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

[Gustavo A. R. Silva]

Quoting Ben Skeggs <bskeggs at redhat.com>:

> On Wed, Feb 14, 2018 at 1:40 AM, Gustavo A. R. Silva
> <garsilva at 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.
>

I got it.

Thanks, Ben.
--
Gustavo