radeon_vce: unsigned underflow in firmware header scan when datasize < strlen(prefix)+9
Summary
radeon_vce.c:60 unsigned long size; 97 size=datasize-strlen(fw_version)-9. strlen([ATI LIB=VCEFW,)=15 so datasize<24 wraps to ~ULONG_MAX. Loop 99-101 advances c past firmware buffer until page fault. Same at 112 size=datasize-strlen(fb_version)-3. Root only (firmware file replacement).
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-1756 · 2 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | Validate datasize >= strlen(fw_version)+9 before the subtraction. | 402 B | view raw |
| VERDICT.md | verdict | full analysis | 1.1 KB | ↓ raw |
DF-1756 — Verdict
Severity: Low Status: REPRODUCED (source-only confirmation — driver/HW-gated, not runtime-triggered on QEMU guest) Impact: panic Confidence: certain
Verdict
REPRODUCED. The cited bug is confirmed real in the audited source at sys/dev/drm/radeon/radeon_vce.c:97.
Mechanism
radeon_vce_init computes size=datasize-strlen(fw_version)-9 as unsigned long; datasize<24 wraps to ~ULONG_MAX, loop advances c past firmware buffer causing page fault.
Fix
Validate datasize >= strlen(fw_version)+9 before the subtraction.
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.
radeon sources compiled with -Werror.
Confirmed kernel references
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Detail
Exploit chain
none
Evidence (decisive lines)
Source traced at sys/dev/drm/radeon/radeon_vce.c:97. Fix compiled clean.
PoC changes
authored fix.diff: validate datasize >= strlen+9 before subtraction
Verified recommended fix
Validate datasize before subtraction. Matches finding proposal.
Verdict
REPRODUCED (source-only). size=datasize-strlen(fw_version)-9 wraps unsigned long when datasize<24; loop advances past buffer causing page fault.
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