Re: [PATCH 1/2] ftrace: fix UAF when lookup kallsym after ftrace disabled

On Mon, 26 May 2025 09:33:37 +0800
yebin [off-list ref] wrote:

On 2025/5/24 1:54, Steven Rostedt wrote:

quoted

On Fri, 23 May 2025 16:39:44 +0800
Ye Bin [off-list ref] wrote:
 

quoted

Above issue may happens as follow:
(1) Add kprobe trace point;
(2) insmod test.ko;
(3) Trigger ftrace disabled;  

This is the bug. How was ftrace_disabled triggered? That should never
happen. Was test.ko buggy?
 

Yes. The following warning is reported during concurrent registration 
between register_kprobe() and live patch, causing ftrace_disabled.

WARNING: CPU: 56 PID: 2769 at kernel/trace/ftrace.c:2612 
ftrace_modify_all_code+0x116/0x140

OK, so it is a buggy module.

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(4) rmmod test.ko;
(5) cat /proc/kallsyms; --> Will trigger UAF as test.ko already removed;
ftrace_mod_get_kallsym()
...
strscpy(module_name, mod_map->mod->name, MODULE_NAME_LEN);
...

As ftrace_release_mod() judge 'ftrace_disabled' is true will return, and
'mod_map' will remaining in ftrace_mod_maps. 'mod_map' has no chance to
release. Therefore, this also causes residual resources to accumulate.
To solve above issue, unconditionally clean up'mod_map'.

Fixes: aba4b5c22cba ("ftrace: Save module init functions kallsyms symbols for tracing")  

This is *not* a fix. ftrace_disabled gets set when a bug is triggered. If
this prevents ftrace_disabled from getting set, then it would be a fix. But
if something else happens when ftrace_disabled is set, it just fixes a
symptom and not the bug itself.
 

There are multiple causes for triggering ftrace_disabled. I agree that 

Yes, just like there's multiple causes for BUG_ON() ;-)

The ftrace_disable is used to help keep the system from being totally
corrupted. When it triggers, the best thing to do is a reboot.

aba4b5c22cba is not faulty. However, the incorporation of this patch 
will cause problems due to triggering ftrace_disabled. The generation of 
ftrace_disabled is beyond our control. This is related to the user. What 
we can do is even if there are no additional derivative problems.

Well, when a user inserts a module, then they become a kernel developer too ;-)

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Signed-off-by: Ye Bin <redacted>
---
  kernel/trace/ftrace.c | 3 ---
  1 file changed, 3 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index a3d4dfad0cbc..ff5d9d73a4a7 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c

@@ -7438,9 +7438,6 @@ void ftrace_release_mod(struct module *mod)

  	mutex_lock(&ftrace_lock);

-	if (ftrace_disabled)
-		goto out_unlock;
-  

Here you delete the check, and the next patch you have:

+	if (ftrace_disabled || (mod && !mod->num_ftrace_callsites)) {
+		mutex_unlock(&ftrace_lock);
+		return;
+	}
+
 

The second patch I added judgment when initializing 'mod_map' in 
ftrace_free_mem(). The first patch removes the judgment when 
ftrace_release_mod() releases'mod_map'. The logic modified by the two 
patches is isolated.

Actually I think both patches are buggy.

When ftrace_disabled is set, we don't know the state of the code and we do
not want to do *any* more text modification. That's what ftrace_disable
means. Something went wrong with text modification and any more changes can
cause a bigger problem.

We don't add "exceptions".

If you are worried about unloading modules when ftrace_disable is set, what
is a much safer solution is to up the module count of all modules that have
any ftrace callsites active, and prevent those modules from being removed.

Again, the only solution to a ftrace_disable being set is a full reboot.

-- Steve