linux-kernel - Re: [PATCH -next] drm/amdgpu: double free error and freeing uninitialized null pointer

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Date:   Thu, 14 Jul 2022 12:43:19 -0300
From:   André Almeida <andrealmeid@...lia.com>
To:     Sebin Sebastian <mailmesebin00@...il.com>
Cc:     Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Nirmoy Das <nirmoy.das@....com>,
        Lijo Lazar <lijo.lazar@....com>,
        Tom St Denis <tom.stdenis@....com>,
        Evan Quan <evan.quan@....com>,
        Somalapuram Amaranath <Amaranath.Somalapuram@....com>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] drm/amdgpu: double free error and freeing
 uninitialized null pointer

Às 12:06 de 14/07/22, Sebin Sebastian escreveu:
> On Tue, Jul 12, 2022 at 12:14:27PM -0300, André Almeida wrote:
>> Hi Sebin,
>>
>> Às 10:29 de 10/07/22, Sebin Sebastian escreveu:
>>> Fix two coverity warning's double free and and an uninitialized pointer
>>> read. Both tmp and new are pointing at same address and both are freed
>>> which leads to double free. Freeing tmp in the condition after new is
>>> assigned with new address fixes the double free issue. new is not
>>> initialized to null which also leads to a free on an uninitialized
>>> pointer.
>>> Coverity issue: 1518665 (uninitialized pointer read)
>>> 		1518679 (double free)
>>
>> What are those numbers?
>>
> These numbers are the issue ID's for the errors that are being reported
> by the coverity static analyzer tool.
> 

I see, but I don't know which tool was used, so those seem like random
number to me. I would just remove this part of your commit message, but
if you want to keep it, you need to at least mention what's the tool.

>>>
>>> Signed-off-by: Sebin Sebastian <mailmesebin00@...il.com>
>>> ---
>>>  drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c | 8 +++++---
>>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>>> index f3b3c688e4e7..d82fe0e1b06b 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>>> @@ -1660,7 +1660,7 @@ static ssize_t amdgpu_reset_dump_register_list_write(struct file *f,
>>>  {
>>>  	struct amdgpu_device *adev = (struct amdgpu_device *)file_inode(f)->i_private;
>>>  	char reg_offset[11];
>>> -	uint32_t *new, *tmp = NULL;
>>> +	uint32_t *new = NULL, *tmp = NULL;
>>>  	int ret, i = 0, len = 0;
>>>  
>>>  	do {
>>> @@ -1692,17 +1692,19 @@ static ssize_t amdgpu_reset_dump_register_list_write(struct file *f,
>>>  		goto error_free;
>>>  	}
>>
>> If the `if (!new) {` above this line is true, will be tmp freed?
>>
> Yes, It doesn't seem to free tmp here. Should I free tmp immediately
> after the do while loop and remove `kfree(tmp)` from the `if (ret)`
> block? Thanks for pointing out the errors.

If you free immediately after the while loop, then you would risk a use
after free here:

	swap(adev->reset_dump_reg_list, tmp);

So this isn't the solution either.

> 
>>>  	ret = down_write_killable(&adev->reset_domain->sem);
>>> -	if (ret)
>>> +	if (ret) {
>>> +		kfree(tmp);
>>>  		goto error_free;
>>> +	}
>>>  
>>>  	swap(adev->reset_dump_reg_list, tmp);
>>>  	swap(adev->reset_dump_reg_value, new);
>>>  	adev->num_regs = i;
>>>  	up_write(&adev->reset_domain->sem);
>>> +	kfree(tmp);
>>>  	ret = size;
>>>  
>>>  error_free:
>>> -	kfree(tmp);
>>>  	kfree(new);
>>>  	return ret;
>>>  }