linux-kernel - Re: [PATCH] drm/msm/adreno/a5xx_debugfs: fix potential NULL pointer dereference

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Date:   Fri, 2 Feb 2018 14:04:32 -0500
From:   Rob Clark <robdclark@...il.com>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>,
        Rob Clark <robdclark@...il.com>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm/adreno/a5xx_debugfs: fix potential NULL pointer dereference

On Fri, Feb 2, 2018 at 11:30 AM, Jordan Crouse <jcrouse@...eaurora.org> wrote:
> On Fri, Feb 02, 2018 at 06:32:23AM -0600, Gustavo A. R. Silva wrote:
>> _minor_ is being dereferenced before it is null checked, hence there
>> is a potential null pointer dereference. Fix this by moving the pointer
>> dereference after _minor_ has been null checked.
>>
>> Fixes: 024ad8df763f ("drm/msm: add a5xx specific debugfs")
>> Signed-off-by: Gustavo A. R. Silva <garsilva@...eddedor.com>
>> ---
>>
>> I wonder if a better solution for this would be to WARN_ON in case _minor_
>> happens to be NULL and return -EINVAL, instead of just returning zero.
>>
>> Something like:
>>
>> struct drm_device *dev;
>>
>> if (WARN_ON(!minor)
>>       return -EINVAL;
>>
>> dev = minor->dev;
>>
>> What do you think?
>
> In my opinion everything in debugfs is optional. I'm not sure if it is even
> possible for dev->primary, dev->render or dev->control to be NULL from the DRM
> core but if so I think the failure should be silent.
>

Don't have code in front of me atm, but I think this is one of those
things you can hit both before and after we have the minor, depending
on whether filesystem and fw are present when the driver loads or
not.. so I don't think it should be a WARN_ON().

BR,
-R


> Jordan
>>
>>  drivers/gpu/drm/msm/adreno/a5xx_debugfs.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/msm/adreno/a5xx_debugfs.c b/drivers/gpu/drm/msm/adreno/a5xx_debugfs.c
>> index 6b27941..059ec7d 100644
>> --- a/drivers/gpu/drm/msm/adreno/a5xx_debugfs.c
>> +++ b/drivers/gpu/drm/msm/adreno/a5xx_debugfs.c
>> @@ -159,13 +159,15 @@ DEFINE_SIMPLE_ATTRIBUTE(reset_fops, NULL, reset_set, "%llx\n");
>>
>>  int a5xx_debugfs_init(struct msm_gpu *gpu, struct drm_minor *minor)
>>  {
>> -     struct drm_device *dev = minor->dev;
>> +     struct drm_device *dev;
>>       struct dentry *ent;
>>       int ret;
>>
>>       if (!minor)
>>               return 0;
>>
>> +     dev = minor->dev;
>> +
>>       ret = drm_debugfs_create_files(a5xx_debugfs_list,
>>                       ARRAY_SIZE(a5xx_debugfs_list),
>>                       minor->debugfs_root, minor);
>> --
>> 2.7.4
>>
>> --
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>
> --
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