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DF-2479

Divide-by-zero in mmcsd_delete via attacker-controlled CSD erase-sector size

Summary

mmcsd_delete() uses mmc_get_erase_sector(dev) directly as divisor and modulo operand with no validation. mmc layer computes ivar->erase_sector=csd.erase_sector*csd.write_bl_len/512 from raw attacker-controlled CSD fields. When crafted card reports small product (<512) integer division yields 0 and any subsequent BIO_DELETE/TRIM panics kernel with division-by-zero trap. csd.erase_sector in [1 128] csd.write_bl_len in [1 65536]. If write_bl_len exponent=0 (write_bl_len=1) and erase_sector bits=0 (erase_sector=1) product=1 ivar->erase_sector=1/512=0. Override paths conditional (au_size!=0 / ERASE_GRP_SIZE!=0) crafted card reporting 0 leaves ivar=0. BUF_CMD_FREEBLKS reachable from unprivileged user deleting file on UFS mount with trim. Impact: immediate kernel panic.

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PoC verification

Evidence pack

findings/poc/DF-2479 · 5 files
FileTypeDescriptionSize
VERDICT.md verdict analysis: code bug confirmed, gate confirmed, fix authored 1.4 KB ↓ raw
fix.diff suggested-fix git-apply-able defense-in-depth fix (verified --check clean) 639 B view raw
env.txt environment guest gate-proof: usbconfig/pciconf/ifconfig/devinfo/sysctl output 2.0 KB view raw
build.sh build-script no-op (no live-device PoC) 255 B view raw
run.sh run-script gate check (required device is absent) 333 B view raw
VERDICT.md verdict analysis: code bug confirmed, gate confirmed, fix authored
↓ download raw

DF-2479 — PoC verdict

File: sys/dev/disk/mmcsd/mmcsd.c

Verdict: NOT REPRODUCED on this QEMU guest — confirmed HW/ACPI/device-gated; the code bug is REAL and a defense-in-depth fix.diff is attached.

Mechanism (confirmed in source)

mmcsd_delete() uses mmc_get_erase_sector(dev) directly as a divisor and modulo operand with no validation. The mmc layer derives erase_sector from raw attacker-controlled CSD fields; a crafted card reporting a small product (<512) yields 0, and BIO_DELETE/TRIM panics with divide-by-zero.

Cited lines

Why it does not reproduce on this guest

No mmc/sd controller or card on the QEMU guest (no /dev/mmcsd*; mmcsd module not loaded).

Guest gate-proof (full usbconfig/pciconf/ifconfig/devinfo/sysctl/kldstat output) is in env.txt.

Defense-in-depth fix

Guard erase_sector==0 in mmcsd_delete: set b_error=EINVAL, B_ERROR, biodone, and return before the divisions.

The git-apply-able diff is in fix.diff (verified git apply --check clean).

Classification

  • status: not_reproduced
  • reproduced: 0
  • impact: none (not reachable on this guest; latent code bug confirmed in source)
  • confidence: certain (code bug + gate both confirmed by direct source trace and guest enumeration)
  • fix_status: not_testable (patch applies + compiles-correct by inspection, but no live device to exercise on this guest)

Fix verification

not_testable
baseline no→ patch + rebuild →patched clean

not_testable: target device absent on this guest. fix.diff applies clean and is source-correct; no live device to exercise.

git apply --check findings/poc/DF-2479/fix.diff -> OK. No runtime test possible (HW/ACPI/device-gated).
↓ fix.diffper-fix-DF-2479

Confirmed kernel references

Detail

Exploit chain

none — valid hard blocker (driver/device path dead at runtime on this guest: no target HW/ACPI/device). No unprivileged->root path.

Evidence (decisive lines)

usbconfig list -> No device match; pciconf -l -> no target HW/capability; ifconfig -> vtnet0 lo0; kldstat -> kernel/ehci/xhci only; sysctl/devinfo -> no target OIDs. Source confirmed at cited lines.

PoC changes

Created findings/poc/DF-2479/{VERDICT.md,fix.diff,manifest.json,env.txt,build.sh,run.sh}. No PoC source (HW/ACPI/device-gated).

Verified recommended fix

Defense-in-depth fix.diff closes the cited path (see findings/poc/DF-2479/fix.diff; git apply --check OK).

Verdict

NOT REPRODUCED (HW/ACPI/device-gated on this guest). The bug is REAL in source (traced line-by-line): mmcsd divide-by-zero in mmcsd_delete via attacker-controlled csd (no mmc/sd). Gate confirmed via usbconfig list (no devices), pciconf -l (no target HW/capability), ifconfig (vtnet0 lo0 only), kldstat (no target module), sysctl/devinfo (no target ACPI/device OIDs). The benign QEMU environment cannot produce the malicious device/ACPI/descriptor the bug requires.