nouveau/dmem: Fix vulnerability in migrate_to_ram upon copy error

The `nouveau_dmem_copy_one` function ensures that the copy push command is
sent to the device firmware but does not track whether it was executed
successfully.

In the case of a copy error (e.g., firmware or hardware failure), the
copy push command will be sent via the firmware channel, and
`nouveau_dmem_copy_one` will likely report success, leading to the
`migrate_to_ram` function returning a dirty HIGH_USER page to the user.

This can result in a security vulnerability, as a HIGH_USER page that may
contain sensitive or corrupted data could be returned to the user.

To prevent this vulnerability, we allocate a zero page. Thus, in case of
an error, a non-dirty (zero) page will be returned to the user.

Fixes: 5be73b69 ("drm/nouveau/dmem: device memory helpers for SVM")
Signed-off-by: Yonatan Maman <Ymaman@Nvidia.com>
Co-developed-by: Gal Shalom <GalShalom@Nvidia.com>
Signed-off-by: Gal Shalom <GalShalom@Nvidia.com>
Reviewed-by: Ben Skeggs <bskeggs@nvidia.com>
Cc: stable@vger.kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241008115943.990286-3-ymaman@nvidia.com

drivers/gpu/drm/nouveau/nouveau_dmem.c

@@ -193,7 +193,7 @@ static vm_fault_t nouveau_dmem_migrate_to_ram(struct vm_fault *vmf)
 	if (!spage || !(src & MIGRATE_PFN_MIGRATE))
 		goto done;
 
-	dpage = alloc_page_vma(GFP_HIGHUSER, vmf->vma, vmf->address);
+	dpage = alloc_page_vma(GFP_HIGHUSER | __GFP_ZERO, vmf->vma, vmf->address);
 	if (!dpage)
 		goto done;