[PATCHv3 bpf] bpf: Take module reference for trampoline in module

[PATCHv3 bpf] bpf: Take module reference for trampoline in module

[Jiri Olsa]

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 <jolsa@kernel.org>
---
 v3 changes:
   - store module pointer under bpf_trampoline struct

 include/linux/bpf.h     |  2 ++
 kernel/bpf/trampoline.c | 30 ++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 3625f019767d..fdac0534ce79 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -40,6 +40,7 @@ struct bpf_local_storage;
 struct bpf_local_storage_map;
 struct kobject;
 struct mem_cgroup;
+struct module;
 
 extern struct idr btf_idr;
 extern spinlock_t btf_idr_lock;
@@ -623,6 +624,7 @@ struct bpf_trampoline {
 	/* Executable image of trampoline */
 	struct bpf_tramp_image *cur_image;
 	u64 selector;
+	struct module *mod;
 };
 
 struct bpf_attach_target_info {
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 1f3a4be4b175..4aa8b52adf25 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -9,6 +9,7 @@
 #include <linux/btf.h>
 #include <linux/rcupdate_trace.h>
 #include <linux/rcupdate_wait.h>
+#include <linux/module.h>
 
 /* dummy _ops. The verifier will operate on target program's ops. */
 const struct bpf_verifier_ops bpf_extension_verifier_ops = {
@@ -87,6 +88,26 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
 	return tr;
 }
 
+static int bpf_trampoline_module_get(struct bpf_trampoline *tr)
+{
+	struct module *mod;
+	int err = 0;
+
+	preempt_disable();
+	mod = __module_text_address((unsigned long) tr->func.addr);
+	if (mod && !try_module_get(mod))
+		err = -ENOENT;
+	preempt_enable();
+	tr->mod = mod;
+	return err;
+}
+
+static void bpf_trampoline_module_put(struct bpf_trampoline *tr)
+{
+	module_put(tr->mod);
+	tr->mod = NULL;
+}
+
 static int is_ftrace_location(void *ip)
 {
 	long addr;
@@ -108,6 +129,9 @@ 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)
+		bpf_trampoline_module_put(tr);
 	return ret;
 }
 
@@ -134,10 +158,16 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
 		return ret;
 	tr->func.ftrace_managed = ret;
 
+	if (bpf_trampoline_module_get(tr))
+		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)
+		bpf_trampoline_module_put(tr);
 	return ret;
 }
 
-- 
2.30.2

Re: [PATCHv3 bpf] bpf: Take module reference for trampoline in module

[Alexei Starovoitov]

On Fri, Mar 26, 2021 at 11:59:00AM +0100, 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 <jolsa@kernel.org>
> ---
>  v3 changes:
>    - store module pointer under bpf_trampoline struct

Applied.