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

Quoting Ben Skeggs <bskeggs@redhat.com>:

> 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.
>

I got it.

Thanks, Ben.
--
Gustavo






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