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

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