DF-1341 — r600_texture_size nlevels unsigned underflow

Summary

Return -EINVAL when blevel > llevel (currently only dev_warn then continues).

How to reproduce

This bug lives in a device driver not reachable from the booted QEMU guest as an unprivileged user (maxx) because the required hardware is absent (AMD GPU, RAID HBA, sound PCI, AMD SCSI) or the trigger requires a malicious hypervisor (virtio_net, virtio_scsi). The bug is reproduced at the function level by porting the cited code path into a userspace harness that drives it with the attacker-controlled inputs the original code fails to validate.

Build

cc -O2 -Wall -o trigger trigger.c

Run

./trigger

Expected

• BEFORE-FIX section shows the bug signature (SIGFPE for div-by-zero, OOB index report for overflows, wraparound count for underflows, over-read length for info leaks).
• AFTER-FIX section shows the guard from fix.diff cleanly rejecting the attacker input.

The same harness was compiled and run on the patched single-fix kernel (DragonFly 6.5-DEVELOPMENT #1) — output is identical because the harness intentionally demonstrates both the unpatched and patched function logic side by side, and the userspace behavior of those branches is independent of the kernel. The patched kernel build (fix_build.log) confirms all 15 fix.diffs compile cleanly in the real kernel / module context.

Impact classification

dos — gated by absent hardware / malicious-hypervisor precondition on this guest; live trigger from maxx is not possible. See VERDICT.md for the threat-model analysis.

Files

• trigger.c — function-level harness porting the cited code path.
• fix.diff — git-apply-able unified diff against sys/.
• build.sh / run.sh — exact repro commands.
• run.log — decisive harness output (BEFORE-FIX + AFTER-FIX).
• fix_build.log — patched kernel build log (proves all 15 fixes compile).
• VERDICT.md — full narrative analysis.
• manifest.json — machine-readable catalog.

Host has no gcc; harnesses built in guest as maxx with cc (DragonFly gcc 8.3).

DF-1341 — VERDICT

REPRODUCED at the function level (impact: dos).

Mechanism

r600_texture_size() at r600_cs.c:1418 computes 'unsigned nlevels = llevel - blevel + 1'. If blevel > llevel, nlevels wraps to ~4 billion. The for loop at :1426 iterates nlevels times computing per-level surface sizes. r600_check_texture_resource at :1580 only does dev_warn() and does NOT return -EINVAL. The MSAA path at :1542-1543 reads blevel from word4 then forces llevel = 0, so any user-supplied blevel > 0 (e.g. from a DRM command-submission ioctl with DRM_AUTH) triggers the wrap and a multi-minute spin per submission.

Why not live-reproduced on the QEMU guest

AMD/ATI r600-era (Radeon HD 2xxx-4xxx) GPU absent from QEMU guest. radeon module loads only on matching HW. The bug fires on the command-submission validation path (DRM ioctl with DRM_AUTH — local authenticated user).

Recommended fix

In r600_check_texture_resource, change the 'if (blevel > llevel) dev_warn(...)' block to also 'return -EINVAL'. This prevents the unsigned underflow in r600_texture_size.

Kernel references (confirmed during verification)

• sys/dev/drm/radeon/r600_cs.c:1418 (nlevels = llevel - blevel + 1)
• sys/dev/drm/radeon/r600_cs.c:1426 (for-loop with nlevels iterations)
• sys/dev/drm/radeon/r600_cs.c:1542-1543 (MSAA path forces llevel = 0 after blevel is parsed)
• sys/dev/drm/radeon/r600_cs.c:1580 (only dev_warn, no return -EINVAL)

Build/run

• Build harness: cc -O2 -Wall -o trigger trigger.c
• Run harness: ./trigger
• Apply fix: cd /usr/src && patch -p1 < fix.diff
• Build single-fix kernel: make -j6 nativekernel KERNCONF=X86_64_GENERIC (validated — see fix_build.log; all 15 fixes compile cleanly in one batched build, rc=0).

Tested kernels

• baseline: 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
• patched : DragonFly 6.5-DEVELOPMENT #1: Mon Jul 20 21:51:01 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64