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

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Message-ID: <YtUPjqJyB/X+TnTi@sebin-inspiron>
Date:   Mon, 18 Jul 2022 13:15:18 +0530
From:   Sebin Sebastian <mailmesebin00@...il.com>
To:     "Somalapuram, Amaranath" <asomalap@....com>
Cc:     André Almeida <andrealmeid@...lia.com>,
        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 Fri, Jul 15, 2022 at 01:48:56PM +0530, Somalapuram, Amaranath wrote:
> 
> On 7/14/2022 9:13 PM, André Almeida wrote:
> > 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.
> 
> new variable is not needed to initialize.
>
But if new is not initialized to null, won't it trigger a free on an
uninitialized pointer in the first if block inside the do while loop?

> The only condition double free happens is:
> 
> tmp = new;
>                 if (sscanf(reg_offset, "%X %n", &tmp[i], &ret) != 1) {
>                         ret = -EINVAL;
>                         goto error_free; *//    if it hits this*
>                 }/
> /
> 
> and can be avoided like:
> 
>  error_free:
> -       kfree(tmp);
> +       if (tmp != new)
> +               kfree(tmp);
>         kfree(new);
>         return ret;
>  }
> 
> 
> Regards,
> 
> S.Amarnath
>
This seem's like the best way to avoid the double free. Thanks for the
suggestions.

> > > > > 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;
> > > > >   }