Re: [PATCH v8 18/22] tpm: add session encryption protection to tpm2_get_random()

Nícolas F. R. A. Prado
2024-05-17T00:25:59Z

On Mon, Apr 29, 2024 at 04:28:07PM -0400, James Bottomley wrote:
> If some entity is snooping the TPM bus, they can see the random
> numbers we're extracting from the TPM and do prediction attacks
> against their consumers. Foil this attack by using response
> encryption to prevent the attacker from seeing the random sequence.
>
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
>
> ---
> v7: add review
> ---
> drivers/char/tpm/tpm2-cmd.c | 21 +++++++++++++++++----
> 1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index a53a843294ed..0cdf892ec2a7 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -292,25 +292,35 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
> if (!num_bytes || max > TPM_MAX_RNG_DATA)
> return -EINVAL;
>
> - err = tpm_buf_init(&buf, 0, 0);
> + err = tpm2_start_auth_session(chip);
> if (err)
> return err;
>
> + err = tpm_buf_init(&buf, 0, 0);
> + if (err) {
> + tpm2_end_auth_session(chip);
> + return err;
> + }
> +
> do {
> - tpm_buf_reset(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_GET_RANDOM);
> + tpm_buf_reset(&buf, TPM2_ST_SESSIONS, TPM2_CC_GET_RANDOM);
> + tpm_buf_append_hmac_session_opt(chip, &buf, TPM2_SA_ENCRYPT
> + | TPM2_SA_CONTINUE_SESSION,
> + NULL, 0);
> tpm_buf_append_u16(&buf, num_bytes);
> + tpm_buf_fill_hmac_session(chip, &buf);
> err = tpm_transmit_cmd(chip, &buf,
> offsetof(struct tpm2_get_random_out,
> buffer),
> "attempting get random");
> + err = tpm_buf_check_hmac_response(chip, &buf, err);
> if (err) {
> if (err > 0)
> err = -EIO;
> goto out;
> }
>
> - out = (struct tpm2_get_random_out *)
> - &buf.data[TPM_HEADER_SIZE];
> + out = (struct tpm2_get_random_out *)tpm_buf_parameters(&buf);
> recd = min_t(u32, be16_to_cpu(out->size), num_bytes);
> if (tpm_buf_length(&buf) <
> TPM_HEADER_SIZE +
> @@ -327,9 +337,12 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
> } while (retries-- && total < max);
>
> tpm_buf_destroy(&buf);
> + tpm2_end_auth_session(chip);
> +
> return total ? total : -EIO;
> out:
> tpm_buf_destroy(&buf);
> + tpm2_end_auth_session(chip);
> return err;
> }
>
> --
> 2.35.3
>

Hi,

KernelCI has identified a new warning and I tracked it down to this commit. It
was observed on the following platforms:
* mt8183-kukui-jacuzzi-juniper-sku16
* sc7180-trogdor-kingoftown
(but probably affects all platforms that have a tpm driver with async probe)

The warning is the following:

