drm/amd: Guard against bad data for ATIF ACPI method

If a BIOS provides bad data in response to an ATIF method call
this causes a NULL pointer dereference in the caller.

```
? show_regs (arch/x86/kernel/dumpstack.c:478 (discriminator 1))
? __die (arch/x86/kernel/dumpstack.c:423 arch/x86/kernel/dumpstack.c:434)
? page_fault_oops (arch/x86/mm/fault.c:544 (discriminator 2) arch/x86/mm/fault.c:705 (discriminator 2))
? do_user_addr_fault (arch/x86/mm/fault.c:440 (discriminator 1) arch/x86/mm/fault.c:1232 (discriminator 1))
? acpi_ut_update_object_reference (drivers/acpi/acpica/utdelete.c:642)
? exc_page_fault (arch/x86/mm/fault.c:1542)
? asm_exc_page_fault (./arch/x86/include/asm/idtentry.h:623)
? amdgpu_atif_query_backlight_caps.constprop.0 (drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:387 (discriminator 2)) amdgpu
? amdgpu_atif_query_backlight_caps.constprop.0 (drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:386 (discriminator 1)) amdgpu
```

It has been encountered on at least one system, so guard for it.

Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
index f85ace0384d21893c94171d541f382a2ae3932a7..1f5a296f5ed2f4773fe35293049a7262a0c46ecc 100644 (file)
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
@@ -147,6 +147,7 @@ static union acpi_object *amdgpu_atif_call(struct amdgpu_atif *atif,
                                           struct acpi_buffer *params)
 {
        acpi_status status;
+       union acpi_object *obj;
        union acpi_object atif_arg_elements[2];
        struct acpi_object_list atif_arg;
        struct acpi_buffer buffer = { ACPI_ALLOCATE_BUFFER, NULL };
@@ -169,16 +170,24 @@ static union acpi_object *amdgpu_atif_call(struct amdgpu_atif *atif,
 
        status = acpi_evaluate_object(atif->handle, NULL, &atif_arg,
                                      &buffer);
+       obj = (union acpi_object *)buffer.pointer;
 
-       /* Fail only if calling the method fails and ATIF is supported */
+       /* Fail if calling the method fails and ATIF is supported */
        if (ACPI_FAILURE(status) && status != AE_NOT_FOUND) {
                DRM_DEBUG_DRIVER("failed to evaluate ATIF got %s\n",
                                 acpi_format_exception(status));
-               kfree(buffer.pointer);
+               kfree(obj);
                return NULL;
        }
 
-       return buffer.pointer;
+       if (obj->type != ACPI_TYPE_BUFFER) {
+               DRM_DEBUG_DRIVER("bad object returned from ATIF: %d\n",
+                                obj->type);
+               kfree(obj);
+               return NULL;
+       }
+
+       return obj;
 }
 
 /**