Re: [drm-nouveau-mmu] question about potential NULL pointer dereference

[Gustavo A. R. Silva]

Quoting Ben Skeggs <bskeggs@xxxxxxxxxx>:

> On Wed, Feb 14, 2018 at 1:40 AM, Gustavo A. R. Silva
> <garsilva@xxxxxxxxxxxxxx> wrote:
> > 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.
> It's not checked because it can't happen.  If vma->part is set, there
> will be a previous node that it was split from.

I got it.

Thanks, Ben.
--
Gustavo