linux-kernel - Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm

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Date:   Thu, 4 Feb 2021 20:44:48 -0500
From:   Stefan Berger <stefanb@...ux.ibm.com>
To:     Lino Sanfilippo <LinoSanfilippo@....de>, peterhuewe@....de,
        jarkko@...nel.org
Cc:     jgg@...pe.ca, stefanb@...ux.vnet.ibm.com,
        James.Bottomley@...senpartnership.com, stable@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip

On 2/4/21 7:46 PM, Stefan Berger wrote:
> On 2/4/21 6:50 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 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.
>>
>> 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@...bus.com>
>
> Tested-by: Stefan Berger <stefanb@...ux.ibm.com>
>
> Steps:
>
> modprobe tpm_vtpm_proxy
>
> swtpm chardev --vtpm-proxy --tpm2 --tpmstate dir=./ &
>
> exec 100<>/dev/tpmrm1
>
> kill -9 <swtpm pid>
>
> rmmod tpm_vtpm_proxy
>
> exec 100>&-   # fails before, works after   --> great job! :-)
>
>
To clarify: When I tested this I had *both* patches applied. Without the 
patches I got the null pointer exception in tpm2_del_space(). The 2nd 
patch alone solves that issue when using the steps above.

[  525.647443] [c000000005d3bba0] [c000000000e81d78] 
mutex_lock+0x28/0x90 (unreliable)
[  525.647539] [c000000005d3bbd0] [c0080000001f5da0] 
tpm2_del_space+0x48/0x130 [tpm]
[  525.647635] [c000000005d3bc20] [c0080000001f56b8] 
tpmrm_release+0x40/0x70 [tpm]
[  525.647746] [c000000005d3bc50] [c0000000004bf718] __fput+0xb8/0x340
[  525.647842] [c000000005d3bca0] [c00000000017def4] 
task_work_run+0xe4/0x150
[  525.647930] [c000000005d3bcf0] [c00000000001feb4] 
do_notify_resume+0x484/0x4f0
[  525.648023] [c000000005d3bdb0] [c000000000033a64] 
syscall_exit_prepare+0x1d4/0x330
[  525.648115] [c000000005d3be20] [c00000000000d96c] 
system_call_common+0xfc/0x27c