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SubjectRe: [drm-nouveau-mmu] question about potential NULL pointer dereference
On Wed, Feb 14, 2018 at 1:40 AM, Gustavo A. R. Silva
<garsilva@embeddedor.com> 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.

Ben.

>
> I appreciate your feedback
> Thank you
> --
> Gustavo
>
>
>
>
>
>

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