[PATCH bpf-next] kallsyms: Disable preemption for find_kallsyms_symbol_value

Jiri Olsa
2023-04-03T22:03:05Z

Artem reported suspicious RCU usage [1]. The reason is that verifier
calls find_kallsyms_symbol_value with preemption enabled which will
trigger suspicious RCU usage warning in rcu_dereference_sched call.

Disabling preemption in find_kallsyms_symbol_value and adding
__find_kallsyms_symbol_value function.

Fixes: 31bf1dbccfb0 ("bpf: Fix attaching fentry/fexit/fmod_ret/lsm to modules")
[1] https://lore.kernel.org/bpf/ZBrPMkv8YVRiWwCR@samus.usersys.redhat.com/
Reported-by: Artem Savkov <asavkov@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
kernel/module/kallsyms.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
index ab2376a1be88..bdc911dbcde5 100644
--- a/kernel/module/kallsyms.c
+++ b/kernel/module/kallsyms.c
@@ -442,7 +442,7 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
}

/* Given a module and name of symbol, find and return the symbol's value */
-unsigned long find_kallsyms_symbol_value(struct module *mod, const char *name)
+static unsigned long __find_kallsyms_symbol_value(struct module *mod, const char *name)
{
unsigned int i;
struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
@@ -466,7 +466,7 @@ static unsigned long __module_kallsyms_lookup_name(const char *name)
if (colon) {
mod = find_module_all(name, colon - name, false);
if (mod)
- return find_kallsyms_symbol_value(mod, colon + 1);
+ return __find_kallsyms_symbol_value(mod, colon + 1);
return 0;
}

@@ -475,7 +475,7 @@ static unsigned long __module_kallsyms_lookup_name(const char *name)

if (mod->state == MODULE_STATE_UNFORMED)
continue;
- ret = find_kallsyms_symbol_value(mod, name);
+ ret = __find_kallsyms_symbol_value(mod, name);
if (ret)
return ret;
}
@@ -494,6 +494,16 @@ unsigned long module_kallsyms_lookup_name(const char *name)
return ret;
}

+unsigned long find_kallsyms_symbol_value(struct module *mod, const char *name)
+{
+ unsigned long ret;
+
+ preempt_disable();
+ ret = __find_kallsyms_symbol_value(mod, name);
+ preempt_enable();
+ return ret;
+}
+
int module_kallsyms_on_each_symbol(const char *modname,
int (*fn)(void *, const char *,
struct module *, unsigned long),
--
2.39.2