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

[Stefan Berger]

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@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
>
> Tested-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
>
> 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