drm/amdgpu: Fix potential out-of-bounds access in 'amdgpu_discovery_reg_base_init()'

jira VULN-37713
cve CVE-2024-27042
commit-author Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
commit cdb637d

The issue arises when the array 'adev->vcn.vcn_config' is accessed
before checking if the index 'adev->vcn.num_vcn_inst' is within the
bounds of the array.

The fix involves moving the bounds check before the array access. This
ensures that 'adev->vcn.num_vcn_inst' is within the bounds of the array
before it is used as an index.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c:1289 amdgpu_discovery_reg_base_init() error: testing array offset 'adev->vcn.num_vcn_inst' after use.

Fixes: a0ccc71 ("drm/amdgpu/discovery: validate VCN and SDMA instances")
	Cc: Christian König <christian.koenig@amd.com>
	Cc: Alex Deucher <alexander.deucher@amd.com>
	Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
	Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
	Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit cdb637d)
	Signed-off-by: CIQ Kernel Automation <ciq_kernel_automation@ciq.com>

Author: CIQ Kernel Automation

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

@@ -1270,11 +1270,10 @@ static int amdgpu_discovery_reg_base_init(struct amdgpu_device *adev)
 				 *     0b10 : encode is disabled
 				 *     0b01 : decode is disabled
 				 */
-				adev->vcn.vcn_config[adev->vcn.num_vcn_inst] =
-					ip->revision & 0xc0;
-				ip->revision &= ~0xc0;
 				if (adev->vcn.num_vcn_inst <
 				    AMDGPU_MAX_VCN_INSTANCES) {
+					adev->vcn.vcn_config[adev->vcn.num_vcn_inst] =
+						ip->revision & 0xc0;
 					adev->vcn.num_vcn_inst++;
 					adev->vcn.inst_mask |=
 						(1U << ip->instance_number);
@@ -1285,6 +1284,7 @@ static int amdgpu_discovery_reg_base_init(struct amdgpu_device *adev)
 						adev->vcn.num_vcn_inst + 1,
 						AMDGPU_MAX_VCN_INSTANCES);
 				}
+				ip->revision &= ~0xc0;
 			}
 			if (le16_to_cpu(ip->hw_id) == SDMA0_HWID ||
 			    le16_to_cpu(ip->hw_id) == SDMA1_HWID ||