[PATCH] net: eth-uclass: change state before stop() in eth_halt()

[Niel Fourie]

Hi Marek,

On 04/12/2022 03:17, Marek Vasut wrote:
> On 12/2/22 11:36, Niel Fourie wrote:
>> Hi Marek,
> 
> Hi,
> 
>> On 01/12/2022 11:44, Marek Vasut wrote:
>>> On 12/1/22 09:24, Lukasz Majewski wrote:
>>>> On Wed, 30 Nov 2022 17:42:25 +0100
>>>> Niel Fourie <lusus at denx.de> wrote:
>>>>
>>>>> In eth_halt(), change the private uclass state before calling
>>>>> stop() instead of afterwards, to avoid writing to memory which
>>>>> may have been freed during stop().
>>>>>
>>>>> In the ethernet gadget implementation, the gadget device gets
>>>>> probed during start() and removed during stop(), which includes
>>>>> freeing `uclass_priv_` to which `priv` is pointing. Writing to
>>>>> `priv` after stop() may corrupt the `fd` member of `struct
>>>>> malloc_chunk`, which represents the freed block, and could cause
>>>>> hard-to-debug crashes on subsequent calls to malloc()/free().
>>>>>
>>>>> Signed-off-by: Niel Fourie <lusus at denx.de>
>>>>> Cc: Ramon Fried <rfried.dev at gmail.com>
>>>>> Cc: Marek Vasut <marex at denx.de>
>>>>> Cc: Lukasz Majewski <lukma at denx.de>
>>>>> ---
>>>>>   net/eth-uclass.c | 2 +-
>>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/net/eth-uclass.c b/net/eth-uclass.c
>>>>> index f41da4b37b3..bc3b9751e32 100644
>>>>> --- a/net/eth-uclass.c
>>>>> +++ b/net/eth-uclass.c
>>>>> @@ -342,9 +342,9 @@ void eth_halt(void)
>>>>>       if (!priv || !priv->running)
>>>>>           return;
>>>>> -    eth_get_ops(current)->stop(current);
>>>>>       priv->state = ETH_STATE_PASSIVE;
>>>>>       priv->running = false;
>>>>> +    eth_get_ops(current)->stop(current);
>>>>>   }
>>>>>   int eth_is_active(struct udevice *dev)
>>>>
>>>> Reviewed-by: Lukasz Majewski <lukma at denx.de>
>>>
>>> How come nobody triggered this problem with regular ethernet in U-Boot ?
>>>
>>> If this is isolated to USB gadget ethernet, then please do not hack 
>>> around this in core networking code, but rather fix the USB ethernet 
>>> gadget itself. It seems that gadget code should not unregister the 
>>> gadget in drivers/usb/gadget/ether.c _usb_eth_halt() , at least not 
>>> fully.
>>
>> The reason is simple, the regular ethernet drivers do not get removed 
>> on stop() like for the gadget ethernet driver, and in their case 
>> `priv` is still valid.
> 
> The suggestion for a proper fix is in the last paragraph above -- do not 
> unregister the usb ethernet gadget device in halt(), keep the gadget 
> device registered. Then the priv pointer would be valid. (*)
> 
>> I agree on your point that reworking the ethernet gadget code would be 
>> preferable, but this would be a much bigger effort (and if I were to 
>> do it, I would probably introduce even more bugs). I am not certain 
>> whether this would not also affect the non-DM gadget implementation as 
>> well, which still contain drivers like ci_udc.c which does not appear 
>> to have been ported to DM gadget yet? (I only see DM USB there...)
>>
>> That said, I am not certain whether this is not also bug, as I am not 
>> certain whether the assumption that `priv` should be available after 
>> stop() is valid or not.
>>
>> The documentation at 
>> https://source.denx.de/u-boot/u-boot/-/blob/master/doc/develop/driver-model/ethernet.rst?plain=1#L135 states:
>>
>> The **stop** function should turn off / disable the hardware and place 
>> it back in its reset state.  It can be called at any time (before any 
>> call to the related start() function), so make sure it can handle this 
>> sort of thing.
> 
> The ->stop callback is supposed to stop the interface, turn off its DMA, 
> but NOT deallocate the device (and its associated data) behind it.
> 
>> In such a complete reset state I am not certain whether the assumption 
>> that `priv` should exist is still valid, at least not without another 
>> call to dev_get_uclass_priv() and revalidating it first?
> 
> In case a device is probe()d, its private data are also allocated and 
> available, so yes, 'priv' pointer should still be valid.

