linux-kernel - Re: [PATCH v4] tpm: fix reference counting for struct tpm

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Message-ID: <5657f8c4-e85e-ad9f-fa1f-ec5d6b659423@gmx.de>
Date:   Tue, 16 Feb 2021 20:17:41 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Stefan Berger <stefanb@...ux.ibm.com>, peterhuewe@....de,
        jarkko@...nel.org, jgg@...pe.ca
Cc:     stefanb@...ux.vnet.ibm.com, James.Bottomley@...senpartnership.com,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lino Sanfilippo <l.sanfilippo@...bus.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v4] tpm: fix reference counting for struct tpm_chip


Hi Stefan,

On 16.02.21 at 17:52, Stefan Berger wrote:
> On 2/15/21 7:31 PM, Lino Sanfilippo wrote:
>> From: Lino Sanfilippo <l.sanfilippo@...bus.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.
>>
>> ------------[ 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 ]---
>>
>> 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 moving the TPM 2 character device handling from
>> tpm_chip_alloc() to tpm_add_char_device() which is called at a later point
>> in time when the flag has been set in case of TPM2.
>>
>> 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.
>>
>> Fix this by putting chip->devs in tpm_chip_unregister().
>>
>> Finally move the new implemenation for the TPM 2 handling into a new
>> function to avoid multiple checks for the TPM_CHIP_FLAG_TPM2 flag in the
>> good case and error cases.
>>
>> Fixes: fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
>> Fixes: 8979b02aaf1d ("tpm: Fix reference count to main device")
>> Co-developed-by: Jason Gunthorpe <jgg@...pe.ca>
>> Signed-off-by: Jason Gunthorpe <jgg@...pe.ca>
>> Signed-off-by: Lino Sanfilippo <l.sanfilippo@...bus.com>
>> Cc: stable@...r.kernel.org
>
>
> I know you'll post another version, but anyway:
>
> Tested-by: Stefan Berger <stefanb@...ux.ibm.com>

Thank you for testing this, I will send a v5 shortly.

Regards,
Lino