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

[Sebastian Reichel]

Hi,

On Wed, May 24, 2023 at 11:28:41AM -0700, Linus Torvalds wrote:
> On Wed, May 24, 2023 at 10:12 AM Steven Rostedt <rostedt@xxxxxxxxxxx> 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

I have a fix for that in my fixes branch, that I planned to send
this week:

https://git.kernel.org/pub/scm/linux/kernel/git/sre/linux-power-supply.git/commit/?h=fixes&id=44c524b642996148a8e94f1a1b8751076edcf577

-- Sebastian

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