On Tue, Feb 02, 2021 at 11:09:01PM +0100, Lino Sanfilippo wrote:
> From: Lino Sanfilippo <l.sanfilippo@xxxxxxxxxx>
>
> The following sequence of operations
>
> 1. open device /dev/tpmrm
> 2. remove the registered tpm chip driver
What is "tpm chip driver"? Please just refer to the exact thing
(e.g. tpm_tis_spi is the one you should refer to in your case).
> 3. perform a write() to /dev/tpmrm
Just a nit: Use capital letter as the first letter in sentences.
If I do "rmmod tpm_tis" the device is gone.
> results in a refcount warning:
>
> ------------[ cut here ]------------
> 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
> ---[ end trace d4b8409def9b8b1f ]---
I guess this is happening with tpm_tis_spi. Unfortunately I don't have
anything available that would use it.
I did testing with tpm_tis but so far no success reproducing.
>
> The reason is an attempt to get the chip->dev reference (see
> tpm_try_get_ops()) although the last reference to this device has already
> been released with the unregistration of the chip driver.
>
> As far as I understand, the history of this bug is as follows:
Generally you don't write in person to a commit message. This not an
email in that sense. You should write in imperative form.
> Commit fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
> introduced the support for /dev/tpmrm. Beside the new device chip->devs
> this code introduced the taking of an extra reference to chip->dev to
> ensure that this device is not released as long as /dev/tpmrm is still in
> use:
>
> + chip->devs.release = tpm_devs_release;
> + /* get extra reference on main device to hold on
> + * behalf of devs. This holds the chip structure
> + * while cdevs is in use. The corresponding put
> + * is in the tpm_devs_release
> + */
Copy pasting this is probably unnecessary.
> The extra reference to chip->dev was supposed to be released as soon as the
> reference to chip->devs was freed:
>
> +static void tpm_devs_release(struct device *dev)
> +{
> + struct tpm_chip *chip = container_of(dev, struct tpm_chip, devs);
> +
> + /* release the master device reference */
> + put_device(&chip->dev);
> +}
OK, what you really should explain in the commit message is:
1. Why this function *is not* called.
2. What your fix does to make things better.
> However the code did not work as expected, because the reference to
> chip->devs was never released. Thus tpm_devs_release() was never called
> which in turn also prevented the extra reference to chip->dev from being
> released.
If it did not work, it works now :-) So any fix would not be required.
Just making a point here.
> This led to behaviour which commit 8979b02aaf1d ("tpm: Fix reference count
> to main device") tried to fix as an excerpt of the commit message shows:
>
> "The main device is currently not properly released due to one additional
> reference to the 'devs' device which is only released in case of a TPM 2.
> So, also get the additional reference only in case of a TPM2."
>
> Instead of adding the missing put for chip->devs, this patch chose another
> approach by only taking the reference in case of TPM2:
>
> + if (chip->flags & TPM_CHIP_FLAG_TPM2)
> + get_device(&chip->dev);
>
> This fixed the non-TPM2 case but left the TPM2 case without the required
> extra reference, since there is no code path that ever sets the flag
> TPM_CHIP_FLAG_TPM2, which is the situation we have todoay.
You should not need this explanation to explain refcount mismatch. Also
past tense should not be used.
I appreciate the time taken to write all this but you should just explain
what you do and why. Let's just say this explanation is very obfuscted and
hard to follow.
> To fix this
>
> 1. revert commit 8979b02aaf1d ("tpm: Fix reference count to main device")
> to make sure the extra reference is taken unconditionally as proposed by
> commit fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
> 2. fix commit fdc915f7f719 ("tpm: expose spaces via a device link
>
> /dev/tpmrm<n>") by adding an action handler to do the missing put to
> chip->devs. This eventually results in the call of function
> tpm_devs_release() which in turn performs the final put to the extra
> reference to chip->dev.
>
> Fixes: fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
> Fixes: 8979b02aaf1d ("tpm: Fix reference count to main device")
If the fix makes sense a Cc stable is also required.
> Signed-off-by: Lino Sanfilippo <l.sanfilippo@xxxxxxxxxx>
> ---
> 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
> --- a/drivers/char/tpm/tpm-chip.c
> +++ 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);
> + }
> +
> + rc = devm_add_action_or_reset(pdev,
> + (void (*)(void *)) put_device,
> + &chip->devs);
> + if (rc) {
> + devm_remove_action(pdev,
> + (void (*)(void *)) put_device,
> + &chip->dev);
> + put_device(&chip->dev);
> + return ERR_PTR(rc);
> + }
>
> dev_set_drvdata(pdev, chip);
>
> diff --git a/drivers/char/tpm/tpm_ftpm_tee.c b/drivers/char/tpm/tpm_ftpm_tee.c
> index 2ccdf8a..82858c2 100644
> --- a/drivers/char/tpm/tpm_ftpm_tee.c
> +++ b/drivers/char/tpm/tpm_ftpm_tee.c
> @@ -286,6 +286,7 @@ static int ftpm_tee_probe(struct device *dev)
>
> out_chip:
> put_device(&pvt_data->chip->dev);
> + put_device(&pvt_data->chip->devs);
> out_chip_alloc:
> tee_shm_free(pvt_data->shm);
> out_shm_alloc:
> @@ -318,6 +319,7 @@ static int ftpm_tee_remove(struct device *dev)
> tpm_chip_unregister(pvt_data->chip);
>
> /* frees chip */
> + put_device(&pvt_data->chip->devs);
> put_device(&pvt_data->chip->dev);
>
> /* Free the shared memory pool */
> diff --git a/drivers/char/tpm/tpm_vtpm_proxy.c b/drivers/char/tpm/tpm_vtpm_proxy.c
> index 91c772e3..97b60f8 100644
> --- a/drivers/char/tpm/tpm_vtpm_proxy.c
> +++ b/drivers/char/tpm/tpm_vtpm_proxy.c
> @@ -520,6 +520,7 @@ static struct proxy_dev *vtpm_proxy_create_proxy_dev(void)
> */
> static inline void vtpm_proxy_delete_proxy_dev(struct proxy_dev *proxy_dev)
> {
> + put_device(&proxy_dev->chip->devs);
> put_device(&proxy_dev->chip->dev); /* frees chip */
> kfree(proxy_dev);
> }
> --
> 2.7.4
>
Hope this feedback helps to improve. You really need to rewrite the whole
commit message. I wonder who could try to reproduce this. With a quick
skim I get the issue.
Also you don't have James Bottomley in the CC list of the patch who is
the author of the original commit. Please sort that out too...
/Jarkko
