linux-kernel - [drm-nouveau-mmu] question about potential NULL pointer dereference

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Date:   Tue, 13 Feb 2018 09:40:16 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Ben Skeggs <bskeggs@...hat.com>, David Airlie <airlied@...ux.ie>
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: [drm-nouveau-mmu] question about potential NULL pointer dereference


Hi all,

While doing some static analysis I ran into the following piece of  
code at drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c:957:

  957#define node(root, dir) ((root)->head.dir == &vmm->list) ? NULL :  
              \
  958        list_entry((root)->head.dir, struct nvkm_vma, head)
  959
  960void
  961nvkm_vmm_unmap_region(struct nvkm_vmm *vmm, struct nvkm_vma *vma)
  962{
  963        struct nvkm_vma *next;
  964
  965        nvkm_memory_tags_put(vma->memory,  
vmm->mmu->subdev.device, &vma->tags);
  966        nvkm_memory_unref(&vma->memory);
  967
  968        if (vma->part) {
  969                struct nvkm_vma *prev = node(vma, prev);
  970                if (!prev->memory) {
  971                        prev->size += vma->size;
  972                        rb_erase(&vma->tree, &vmm->root);
  973                        list_del(&vma->head);
  974                        kfree(vma);
  975                        vma = prev;
  976                }
  977        }
  978
  979        next = node(vma, next);
  980        if (next && next->part) {
  981                if (!next->memory) {
  982                        vma->size += next->size;
  983                        rb_erase(&next->tree, &vmm->root);
  984                        list_del(&next->head);
  985                        kfree(next);
  986                }
  987        }
  988}

The issue here is that in case _node_ returns NULL, _prev_ is not  
being null checked, hence there is a potential null pointer  
dereference at line 970.

Notice that _next_ is being null checked at line 980, so I wonder if  
_prev_ should be checked the same as _next_.

The fact that both _next_ and next->part are null checked, makes me  
wonder if in case _prev_ actually needs to be checked, there is  
another pointer contained into _prev_ to be validated as well? I'm  
sorry, this is not clear to me at this moment.

I appreciate your feedback
Thank you
--
Gustavo