EDK II - NULL Pointer Dereference in XhcPeiSetConfigCmd

Executive Summary

The PEI-phase counterpart of XhcInitializeEndpointContext. Two EndpointTransferRing allocations at lines 1800 (BULK) and 1866 (INTERRUPT) have NO NULL CHECK AT ALL before the result is stored and passed to XhcPeiCreateTransferRing which dereferences it.

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:1800
• Function: XhcPeiSetConfigCmd
• Category: error_handling

Technical Details

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

Data Flow

At line 1800 (BULK case), EndpointTransferRing = AllocateZeroPool(...) has NO CHECK. At line 1802, the NULL pointer is passed to XhcPeiCreateTransferRing, which writes through it at lines 2850-2854. Line 1866 (INTERRUPT case) follows the identical pattern. The allocation is fixed-size (sizeof (TRANSFER_RING)) and is only made when EndpointTransferRing[Dci-1] is still NULL, so a device's configuration descriptor can cause at most 31 small allocations per slot (EndpointTransferRing[31], XhcPeim.h:125) and cannot force a failure; the trigger is general pool exhaustion.

Affected Callers

• XhcPeiControlTransfer (XhcPeim.c:754), from its SET_CONFIGURATION hook.

Sanitization / Security Controls

• None.

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; memory pressure.
2. Attacker connects physical USB device and triggers endpoint configuration during PEI-phase boot.
3. XhcPeiSetConfigCmd configures endpoints.
4. At line 1800 (BULK endpoint), AllocateZeroPool for EndpointTransferRing returns NULL with NO CHECK.
5. At line 1802, XhcPeiCreateTransferRing receives the NULL pointer and dereferences it at line 2850. Line 1866 (INTERRUPT case) follows the identical pattern.

Impact

Denial of service — firmware crash during PEI USB configuration.

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