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

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Date:   Thu, 14 Jul 2022 20:36:09 +0530
From:   Sebin Sebastian <mailmesebin00@...il.com>
To:     André Almeida <andrealmeid@...lia.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

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.

> > 
> > 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.

> >  	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;
> >  }