Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Feb 05, 2021 at 12:50:42AM +0100, Lino Sanfilippo wrote:
> From: Lino Sanfilippo <l.sanfilippo@kunbus.com>
> 
> The following sequence of operations results in a refcount warning:
> 
> 1. Open device /dev/tpmrm
> 2. Remove module tpm_tis_spi
> 3. Write a TPM command to the file descriptor opened at step 1.
> 
> WARNING: CPU: 3 PID: 1161 at lib/refcount.c:25 kobject_get+0xa0/0xa4
> refcount_t: addition on 0; use-after-free.
> Modules linked in: tpm_tis_spi tpm_tis_core tpm mdio_bcm_unimac brcmfmac
> sha256_generic libsha256 sha256_arm hci_uart btbcm bluetooth cfg80211 vc4
> brcmutil ecdh_generic ecc snd_soc_core crc32_arm_ce libaes
> raspberrypi_hwmon ac97_bus snd_pcm_dmaengine bcm2711_thermal snd_pcm
> snd_timer genet snd phy_generic soundcore [last unloaded: spi_bcm2835]
> CPU: 3 PID: 1161 Comm: hold_open Not tainted 5.10.0ls-main-dirty #2
> Hardware name: BCM2711
> [<c0410c3c>] (unwind_backtrace) from [<c040b580>] (show_stack+0x10/0x14)
> [<c040b580>] (show_stack) from [<c1092174>] (dump_stack+0xc4/0xd8)
> [<c1092174>] (dump_stack) from [<c0445a30>] (__warn+0x104/0x108)
> [<c0445a30>] (__warn) from [<c0445aa8>] (warn_slowpath_fmt+0x74/0xb8)
> [<c0445aa8>] (warn_slowpath_fmt) from [<c08435d0>] (kobject_get+0xa0/0xa4)
> [<c08435d0>] (kobject_get) from [<bf0a715c>] (tpm_try_get_ops+0x14/0x54 [tpm])
> [<bf0a715c>] (tpm_try_get_ops [tpm]) from [<bf0a7d6c>] (tpm_common_write+0x38/0x60 [tpm])
> [<bf0a7d6c>] (tpm_common_write [tpm]) from [<c05a7ac0>] (vfs_write+0xc4/0x3c0)
> [<c05a7ac0>] (vfs_write) from [<c05a7ee4>] (ksys_write+0x58/0xcc)
> [<c05a7ee4>] (ksys_write) from [<c04001a0>] (ret_fast_syscall+0x0/0x4c)
> Exception stack(0xc226bfa8 to 0xc226bff0)
> bfa0:                   00000000 000105b4 00000003 beafe664 00000014 00000000
> bfc0: 00000000 000105b4 000103f8 00000004 00000000 00000000 b6f9c000 beafe684
> bfe0: 0000006c beafe648 0001056c b6eb6944
> 
> The reason for this warning is the attempt to get the chip->dev reference
> in tpm_common_write() although the reference counter is already zero.


> Since commit 8979b02aaf1d ("tpm: Fix reference count to main device") the
> extra reference used to prevent a premature zero counter is never taken,
> because the required TPM_CHIP_FLAG_TPM2 flag is never set.
> 
> Fix this by removing the flag condition.
> 
> Commit fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
> already introduced function tpm_devs_release() to release the extra
> reference but did not implement the required put on chip->devs that results
> in the call of this function.

Seems wonky, the devs is just supposed to be a side thing, nothing
should be using it as a primary reference count for a tpm.

The bug here is only that tpm_common_open() did not get a kref on the
chip before putting it in priv and linking it to the fd. See the
comment before tpm_try_get_ops() indicating the caller must already
have taken care to ensure the chip is valid.

This should be all you need to fix the oops:

diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
index 1784530b8387bb..1b738dca7fffb5 100644
--- a/drivers/char/tpm/tpm-dev-common.c
+++ b/drivers/char/tpm/tpm-dev-common.c
@@ -105,6 +105,7 @@ static void tpm_timeout_work(struct work_struct *work)
 void tpm_common_open(struct file *file, struct tpm_chip *chip,
                     struct file_priv *priv, struct tpm_space *space)
 {
+       get_device(&priv->chip.dev);
        priv->chip = chip;
        priv->space = space;
        priv->response_read = true;
@@ -261,6 +262,7 @@ void tpm_common_release(struct file *file, struct file_priv *priv)
        flush_work(&priv->timeout_work);
        file->private_data = NULL;
        priv->response_length = 0;
+       put_device(&chip->dev);
 }
 
 int __init tpm_dev_common_init(void)

> Fix this also by installing an action handler that puts chip->devs as soon
> as the chip is unregistered.
> 
> Fixes: fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
> Fixes: 8979b02aaf1d ("tpm: Fix reference count to main device")
> Signed-off-by: Lino Sanfilippo <l.sanfilippo@kunbus.com>
>  drivers/char/tpm/tpm-chip.c       | 18 +++++++++++++++---
>  drivers/char/tpm/tpm_ftpm_tee.c   |  2 ++
>  drivers/char/tpm/tpm_vtpm_proxy.c |  1 +
>  3 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> index ddaeceb..3ace199 100644
> +++ b/drivers/char/tpm/tpm-chip.c
> @@ -360,8 +360,7 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
>  	 * while cdevs is in use.  The corresponding put
>  	 * is in the tpm_devs_release (TPM2 only)
>  	 */
> -	if (chip->flags & TPM_CHIP_FLAG_TPM2)
> -		get_device(&chip->dev);
> +	get_device(&chip->dev);
>  
>  	if (chip->dev_num == 0)
>  		chip->dev.devt = MKDEV(MISC_MAJOR, TPM_MINOR);
> @@ -422,8 +421,21 @@ struct tpm_chip *tpmm_chip_alloc(struct device *pdev,
>  	rc = devm_add_action_or_reset(pdev,
>  				      (void (*)(void *)) put_device,
>  				      &chip->dev);
> -	if (rc)
> +	if (rc) {
> +		put_device(&chip->devs);
>  		return ERR_PTR(rc);

This isn't right read what 'or_reset' does

Jason