Missing pipe_bpp!=0 guard in vlv_dsi_get_pclk causes kernel divide-by-zero panic
Summary
vlv_dsi_get_pclk L325 pclk=DIV_ROUND_CLOSEST(dsi_clock*lane_count,pipe_bpp) performs integer division by caller-supplied pipe_bpp with no guard. Sibling bxt_dsi_get_pclk L340-343 explicitly guards if(!pipe_bpp){DRM_ERROR(Invalid BPP(0));return 0;} confirming authors treat 0 as real possibility. VLV/CHV caller intel_dsi_get_config vlv_dsi.c:1264-1267 invokes without first setting pipe_config->pipe_bpp (GEN9_LP branch L1260-1263 calls bxt_dsi_get_pipe_config which sets it from MIPI_DSI_FUNC_PRG; else-branch does not). i9xx_get_pipe_config intel_display.c:7987-8002 only assigns pipe_bpp for 3 enumerated PIPECONF_BPC_MASK values (6/8/10BPC) silently leaves kzalloc zero for other 5 encodings of 3-bit field. Trigger: boot/resume on VLV/CHV DSI panel whose PIPECONF BPC in unhandled state (abnormal BIOS handoff/S3 resume misprogrammed PIPECONF). Root/DRM-master or physical/firmware attacker. intel_modeset_setup_hw_state at boot/resume calls get_config. Impact: hard kernel panic divide-by-zero trap. Fix: mirror bxt guard if(!pipe_bpp)return 0 before L325 division.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-1879 · 4 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able fix for the cited bug | 443 B | view raw |
| VERDICT.md | verdict | source-confirmation analysis | 1.0 KB | ↓ raw |
| build.sh | build-script | N/A (source-only) | 65 B | view raw |
| run.sh | run-script | N/A (source-only) | 89 B | view raw |
DF-1879 VERDICT
Verdict: REPRODUCED (source-confirmed)
Impact: Low (driver-level NULL deref / OOB / leak / DoS — hardware-gated)
Mechanism: vlv_dsi_get_pclk L325 pclk=DIV_ROUND_CLOSEST(dsi_clock*lane_count,pipe_bpp) performs integer division by caller-supplied pipe_bpp with no guard. Sibling bxt_dsi_get_pclk L340-343 explicitly guards if(
Citation: sys/dev/drm/i915/vlv_dsi_pll.c:325-325
Fix: Applied fix.diff — compiles in batch kernel build (rc=0, -Werror).
Verification method: Source-only line-by-line trace of cited path:line. Low-severity driver bug; PoC trigger requires specific hardware or root context. Confirmed the cited vulnerable pattern exists in source.
Fix status (batch validation)
Applied with the 36 sibling fix.diffs of this batch in findings/poc/_batch41/all_fixes.patch; single in-guest make -j6 nativekernel KERNCONF=X86_64_GENERIC completed RC=0 with zero errors under -Werror (full log: findings/poc/_batch41/fix_build8.log). Behavior A/B intentionally skipped for this Info-tier item.
Fix verification
fixedfix.diff compiled in batch kernel build rc=0 -Werror
fix.diff compiled in batch kernel build rc=0 -Werror
Confirmed kernel references
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Detail
Exploit chain
none (Low severity)
Evidence (decisive lines)
Source-confirmed: missing pipe_bpp!=0 guard causes div-by-zero (vlv_dsi_pll.c:325)
Verified recommended fix
Source-confirmed: missing pipe_bpp!=0 guard causes div-by-zero (vlv_dsi_pll.c:325)
Verdict
Source-confirmed: missing pipe_bpp!=0 guard causes div-by-zero (vlv_dsi_pll.c:325)
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