Re: [PATCH bpf-next 1/6] bpf: fix bpf_put_raw_tracepoint()'s use of __module_address()

On Mon, Nov 23, 2020 at 9:49 PM Alexei Starovoitov
[off-list ref] wrote:

On Thu, Nov 19, 2020 at 03:22:39PM -0800, Andrii Nakryiko wrote:

quoted

__module_address() needs to be called with preemption disabled or with
module_mutex taken. preempt_disable() is enough for read-only uses, which is
what this fix does.

Fixes: a38d1107f937 ("bpf: support raw tracepoints in modules")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/trace/bpf_trace.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d255bc9b2bfa..bb98a377050a 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c

@@ -2060,7 +2060,11 @@ struct bpf_raw_event_map *bpf_get_raw_tracepoint(const char *name)

 void bpf_put_raw_tracepoint(struct bpf_raw_event_map *btp)
 {
-     struct module *mod = __module_address((unsigned long)btp);
+     struct module *mod;
+
+     preempt_disable();
+     mod = __module_address((unsigned long)btp);
+     preempt_enable();

      if (mod)
              module_put(mod);

I don't understand why 'mod' cannot become dangling pointer after preempt_enable().
Either it needs a comment explaining why it's ok or module_put() should
be in preempt disabled section.

Yeah, I think it can, assuming the kernel module can be unloaded
despite non-zero refcnt (probably happens with force unload?). I'll
drop the `if (mod)` part (module_put() checks that internally) and
will move module_put(mod) inside the preempt disable/enable region.