EDK II - NULL Pointer Dereference in XhcPeiCreateTransferRing

Executive Summary

The PEI-phase counterpart of CreateTransferRing. UsbHcAllocateMem is guarded only by ASSERT(Buf != NULL) at line 2846 — a no-op in RELEASE builds. Line 2848: ZeroMem(Buf, sizeof(TRB_TEMPLATE) * TrbNum) dereferences NULL. This runs during pre-EFI initialization phase USB device enumeration. Every caller passes a constant ring size (TR_RING_TRB_NUMBER or CMD_RING_TRB_NUMBER), so the allocation size is fixed and a USB device cannot influence it; the NULL return only happens when the PEI USB memory pool is exhausted.

This advisory is one of a set reported to VulnCheck on 2026-06-25 for coordinated disclosure to the EDK II maintainers. After several follow-ups over the following months no response was received. 126 days after the initial report, ByteRay is publishing the findings in good faith so that affected integrators can assess and remediate them. All findings were identified against EDK II commit 70741a15d9a714323a5598eac47727e60507e22f (master, 2026-06-21).

Affected Products and Versions

• Vendor: TianoCore
• Product: EDK II (MdeModulePkg)
• Affected commit: 70741a15d9a714323a5598eac47727e60507e22f (master, 2026-06-21). The code is present on that commit and later revisions that have not changed it.
• Location: MdeModulePkg/Bus/Pci/XhciPei/XhciSched.c:2845
• Function: XhcPeiCreateTransferRing
• Category: error_handling

Technical Details

Controllability: SEMI_TRUSTED — confidence: high. Physical USB device triggers PEI-phase enumeration.

Data Flow

At line 2845, Buf = UsbHcAllocateMem(...) returns NULL on failure. At line 2846, ASSERT(Buf != NULL) is a no-op in RELEASE. At line 2848, ZeroMem(Buf, ...) dereferences NULL. This runs during pre-EFI USB device enumeration.

Affected Callers

• XhcPeiInitializeDeviceSlot
• XhcPeiInitializeDeviceSlot64
• XhcPeiSetConfigCmd
• XhcPeiSetConfigCmd64
• XhcPeiInitSched (command ring, line 3007)

Sanitization / Security Controls

• ASSERT — DEBUG-only.

Reachability and Preconditions

Attacker position and trust level for this issue are stated in the Controllability line and Data Flow above, and the concrete trigger is in Reproduction below.

Exploitability: Low
Realism: Low

Reproduction

1. Physical access to USB port; PEI-phase boot; RELEASE build; memory pressure.
2. Attacker connects physical USB device during PEI-phase boot.
3. PEI-phase USB enumeration calls XhcPeiCreateTransferRing.
4. At line 2845, UsbHcAllocateMem returns NULL on failure.
5. ASSERT(Buf != NULL) at line 2846 compiles to no-op in RELEASE; ZeroMem(Buf, sizeof(TRB_TEMPLATE) * TrbNum) at line 2848 dereferences NULL

Impact

Denial of service — firmware crash during PEI-phase USB enumeration.

Remediation and Mitigation

As of commit 70741a15d9a714323a5598eac47727e60507e22f this issue is unfixed upstream and no CVE has been assigned. Check the allocation result for NULL before dereferencing it and fail the operation cleanly when allocation fails. The specific control that is absent or ineffective is identified in the Sanitization / Security Controls list under Technical Details.

Timeline

• 2026-06-25 - Reported to VulnCheck for coordinated disclosure to the TianoCore / EDK II security team.
• 2026-07 to 2026-10 - Several follow-ups sent; no response received.
• 2026-10-29 - Public disclosure in good faith, 126 days after the initial report.

Credits

Discovered and reported by Argus-Systems.

References

• EDK II source (TianoCore)
• Affected file at the referenced commit