UBUNTU-CVE-2025-40288: Null Pointer Dereference
In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu: Fix NULL pointer dereference in VRAM logic for APU devices Previously, APU platforms (and other scenarios with uninitialized VRAM managers) triggered a NULL pointer dereference in ttmresourcemanagerusage(). The root cause is not that the struct ttmresourcemanager man pointer itself is NULL, but that man->bdev (the backing device pointer within the manager) remains uninitialized (NULL) on APUs—since APUs lack dedicated VRAM and do not fully set up VRAM manager structures. When ttmresourcemanagerusage() attempts to acquire man->bdev->lrulock, it dereferences the NULL man->bdev, leading to a kernel OOPS. 1. amdgpucs.c: Extend the existing bandwidth control check in amdgpucsgetthresholdformoves() to include a check for ttmresourcemanagerused(). If the manager is not used (uninitialized bdev), return 0 for migration thresholds immediately—skipping VRAM-specific logic that would trigger the NULL dereference. 2. amdgpukms.c: Update the AMDGPUINFOVRAMUSAGE ioctl and memory info reporting to use a conditional: if the manager is used, return the real VRAM usage; otherwise, return 0. This avoids accessing man->bdev when it is NULL. 3. amdgpuvirt.c: Modify the vf2pf (virtual function to physical function) data write path. Use ttmresourcemanagerused() to check validity: if the manager is usable, calculate fbusage from VRAM usage; otherwise, set fbusage to 0 (APUs have no discrete framebuffer to report). This approach is more robust than APU-specific checks because it: - Works for all scenarios where the VRAM manager is uninitialized (not just APUs), - Aligns with TTM's design by using its native helper function, - Preserves correct behavior for discrete GPUs (which have fully initialized man->bdev and pass the ttmresourcemanagerused() check). v4: use ttmresourcemanagerused(&adev->mman.vrammgr.manager) instead of checking the adev->gmc.isappapu flag (Christian)
Affected Software
Remediation
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 6.17.0-14.14.1 - Compensating control
In amdgpu_cs_get_threshold_for_moves(), use ttm_resource_manager_used() to check the VRAM manager and immediately return 0 for migration thresholds when the manager is not used, avoiding VRAM-specific logic that can dereference an uninitialized man->bdev.
- Compensating control
In amdgpu_kms.c, update AMDGPU_INFO_VRAM_USAGE ioctl and memory information reporting to use ttm_resource_manager_used(): report real VRAM usage when the manager is used, otherwise report 0.