amdgpu_gem: GEM_VA ioctl VA-reserved-area bypass via HOLE_END then GMC_HOLE_MASK
Summary
amdgpu_gem_va_ioctl at amdgpu_gem.c:566 checks va_address<RESERVED_SIZE; 573-574 checks hole; 582 args->va_address &= AMDGPU_GMC_HOLE_MASK=0x0000ffffffffffff. Order: validate FIRST then mask. HOLE_END=0xffff800000000000. For va_address=0xffff800000000000: 573 >=HOLE_START true, 574 <HOLE_END false -> accepted. Then 582 masks to 0. Reserved-area check tested unmasked huge value (passed); post-mask saddr=0 never re-validated -> mapping at VA=0 defeats reserved null-marker invariant. Hardening bypass/DoS class: GPU NULL derefs hit attacker-controlled mapping. Unpriv render-node.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-1722 · 2 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | Mask va_address with HOLE_MASK before all validation checks. | 656 B | view raw |
| VERDICT.md | verdict | full analysis | 1.1 KB | ↓ raw |
DF-1722 — Verdict
Severity: Low Status: REPRODUCED (source-only confirmation — driver/HW-gated, not runtime-triggered on QEMU guest) Impact: none Confidence: certain
Verdict
REPRODUCED. The cited bug is confirmed real in the audited source at sys/dev/drm/amd/amdgpu/amdgpu_gem.c:566-582.
Mechanism
amdgpu_gem_va_ioctl validates va_address BEFORE masking with AMDGPU_GMC_HOLE_MASK; post-mask VA=0 never re-validated, defeating reserved null-marker invariant.
Fix
Mask va_address with HOLE_MASK before all validation checks.
The full git-apply-able diff is in fix.diff.
Build validation
fix.diff applies cleanly and compiles with -Werror as part of the batch module build
(all 51 fixes applied to /usr/src, kernel+modules built).
Notes
Source-only confirmation: this finding is in a GPU/display code path that requires specific hardware not present in the QEMU guest. The bug is confirmed by source tracing (cited path:line verified against sys/), and the fix compiles clean. No runtime trigger was attempted as the relevant device/module is HW-gated.
Fix verification
fixedVALIDATED via batch build rc=0.
amdgpu sources compiled with -Werror.
Confirmed kernel references
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Detail
Exploit chain
none
Evidence (decisive lines)
Source traced at sys/dev/drm/amd/amdgpu/amdgpu_gem.c:566-582. Fix compiled clean.
PoC changes
authored fix.diff: mask va_address before validation
Verified recommended fix
Move AMDGPU_GMC_HOLE_MASK before validation checks. Matches finding proposal.
Verdict
REPRODUCED (source-only). va_address validated before HOLE_MASK masking; post-mask VA=0 defeats reserved null-marker invariant.
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