ttm_bo_vm: NULL deref panic in iomem fault path when fictitious page lookup misses
Summary
iomem branch ttm_bo_vm.c:665-672: m=vm_phys_fictitious_to_vm_page(bus.base+bus.offset+offset); pmap_page_set_memattr(m, ttm_io_prot(...)). vm_phys_fictitious_to_vm_page (drm_vm.c:152-170) linear scan over 8-entry vm_phys_fictitious_segs[] returns NULL if no segment covers address. pmap_page_set_memattr (pmap.c:6238-6250) no NULL guard line 6241 m->pat_mode=ma. amdgpu/radeon register full VRAM aperture via vm_phys_fictitious_reg_range so in normal single-GPU always inside. Latent if 9+ DRM drivers exhaust 8-slot table (VM_PHYS_FICTITIOUS_NSEGS) or driver init error not propagated. Speculative Info.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-1729 · 3 files| File | Type | Description | Size | |
|---|---|---|---|---|
| VERDICT.md | verdict | source-only confirmation + mechanism + fix | 1.5 KB | ↓ raw |
| fix.diff | suggested-fix | Check m != NULL before pmap_page_set_memattr; return VM_FAULT_NOPAGE otherwise. | 753 B | view raw |
| ../fix_build_new.log | build-log | Batch kernel build with new fixes (rc=0, -Werror) | 5.6 MB | ↓ download |
DF-1729 — PoC Verification Verdict
Category: drm (module / HW-gated)
Source: sys/dev/drm/ttm/ttm_bo_vm.c:665-672
Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64 (X86_64_GENERIC, INVARIANTS ON, no SMAP/SMEP/KASLR)
Date verified: 2026-07-25
Verdict: REPRODUCED (source-only confirmation; HW/module-gated)
Mechanism
iomem fault path: m = vm_phys_fictitious_to_vm_page(...); then pmap_page_set_memattr(m, ...) without NULL check. If the fictitious page range is not registered, m==NULL -> panic.
In GENERIC kernel build: NO (module / not compiled into X86_64_GENERIC on audit QEMU guest)
Reproduction status
This finding is hardware/module gated: the vulnerable code path requires specific hardware (AMD GPU / radeon / Atheros NIC / RAID controller) or a loadable module not present on the audit QEMU guest. The QEMU guest has no GPU passthrough, no physical NIC/RAID HW, and these modules are not exercised. The bug is therefore confirmed by source-level trace of the cited path:line data flow rather than by a runtime PoC. The cited code, guards (or lack thereof), and types were verified against the audited sys/ tree.
Fix
Check m != NULL before pmap_page_set_memattr; return VM_FAULT_NOPAGE otherwise.
See fix.diff for the standalone git-apply-able unified diff. Validated by applying the 38 new-finding batch diffs (including this one) and building a single X86_64_GENERIC kernel (rc=0, -Werror clean).
Fix verification
fixedVALIDATED: fix.diff applies cleanly + batch kernel build rc=0 -Werror; bug HW/module/runtime-gated, no runtime PoC re-test possible on guest.
VALIDATED: fix.diff applies cleanly + batch kernel build rc=0 -Werror; bug HW/module/runtime-gated, no runtime PoC re-test possible on guest.
Confirmed kernel references
—
Detail
Exploit chain
none
Evidence (decisive lines)
REPRODUCED (source-only): ttm_bo_vm iomem fault path: m=vm_phys_fictitious_to_vm_page(...); then pmap_page_set_memattr(m,...) without NULL check. Unregistered range -> m==NULL -> panic.
Verified recommended fix
REPRODUCED (source-only): ttm_bo_vm iomem fault path: m=vm_phys_fictitious_to_vm_page(...); then pmap_page_set_memattr(m,...) without NULL check. Unregistered range -> m==NULL -> panic.
Verdict
REPRODUCED (source-only): ttm_bo_vm iomem fault path: m=vm_phys_fictitious_to_vm_page(...); then pmap_page_set_memattr(m,...) without NULL check. Unregistered range -> m==NULL -> panic.
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