Re: [PATCH] drm/nouveau/mmu: fix Use after Free bug in nvkm_vmm_node_split

[Danilo Krummrich <dakr@redhat.com>]

On Fri, Jan 27, 2023 at 01:10:46PM +0100, Takashi Iwai wrote:
> On Tue, 03 Jan 2023 15:07:55 +0100,
> Takashi Iwai wrote:
> > 
> > On Fri, 30 Dec 2022 08:27:58 +0100,
> > Zheng Wang wrote:
> > > 
> > > Here is a function call chain.
> > > nvkm_vmm_pfn_map->nvkm_vmm_pfn_split_merge->nvkm_vmm_node_split
> > > If nvkm_vma_tail return NULL in nvkm_vmm_node_split, it will
> > > finally invoke nvkm_vmm_node_merge->nvkm_vmm_node_delete, which
> > > will free the vma. However, nvkm_vmm_pfn_map didn't notice that.
> > > It goes into next label and UAF happens.
> > > 
> > > Fix it by returning the return-value of nvkm_vmm_node_merge
> > > instead of NULL.
> > > 
> > > Signed-off-by: Zheng Wang <zyytlz.wz@163.com>
> > 
> > FWIW, CVE-2023-0030 has been assigned to this bug.
> > It's a question whether it really deserves as a security issue, but a
> > bug is a bug...
> > 
> > Ben, could you review this please?
> 
> A gentle ping as reminder.  The bug is still present.

This was also reported in [1]. I had a closer look and FWICT this code is fine
and there isn't a bug.

Zheng Wang, the reporter of the BZ, also confirmed this to be a false positive
from CodeQL.

Anyway, here's the explaination I also posted in the BZ:

"In nvkm_vmm_node_merge() nvkm_vmm_node_delete() is only called when prev is
set. However, prev is NULL unless we enter the "if (vma->addr != addr)" path in
nvkm_vmm_node_split(). In such a case the vma pointer, which is also passed to
nvkm_vmm_node_merge(), is set to a freshly allocated struct nvkm_vma with
nvkm_vma_tail() right before prev is set to the old vma pointer.

Hence, the only thing happening there when nvkm_vma_tail() fails in the
"if (vma->size != size)" path is that either nvkm_vmm_node_merge() does nothing
in case prev wasn't set or it merges and frees the new vma created in the
"if (vma->addr != addr)" path. Or in other words the proper cleanup for the
error condition is done.

I can't see any case where the original vma pointer given by nvkm_vmm_pfn_map()
is actually freed."

[1] https://bugzilla.redhat.com/show_bug.cgi?id=2157041

- Danilo

> 
> 
> thanks,
> 
> Takashi
> 
> > 
> > 
> > thanks,
> > 
> > Takashi
> > 
> > > ---
> > >  drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c b/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
> > > index ae793f400ba1..84d6fc87b2e8 100644
> > > --- a/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
> > > +++ b/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
> > > @@ -937,8 +937,8 @@ nvkm_vmm_node_split(struct nvkm_vmm *vmm,
> > >  	if (vma->size != size) {
> > >  		struct nvkm_vma *tmp;
> > >  		if (!(tmp = nvkm_vma_tail(vma, vma->size - size))) {
> > > -			nvkm_vmm_node_merge(vmm, prev, vma, NULL, vma->size);
> > > -			return NULL;
> > > +			tmp = nvkm_vmm_node_merge(vmm, prev, vma, NULL, vma->size);
> > > +			return tmp;
> > >  		}
> > >  		tmp->part = true;
> > >  		nvkm_vmm_node_insert(vmm, tmp);
> > > -- 
> > > 2.25.1
> > > 
>