drm/amdgpu: Fix a null pointer access when the smc_rreg pointer is NULL

In certain types of chips, such as VEGA20, reading the amdgpu_regs_smc file could result in an abnormal null pointer access when the smc_rreg pointer is NULL. Below are the steps to reproduce this issue and the corresponding exception log:

1. Navigate to the directory: /sys/kernel/debug/dri/0
2. Execute command: cat amdgpu_regs_smc
3. Exception Log::
[4005007.702554] BUG: kernel NULL pointer dereference, address: 0000000000000000
[4005007.702562] #PF: supervisor instruction fetch in kernel mode
[4005007.702567] #PF: error_code(0x0010) - not-present page
[4005007.702570] PGD 0 P4D 0
[4005007.702576] Oops: 0010 [#1] SMP NOPTI
[4005007.702581] CPU: 4 PID: 62563 Comm: cat Tainted: G           OE     5.15.0-43-generic torvalds#46-Ubunt       u
[4005007.702590] RIP: 0010:0x0
[4005007.702598] Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
[4005007.702600] RSP: 0018:ffffa82b46d27da0 EFLAGS: 00010206
[4005007.702605] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffa82b46d27e68
[4005007.702609] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff9940656e0000
[4005007.702612] RBP: ffffa82b46d27dd8 R08: 0000000000000000 R09: ffff994060c07980
[4005007.702615] R10: 0000000000020000 R11: 0000000000000000 R12: 00007f5e06753000
[4005007.702618] R13: ffff9940656e0000 R14: ffffa82b46d27e68 R15: 00007f5e06753000
[4005007.702622] FS:  00007f5e0755b740(0000) GS:ffff99479d300000(0000) knlGS:0000000000000000
[4005007.702626] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[4005007.702629] CR2: ffffffffffffffd6 CR3: 00000003253fc000 CR4: 00000000003506e0
[4005007.702633] Call Trace:
[4005007.702636]  <TASK>
[4005007.702640]  amdgpu_debugfs_regs_smc_read+0xb0/0x120 [amdgpu]
[4005007.703002]  full_proxy_read+0x5c/0x80
[4005007.703011]  vfs_read+0x9f/0x1a0
[4005007.703019]  ksys_read+0x67/0xe0
[4005007.703023]  __x64_sys_read+0x19/0x20
[4005007.703028]  do_syscall_64+0x5c/0xc0
[4005007.703034]  ? do_user_addr_fault+0x1e3/0x670
[4005007.703040]  ? exit_to_user_mode_prepare+0x37/0xb0
[4005007.703047]  ? irqentry_exit_to_user_mode+0x9/0x20
[4005007.703052]  ? irqentry_exit+0x19/0x30
[4005007.703057]  ? exc_page_fault+0x89/0x160
[4005007.703062]  ? asm_exc_page_fault+0x8/0x30
[4005007.703068]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[4005007.703075] RIP: 0033:0x7f5e07672992
[4005007.703079] Code: c0 e9 b2 fe ff ff 50 48 8d 3d fa b2 0c 00 e8 c5 1d 02 00 0f 1f 44 00 00 f3 0f        1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 0f 05 <48> 3d 00 f0 ff ff 77 56 c3 0f 1f 44 00 00 48 83 e       c 28 48 89 54 24
[4005007.703083] RSP: 002b:00007ffe03097898 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[4005007.703088] RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007f5e07672992
[4005007.703091] RDX: 0000000000020000 RSI: 00007f5e06753000 RDI: 0000000000000003
[4005007.703094] RBP: 00007f5e06753000 R08: 00007f5e06752010 R09: 00007f5e06752010
[4005007.703096] R10: 0000000000000022 R11: 0000000000000246 R12: 0000000000022000
[4005007.703099] R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000
[4005007.703105]  </TASK>
[4005007.703107] Modules linked in: nf_tables libcrc32c nfnetlink algif_hash af_alg binfmt_misc nls_       iso8859_1 ipmi_ssif ast intel_rapl_msr intel_rapl_common drm_vram_helper drm_ttm_helper amd64_edac t       tm edac_mce_amd kvm_amd ccp mac_hid k10temp kvm acpi_ipmi ipmi_si rapl sch_fq_codel ipmi_devintf ipm       i_msghandler msr parport_pc ppdev lp parport mtd pstore_blk efi_pstore ramoops pstore_zone reed_solo       mon ip_tables x_tables autofs4 ib_uverbs ib_core amdgpu(OE) amddrm_ttm_helper(OE) amdttm(OE) iommu_v       2 amd_sched(OE) amdkcl(OE) drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops cec rc_core        drm igb ahci xhci_pci libahci i2c_piix4 i2c_algo_bit xhci_pci_renesas dca
[4005007.703184] CR2: 0000000000000000
[4005007.703188] ---[ end trace ac65a538d240da39 ]---
[4005007.800865] RIP: 0010:0x0
[4005007.800871] Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
[4005007.800874] RSP: 0018:ffffa82b46d27da0 EFLAGS: 00010206
[4005007.800878] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffa82b46d27e68
[4005007.800881] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff9940656e0000
[4005007.800883] RBP: ffffa82b46d27dd8 R08: 0000000000000000 R09: ffff994060c07980
[4005007.800886] R10: 0000000000020000 R11: 0000000000000000 R12: 00007f5e06753000
[4005007.800888] R13: ffff9940656e0000 R14: ffffa82b46d27e68 R15: 00007f5e06753000
[4005007.800891] FS:  00007f5e0755b740(0000) GS:ffff99479d300000(0000) knlGS:0000000000000000
[4005007.800895] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[4005007.800898] CR2: ffffffffffffffd6 CR3: 00000003253fc000 CR4: 00000000003506e0

Signed-off-by: Qu Huang <qu.huang@linux.dev>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c

@@ -748,6 +748,9 @@ static ssize_t amdgpu_debugfs_regs_smc_read(struct file *f, char __user *buf,
 	ssize_t result = 0;
 	int r;
 
+	if (!adev->smc_rreg)
+		return -EPERM;
+
 	if (size & 0x3 || *pos & 0x3)
 		return -EINVAL;
 
@@ -804,6 +807,9 @@ static ssize_t amdgpu_debugfs_regs_smc_write(struct file *f, const char __user *
 	ssize_t result = 0;
 	int r;
 
+	if (!adev->smc_wreg)
+		return -EPERM;
+
 	if (size & 0x3 || *pos & 0x3)
 		return -EINVAL;