linux-kernel - Re: [BUG 6.4-rc3] BUG: kernel NULL pointer dereference in __dev

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Date:   Wed, 24 May 2023 11:28:41 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Sebastian Reichel <sre@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Matti Vaittinen <mazziesaccount@...il.com>,
        linux-pm@...r.kernel.org
Subject: Re: [BUG 6.4-rc3] BUG: kernel NULL pointer dereference in __dev_fwnode

On Wed, May 24, 2023 at 10:12 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> I started adding fixes to my urgent branch rebased on top of v6.4-rc3
> and ran my tests. Unfortunately they crashed on unrelated code.
>
> Here's the dump:
>
>  BUG: kernel NULL pointer dereference, address: 00000000000003e8
>  RIP: 0010:__dev_fwnode+0x9/0x2a
>  Code: ff 85 c0 78 16 48 8b 3c 24 89 c6 59 e9 e0 f7 ff ff b8 ea ff ff ff c3 cc cc cc cc 5a c3 cc cc cc cc f3 0f 1e fa 0f 1f 44 00 00 <48> 8b 87 e8 03 00 00 48
>  83 c0 18 c3 cc cc cc cc 48

That disassembles to

    endbr64
    nopl   0x0(%rax,%rax,1)
    mov    0x3e8(%rdi),%rax
    add    $0x18,%rax
    ret

which looks like it must be the

    return dev->fwnode;

with a NULL 'dev'. Which makes sense for __dev_fwnode with CONFIG_OF
not enabled.

Except I have no idea what that odd 'add $0x18" is all about. Strange.

Anyway, the caller seems to be this code in power_supply_get_battery_info():

        if (psy->of_node) {
            .. presumably not this ..
        } else {
                err = fwnode_property_get_reference_args(
                                        dev_fwnode(psy->dev.parent),
                                        "monitored-battery", NULL, 0, 0, &args);
                ...

so I suspect we have psy->dev.parent being NULL.

>  I ran a bisect and it found it to be this commit:
>
> 27a2195efa8d2 ("power: supply: core: auto-exposure of simple-battery data")
>
> I checked out that commit and tested it, and it crashed. I then
> reverted that commit, and the crash goes away.

At a guess, it's

 (a) the new code to expose battery info at registration time:

+       /*
+        * Expose constant battery info, if it is available. While there are
+        * some chargers accessing constant battery data, we only want to
+        * expose battery data to userspace for battery devices.
+        */
+       if (desc->type == POWER_SUPPLY_TYPE_BATTERY) {
+               rc = power_supply_get_battery_info(psy, &psy->battery_info);
+               if (rc && rc != -ENODEV && rc != -ENOENT)
+                       goto check_supplies_failed;
+       }

interacting with

 (b) the test_power_init() that does that

                test_power_supplies[i] = power_supply_register(NULL,
                                                &test_power_desc[i],
                                                &test_power_configs[i]);

which passes in NULL for the "parent" pointer.

So it looks like a dodgy test that was a bit lazy. But maybe a NULL
parent is supposed to work.

                Linus