Granted, the usb gadget driver implementation is problematic, and it 
definitely belongs on the TODO list.

That being said, priv not being valid at this stage is not a new 
problem, as validation for it after stop() was explicitly added in 
commit c3211708 ("net: eth-uclass: Fix for DM USB ethernet support") for 
this reason 4 years ago. Fortunately in that case, it just fixed a null 
pointer de-reference, not corruption of freed memory.

>> Lastly, this assumption that priv is still valid is rather new and it 
>> was introduced here:
>>
>> https://source.denx.de/u-boot/u-boot/-/commit/fa795f452541ce07b33be603de36cac3c5d7dfcf
> 
> I disagree, the device private data are valid during the entire lifespan 
> of the device. That assumption has been baked into the driver model 
> itself and far predates that commit.
> 
> In case of a usb ethernet, the lifespan of the device starts with the 
> 'bind' command which triggers the ->bind callback, and first use which 
> triggers its ->probe callback. The lifespan ends with 'unbind' command 
> or OS boot, which triggers ->remove callback and ->unbind callbacks.
> 
>> This commit appears to be a workaround for drivers which cannot deal 
>> with stop() being called at any time as required in the above quoted 
>> documentation.
> 
> This commit prevents network device ->start() from being called multiple 
> times, which is a valid precaution, as calling start while the interface 
> is already up would interfere with existing connection (e.g. the 
> netconsole as mentioned in the commit message). That does not seem to be 
> a workaround to me.

Ah yes, the commit message indeed states that some drivers (like 
fec_mxc) cannot deal with multiple start() calls, not multiple stop() 
calls. My mistake, good point.

But preventing multiple start()s getting called is not what the code 
change in this commit is actually doing. It is instead inhibiting 
calling stop() on devices for which priv is not valid or for which 
priv->running is false. Therefore it is avoiding calling stop() on 
un-started devices.

>> I would consider adding a workaround to a workaround in this case to 
>> be the lesser evil, as tracking down this bug in the first place was 
>> like looking for a needle in a haystack. This change would at least 
>> save everybody else from strange crashes in particular configurations 
>> without any negative impact. But this is fortunately not my decision. :)
> 
> Commit fa795f45254 ("net: eth-uclass: avoid running start() twice 
> without stop()") is as far as I can tell unrelated to this change and 
> does not seem to me like a workaround.

It does introduce writing to priv after stop() is called without 
(re-)validating priv first, though.

The code previously first called stop(), then called 
dev_get_uclass_priv() to get priv and then validated priv before writing 
to it, which avoids exactly the problem of priv no longer being valid.

As mentioned above, this was explicitly addressed by commit c3211708 
("net: eth-uclass: Fix for DM USB ethernet support").

> It does however show that this patch introduces a bug -- this patch 
> changes the order in which priv->state = ETH_STATE_PASSIVE; is assigned 
> from _after_ the ->stop callback to _before_ the -> stop callback. This 
> breaks drivers/net/ldpaa_eth/ldpaa_eth.c which checks the priv->state in 
> its ->stop callback, either on its own in non-DM case, or in 
> eth_is_active() implementation in DM case. With this patch, the 
> interface would never be stopped in the ->stop callback, because the 
> condition (net_dev->state == ETH_STATE_PASSIVE) test in the ldpaa stop 
> callback implementation would always be true.
>

In drivers/net/ldpaa_eth/ldpaa_eth.c:ldpaa_eth_stop(), priv is of type
struct ldpaa_eth_priv*, defined in drivers/net/ldpaa_eth/ldpaa_eth.h and 
is accessed using dev_get_priv().

In net/eth-uclass.c:eht_halt(), priv is of type struct eth_device_priv* 
and defined in the same .c file, and is accessed using 
dev_get_uclass_priv(). As the structure is local to this file, nothing 
outside of this file should have any knowledge of its contents, and 
changing of the order of the calls should only impact this file.

I sincerely hope that these two are not interfering with each other, 
otherwise we have much bigger problems...

> The proper fix is in the usb ethernet gadget code, see (*) above, let's 
> not pile workarounds onto already difficult to maintain code.
> 
> [...]
Agreed. The problem is unfortunately just that I don't see that 
happening simply to fix this bug.

Best regards,
Niel Fourie

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