[ 2.017338] ------------[ cut here ]------------
[ 2.025521] WARNING: CPU: 0 PID: 72 at kernel/module/kmod.c:144 __request_module+0x188/0x1f4
[ 2.039508] Modules linked in:
[ 2.046447] CPU: 0 PID: 72 Comm: kworker/u34:3 Not tainted 6.9.0 #1
[ 2.046455] Hardware name: Google juniper sku16 board (DT)
[ 2.046460] Workqueue: async async_run_entry_fn
[ 2.060094]
[ 2.060097] pstate: 00000005 (nzcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 2.091758] pc : __request_module+0x188/0x1f4
[ 2.096114] lr : __request_module+0x180/0x1f4
[ 2.100468] sp : ffff80008088b400
[ 2.103777] x29: ffff80008088b400 x28: 0000000000281ae0 x27: ffffa13fd366e0d2
[ 2.110915] x26: 0000000000000000 x25: ffff2387008f33c0 x24: 00000000ffffffff
[ 2.118053] x23: 000000000000200f x22: ffffa13fd0ed49de x21: 0000000000000001
[ 2.125190] x20: 0000000000000000 x19: ffffa13fd23f0be0 x18: 0000000000000014
[ 2.132327] x17: 00000000fbdae5e3 x16: 000000005bcbb9f8 x15: 000000008700f694
[ 2.139463] x14: 0000000000000001 x13: ffff80008088b850 x12: 0000000000000000
[ 2.146600] x11: 00000000f8f4a4bb x10: fffffffffdd186ae x9 : 0000000000000004
[ 2.153736] x8 : ffff2387008f33c0 x7 : 3135616873286361 x6 : 0c0406065b07370f
[ 2.160873] x5 : 0f37075b0606040c x4 : 0000000000000000 x3 : 0000000000000000
[ 2.168009] x2 : ffffa13fd0ed49de x1 : ffffa13fcfcc4768 x0 : 0000000000000001
[ 2.175146] Call trace:
[ 2.177587] __request_module+0x188/0x1f4
[ 2.181596] crypto_alg_mod_lookup+0x178/0x21c
[ 2.186042] crypto_alloc_tfm_node+0x58/0x114
[ 2.190396] crypto_alloc_shash+0x24/0x30
[ 2.194404] drbg_init_hash_kernel+0x28/0xdc
[ 2.198673] drbg_kcapi_seed+0x21c/0x420
[ 2.202593] crypto_rng_reset+0x84/0xb4
[ 2.206425] crypto_get_default_rng+0xa4/0xd8
[ 2.210779] ecc_gen_privkey+0x58/0xd0
[ 2.214526] ecdh_set_secret+0x90/0x198
[ 2.218360] tpm_buf_append_salt+0x164/0x2dc
[ 2.222632] tpm2_start_auth_session+0xc8/0x29c
[ 2.227162] tpm2_get_random+0x44/0x204
[ 2.230996] tpm_get_random+0x74/0x90
[ 2.234655] tpm_hwrng_read+0x24/0x30
[ 2.238314] add_early_randomness+0x68/0x118
[ 2.242584] hwrng_register+0x16c/0x218
[ 2.246418] tpm_chip_register+0xf0/0x2cc
[ 2.248143] cros-ec-spi spi2.0: SPI transfer timed out
[ 2.250419] tpm_tis_core_init+0x494/0x7e0
[ 2.255552] spi_master spi2: failed to transfer one message from queue
[ 2.259623] tpm_tis_spi_init+0x54/0x70
[ 2.259629] cr50_spi_probe+0xf4/0x27c
[ 2.266145] spi_master spi2: noqueue transfer failed
[ 2.269961] tpm_tis_spi_driver_probe+0x34/0x64
[ 2.273704] cros-ec-spi spi2.0: spi transfer failed: -110
[ 2.278647] spi_probe+0x84/0xe4
[ 2.291799] really_probe+0xbc/0x2a0
[ 2.295373] __driver_probe_device+0x78/0x12c
[ 2.299725] driver_probe_device+0xdc/0x160
[ 2.303903] __device_attach_driver+0xb8/0x134
[ 2.308342] bus_for_each_drv+0x84/0xe0
[ 2.312174] __device_attach_async_helper+0xac/0xd0
[ 2.317051] async_run_entry_fn+0x34/0xe0
[ 2.321058] process_one_work+0x150/0x294
[ 2.325068] worker_thread+0x304/0x408
[ 2.328816] kthread+0x118/0x11c
[ 2.332045] ret_from_fork+0x10/0x20
[ 2.335621] ---[ end trace 0000000000000000 ]---

Which is generated in __request_module() here:

/*
* We don't allow synchronous module loading from async. Module
* init may invoke async_synchronize_full() which will end up
* waiting for this task which already is waiting for the module
* loading to complete, leading to a deadlock.
*/
WARN_ON_ONCE(wait && current_is_async());

As the comment says this could lead to a deadlock so it seemed worthwhile to
report and get fixed.

The tpm_tis_spi driver probes asynchronously,

.probe_type = PROBE_PREFER_ASYNCHRONOUS,

and as part of its probe registers the tpm device which ultimately leads to a
module being requested synchronously in crypto_larval_lookup():

request_module("crypto-%s-all", name);

and that triggers the warning.

#regzbot introduced: 1b6d7f9eb150
#regzbot title: __request_module warning: sync module loading from async tpm driver probe

Thanks,
Nícolas