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

[Yuran Pereira <yuran.pereira@xxxxxxxxxxx>]

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.

```
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"
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.

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) {
-- 
2.25.1