When booting the kernel v5.9-rc4 on a VM, the kernel would panic when
printing a warning message in swiotlb_map(). It is because dev->dma_mask
can potentially be a null pointer. Using the dma_get_mask() macro can
avoid the NULL pointer dereference.

Fixes: d323bb44e4d2 ("drm/virtio: Call the right shmem helpers")

[drm] Initialized virtio_gpu 0.1.0 0 for virtio0 on minor 0
 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: 0000 [#1] SMP PTI
 CPU: 1 PID: 331 Comm: systemd-udevd Not tainted 5.9.0-rc4 #1
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
 BIOS 1.13.0-1ubuntu1 04/01/2014
 RIP: 0010:swiotlb_map+0x1ac/0x200
 Code: e8 d9 fc ff ff 80 3d 92 ee 4c 01 00 75 51 49 8b 84 24 48 02 00 00
 4d 8b 6c 24 50 c6 05 7c ee 4c 01 01 4d 8b bc 24 58 02 00 00 <4c> 8b 30
 4d 85 ed 75 04 4d 8b 2c 24 4c 89 e7 e8 10 6b 4f 00 4d 89
 RSP: 0018:ffff9f96801af6f8 EFLAGS: 00010246
 RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000080
 RDX: 000000000000007f RSI: 0000000000000202 RDI: 0000000000000202
 RBP: ffff9f96801af748 R08: ffffffffffffffff R09: 0000000000000020
 R10: 0000000000000000 R11: ffff8fabfffa3000 R12: ffff8faad02c7810
 R13: 0000000000000000 R14: 0000000000000020 R15: 0000000000000000
 FS:  00007fabc63588c0(0000) GS:ffff8fabf7c80000(0000)
 knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 0000000151496005 CR4: 0000000000370ee0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  dma_direct_map_sg+0x124/0x210
  dma_map_sg_attrs+0x32/0x50
  drm_gem_shmem_get_pages_sgt+0x6a/0x90 [drm]
  virtio_gpu_object_create+0x140/0x2f0 [virtio_gpu]
  ? ww_mutex_unlock+0x26/0x30
  virtio_gpu_mode_dumb_create+0xab/0x160 [virtio_gpu]
  drm_mode_create_dumb+0x82/0x90 [drm]
  drm_client_framebuffer_create+0xaa/0x200 [drm]
  drm_fb_helper_generic_probe+0x59/0x150 [drm_kms_helper]
  drm_fb_helper_single_fb_probe+0x29e/0x3e0 [drm_kms_helper]
  __drm_fb_helper_initial_config_and_unlock+0x41/0xd0 [drm_kms_helper]
  drm_fbdev_client_hotplug+0xe6/0x1a0 [drm_kms_helper]
  drm_fbdev_generic_setup+0xaf/0x170 [drm_kms_helper]
  virtio_gpu_probe+0xea/0x100 [virtio_gpu]
  virtio_dev_probe+0x14b/0x1e0 [virtio]
  really_probe+0x1db/0x440
  driver_probe_device+0xe9/0x160
  device_driver_attach+0x5d/0x70
  __driver_attach+0x8f/0x150
  ? device_driver_attach+0x70/0x70
  bus_for_each_dev+0x7e/0xc0
  driver_attach+0x1e/0x20
  bus_add_driver+0x152/0x1f0
  driver_register+0x74/0xd0
  ? 0xffffffffc0529000
  register_virtio_driver+0x20/0x30 [virtio]
  virtio_gpu_driver_init+0x15/0x1000 [virtio_gpu]
  do_one_initcall+0x4a/0x1fa
  ? _cond_resched+0x19/0x30
  ? kmem_cache_alloc_trace+0x16b/0x2e0
  do_init_module+0x62/0x240
  load_module+0xe0e/0x1100
  ? security_kernel_post_read_file+0x5c/0x70
  __do_sys_finit_module+0xbe/0x120
  ? __do_sys_finit_module+0xbe/0x120
  __x64_sys_finit_module+0x1a/0x20
  do_syscall_64+0x38/0x50
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Signed-off-by: Thomas Tai <thomas....@oracle.com>
---
 include/linux/dma-direct.h | 2 +-
 kernel/dma/swiotlb.c       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index 6e87225..7556067 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h
@@ -168,7 +168,7 @@ static inline dma_addr_t dma_direct_map_page(struct device 
*dev,
 
                dev_WARN_ONCE(dev, 1,
                             "DMA addr %pad+%zu overflow (mask %llx, bus limit 
%llx).\n",
-                            &dma_addr, size, *dev->dma_mask, 
dev->bus_dma_limit);
+                            &dma_addr, size, dma_get_mask(dev), 
dev->bus_dma_limit);
                return DMA_MAPPING_ERROR;
        }
 
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index c19379fa..aa7727b 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -682,7 +682,7 @@ dma_addr_t swiotlb_map(struct device *dev, phys_addr_t 
paddr, size_t size,
                        attrs | DMA_ATTR_SKIP_CPU_SYNC);
                dev_WARN_ONCE(dev, 1,
                        "swiotlb addr %pad+%zu overflow (mask %llx, bus limit 
%llx).\n",
-                       &dma_addr, size, *dev->dma_mask, dev->bus_dma_limit);
+                       &dma_addr, size, dma_get_mask(dev), dev->bus_dma_limit);
                return DMA_MAPPING_ERROR;
        }
 
-- 
1.8.3.1

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