+       if (mod && !try_module_get(mod))
+               err = -ENOENT;
+       preempt_enable();
+       return err ? ERR_PTR(err) : mod;
+}
+
+static void ip_module_put(unsigned long ip)
+{
+       preempt_disable();
+       module_put(__module_text_address(ip));
+       preempt_enable();

Could we cache the mod pointer in tr instead of doing another addr search
for dropping the ref?

+}
+
  static int is_ftrace_location(void *ip)
  {
        long addr;
@@ -108,6 +128,9 @@ static int unregister_fentry(struct bpf_trampoline *tr, 
void *old_addr)
                ret = unregister_ftrace_direct((long)ip, (long)old_addr);
        else
                ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, NULL);
+
+       if (!ret)
+               ip_module_put((unsigned long) ip);
        return ret;
  }

@@ -126,6 +149,7 @@ static int modify_fentry(struct bpf_trampoline *tr, void *old_addr, void *new_ad

  /* first time registering */
  static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
  {
+       struct module *mod;
        void *ip = tr->func.addr;
        int ret;

@@ -134,10 +158,17 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)

                return ret;
        tr->func.ftrace_managed = ret;

+ mod = ip_module_get((unsigned long) ip);

+       if (IS_ERR(mod))
+               return -ENOENT;
+
        if (tr->func.ftrace_managed)
                ret = register_ftrace_direct((long)ip, (long)new_addr);
        else
                ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
+
+       if (ret)
+               module_put(mod);
        return ret;
  }