Re: [PATCH] drm/nouveau: Prevents NULL pointer dereference in nouveau_uvmm_sm_prepare

[Danilo Krummrich]

Hi Yuran,

On 10/26/23 19:03, Yuran Pereira wrote:
> There are instances where the "args" argument passed to
> nouveau_uvmm_sm_prepare() is NULL.
>
> I.e. when nouveau_uvmm_sm_prepare() is called from
> nouveau_uvmm_sm_unmap_prepare()
>
> ```
> static int
> nouveau_uvmm_sm_unmap_prepare(struct nouveau_uvmm *uvmm,
> ...
> {
>      return nouveau_uvmm_sm_prepare(uvmm, new, ops, NULL);
> }
> ```
>
> The problem is that op_map_prepare() which nouveau_uvmm_sm_prepare
> calls, dereferences this value, which can lead to a NULL pointer
> dereference.

op_map_prepare() can't be called with `args` being NULL, since when called
through nouveau_uvmm_sm_unmap_prepare() we can't hit the DRM_GPUVA_OP_MAP
case at all.

Unmapping something never leads to a new mapping being created, it can lead
to remaps though.

> ```
> static int
> op_map_prepare(struct nouveau_uvmm *uvmm,
> ...
> {
>      ...
>      uvma->region = args->region; <-- Dereferencing of possibly NULL pointer
>      uvma->kind = args->kind;     <--
>      ...
> }
> ```
>
> ```
> static int
> nouveau_uvmm_sm_prepare(struct nouveau_uvmm *uvmm,
> ...
>              struct uvmm_map_args *args)
> {
>      struct drm_gpuva_op *op;
>      u64 vmm_get_start = args ? args->addr : 0;
>      u64 vmm_get_end = args ? args->addr + args->range : 0;
>      int ret;
>
>      drm_gpuva_for_each_op(op, ops) {
>          switch (op->op) {
>          case DRM_GPUVA_OP_MAP: {
>              u64 vmm_get_range = vmm_get_end - vmm_get_start;
>
>              ret = op_map_prepare(uvmm, &new->map, &op->map, args); <---
>              if (ret)
>                  goto unwind;
>
>              if (args && vmm_get_range) {
>                  ret = nouveau_uvmm_vmm_get(uvmm, vmm_get_start,
>                                 vmm_get_range);
>                  if (ret) {
>                      op_map_prepare_unwind(new->map);
>                      goto unwind;
>                  }
>              }
>      ...
> ```
>
> Since the switch "case DRM_GPUVA_OP_MAP", also NULL checks "args"

This check is not required for the reason given above. If you like, you
can change this patch up to remove the args check and add a comment like:

/* args can't be NULL when called for a map operation. */

> after the call to op_map_prepare(), my guess is that we should
> probably relocate this check to a point before op_map_prepare()
> is called.

Yeah, I see how this unnecessary check made you think so.

- Danilo

> This patch ensures that the value of args is checked before
> calling op_map_prepare()
>
> Addresses-Coverity-ID: 1544574 ("Dereference after null check")
> Signed-off-by: Yuran Pereira <yuran.pereira@xxxxxxxxxxx>
> ---
>   drivers/gpu/drm/nouveau/nouveau_uvmm.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_uvmm.c b/drivers/gpu/drm/nouveau/nouveau_uvmm.c
> index aae780e4a4aa..6baa481eb2c8 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_uvmm.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_uvmm.c
> @@ -620,11 +620,14 @@ nouveau_uvmm_sm_prepare(struct nouveau_uvmm *uvmm,
>   		case DRM_GPUVA_OP_MAP: {
>   			u64 vmm_get_range = vmm_get_end - vmm_get_start;
>   
> +			if (!args)
> +				goto unwind;
> +
>   			ret = op_map_prepare(uvmm, &new->map, &op->map, args);
>   			if (ret)
>   				goto unwind;
>   
> -			if (args && vmm_get_range) {
> +			if (vmm_get_range) {
>   				ret = nouveau_uvmm_vmm_get(uvmm, vmm_get_start,
>   							   vmm_get_range);
>   				if (ret) {