linux-kernel - Re: [PATCH] bus: fsl-mc: fix improper free of mc

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Date:   Wed, 28 Apr 2021 10:26:16 -0700
From:   Tom Rix <trix@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     stuyoder@...il.com, laurentiu.tudor@....com,
        Nathan Chancellor <nathan@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] bus: fsl-mc: fix improper free of mc_dev


On 4/27/21 12:19 PM, Nick Desaulniers wrote:
> On Tue, Apr 27, 2021 at 11:36 AM <trix@...hat.com> wrote:
>> From: Tom Rix <trix@...hat.com>
>>
>> Clang static analysis reports this error
>>
>> fsl-mc-bus.c:891:2: warning: Attempt to free released memory
>>          kfree(mc_dev);
>>          ^~~~~~~~~~~~~
>>
>> In this block of code
>>
>> if (strcmp(obj_desc->type, "dprc") == 0) {
>> ..
>>    mc_bus = kzalloc(..)
>>    mc_dev = &mc_bus->mc_dev;
> Thanks for the patch.
>
> Aren't the allocations for mc_bus and mc_dev mutually exclusive based
> on that conditional? If so...
>
>> mc_dev is not alloc-ed, so it should not be freed.
>> Old handler triggers a false positive from checkpatch, so add a
>> comment and change logic a bit.
>>
>> Fixes: a042fbed0290 ("staging: fsl-mc: simplify couple of deallocations")
>> Signed-off-by: Tom Rix <trix@...hat.com>
>> ---
>>   drivers/bus/fsl-mc/fsl-mc-bus.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/bus/fsl-mc/fsl-mc-bus.c b/drivers/bus/fsl-mc/fsl-mc-bus.c
>> index 380ad1fdb745..fb3e1d8a7f63 100644
>> --- a/drivers/bus/fsl-mc/fsl-mc-bus.c
>> +++ b/drivers/bus/fsl-mc/fsl-mc-bus.c
>> @@ -887,8 +887,10 @@ int fsl_mc_device_add(struct fsl_mc_obj_desc *obj_desc,
>>
>>   error_cleanup_dev:
>>          kfree(mc_dev->regions);
>> +       /* mc_dev is only allocated when it is not part of mc_bus */
>> +       if (!mc_bus)
>> +               kfree(mc_dev);
>>          kfree(mc_bus);
>> -       kfree(mc_dev);
> The error handling here seems quite wrong (regardless of your patch).
> mc_dev->regions is allocated by fsl_mc_device_get_mmio_regions() IIUC.
> Wouldn't the first `goto error_cleanup_dev;` taken end up passing an
> uninitialized pointer to kfree()?

On the first goto, mc_dev->regions, because of the kzalloc, the value 
would be

mc_bus->mc_dev.regions , should be 0 or

mc_dev->regions, which also should be 0

and kfree handles 0.

>
> what if `strcmp(obj_desc->type, "dprc") == 0` is false? We allocate
> `mc_dev`, but then call kfree on `mc_bus`?

mc_bus is initialized to NULL, which makes the call to kfree safe.

The original handler was

if (mc_bus)

   kfree(mc_bus)

else

   kfree(mc_dev)

I tried this first, which works, but checkpatch throw a warning for 
kfree(mc_bus).

This change makes the 'else' with the !mc_bus

> I think it would be safer to locally save the result of
> `strcmp(obj_desc->type, "dprc") == 0`, then check that throughout this
the local mc_bus is only set in this block, so I don't think another 
local is needed.
> function, including the error handling at the end, or use multiple
> labels to unwind the allocations correctly.

The goto's could be finer grained because some of the mc_dev->regions 
are known to be unallocated.

Changing these would not be a fix and it could be argued the simpler, 
less efficent error handling works as designed.

Tom

>
>>          return error;
>>   }
>> --
>
> --
> Thanks,
> ~Nick Desaulniers
>