QEMUtiny

Abstract

QEMUtiny is a memory corruption vulnerability in QEMU's implementation of CXL Type-3 device emulation, reported against QEMU master 007b29752e and confirmed working against 5e61afe (May 11, 2026).

QEMUtiny was discovered autonomously with V12 by Aaron Esau of the V12 security team. The PoC was prepared by @xia0o0o0o.

Want to find issues like this in your own code? Try V12 at https://v12.sh.

The PoC chains two CXL mailbox bugs in hw/cxl/cxl-mailbox-utils.c: an out-of-bounds read in GET_LOG, followed by an out-of-bounds write in SET_FEATURE.

OOB read: cmd_logs_get_log() treats the CEL log offset as an array index in the memmove() source expression even though the CXL mailbox offset is in bytes.
OOB write: cmd_features_set_feature() accepts byte offsets into several small feature write-attribute structures without checking that offset + bytes_to_copy stays inside the selected structure.

We reported the bugs upstream. Maintainers state CXL support is currently for at non-virtualization use cases, so we feel comfortable release the PoC publicly.

The included poc.c is a working exploit that drives the emulated CXL mailbox from the guest through the device BAR. It depends on offsets for the specific QEMU build and host libc layout. The exploit can be weaponized to work reliably across many QEMU versions using the OOB read to scan memory. However this is out of scope for this PoC.

"QEMUtiny"?

QEMU + Mutiny.

Building

gcc -O2 -Wall -Wextra -o exp poc.c

The reproducer must be run as root inside the guest because it writes PCI config space and mmaps the CXL device BAR through sysfs.

sudo ./exp

One-line version:

git clone https://github.com/v12-security/pocs.git && cd pocs/qemu && gcc -O2 -Wall -Wextra -o exp poc.c && sudo ./exp

Test Setup

Use ./run_qemu_shell.sh. Then in the guest, use /exp

poc.c assumes the CXL Type-3 device appears in the guest at:

/sys/bus/pci/devices/0000:35:00.0

and that BAR2 is exposed as:

/sys/bus/pci/devices/0000:35:00.0/resource2

If your guest enumerates the device at a different BDF, update the two sysfs paths in main().

How It Works

Mailbox access. The guest enables PCI memory decoding for the CXL device, maps BAR2, and sends CXL mailbox commands by writing the mailbox payload, command, and control registers directly.
CEL out-of-bounds read. cmd_logs_get_log() checks the requested CEL range as if offset were a byte offset, but then performs pointer arithmetic on cci->cel_log as a struct cel_log *. poc.c uses GET_LOG_OOB_BASE_OFFSET to land just past the CEL buffer and read adjacent QEMU CXL state.
QEMU address discovery. The out-of-bounds CEL read leaks a CXL mailbox command handler pointer and the CXLType3Dev heap address. The handler pointer gives the QEMU PIE base for this build.
Rank sparing overflow. The demo sends SET_FEATURE / RANK_SPARING with a non-zero feature offset and a large payload. The rank sparing case copies into ct3d->rank_sparing_wr_attrs + hdr->offset without bounding the copy to sizeof(ct3d->rank_sparing_wr_attrs), so the payload continues into later CXLType3Dev fields.
Fake memory dispatch state. The overflowed payload plants enough fake FlatView, dispatch, section, MemoryRegion, and MemoryRegionOps state for the sanitize path to call a controlled MemoryRegionOps.write callback.
Callback trigger. MEDIA_OPERATIONS / SANITIZE starts a background operation. When the sanitize worker reaches address_space_set(), it walks the corrupted dispatch state and invokes the forged write callback. The demo first uses this to call memmove() and leak libc, then repoints the callback to system("/bin/bash").

Affected Code Paths

The missing SET_FEATURE bounds check affects the PPR paths and the sparing write-attribute paths:

soft_ppr_wr_attrs
hard_ppr_wr_attrs
cacheline_sparing_wr_attrs
row_sparing_wr_attrs
bank_sparing_wr_attrs
rank_sparing_wr_attrs

patrol_scrub_wr_attrs already has the intended style of bounds check.

Affected Versions

The full QEMUtiny chain uses two bugs.

OOB read: the vulnerable GET_LOG path was introduced by 056172691b (hw/cxl/device: Add log commands (8.2.9.4) + CEL), first released in QEMU v7.1.0.
OOB write: the vulnerable PPR and memory sparing SET_FEATURE paths were introduced by 5e5a86bab8 and da5cafdc4d, released in QEMU v11.0.0.

Credit

Found with V12 by Aaron Esau of the V12 security team.

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