Re: [Regression] "tpm: Require that all digests are present in TCG_PCR_EVENT2 structures" causes null pointer dereference

[Tyler Hicks]

On 2020-10-20 17:07:50, Mimi Zohar wrote:
> On Tue, 2020-09-29 at 13:52 -0400, Mimi Zohar wrote:
> > On Mon, 2020-09-28 at 22:16 +0800, Kai-Heng Feng wrote:
> > > Hi Jarkko,
> > > 
> > > > On Sep 28, 2020, at 22:06, Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
> > > > 
> > > > On Mon, Sep 28, 2020 at 08:31:04PM +0800, Kai-Heng Feng wrote:
> > > >> Commit 7f3d176f5f7e "tpm: Require that all digests are present in
> > > >> TCG_PCR_EVENT2 structures" causes a null pointer dereference on all
> > > >> laptops I have:
> > > > 
> > > > ...
> > > > 
> > > >> [   17.868849] BUG: kernel NULL pointer dereference, address: 000000000000002c
> > > >> [   17.868852] #PF: supervisor read access in kernel mode
> > > >> [   17.868854] #PF: error_code(0x0000) - not-present page
> > > >> [   17.868855] PGD 0 P4D 0 
> > > >> [   17.868858] Oops: 0000 [#1] SMP PTI
> > > >> [   17.868860] CPU: 0 PID: 1873 Comm: fwupd Not tainted 5.8.0-rc6+ #25
> > > >> [   17.868861] Hardware name: LENOVO 20LAZ3TXCN/20LAZ3TXCN, BIOS N27ET38W (1.24 ) 11/28/2019
> > > >> [   17.868866] RIP: 0010:tpm2_bios_measurements_start+0x38/0x1f0
> > > >> [   17.868868] Code: 55 41 54 53 48 83 ec 30 4c 8b 16 65 48 8b 04 25 28 00 00 00 48 89 45 d0 48 8b 47 70 4c 8b a0 d0 06 00 00 48 8b 88 d8 06 00 00 <41> 8b 5c 24 1c 48 89 4d b0 48 89 d8 48 83 c3 20 4d 85 d2 75 31 4c
> > > >> [   17.868869] RSP: 0018:ffff9da500a9fde0 EFLAGS: 00010282
> > > >> [   17.868871] RAX: ffff917d03dc4000 RBX: 0000000000000000 RCX: 0000000000000010
> > > >> [   17.868872] RDX: 0000000000001000 RSI: ffff917c99b19460 RDI: ffff917c99b19438
> > > >> [   17.868873] RBP: ffff9da500a9fe38 R08: ffffbda4ffa33fc0 R09: ffff917cbfeae4c0
> > > >> [   17.868874] R10: 0000000000000000 R11: 0000000000000002 R12: 0000000000000010
> > > >> [   17.868875] R13: ffff917c99b19438 R14: ffff917c99b19460 R15: ffff917c99b19470
> > > >> [   17.868876] FS:  00007f9d80988b00(0000) GS:ffff917d07400000(0000) knlGS:0000000000000000
> > > >> [   17.868877] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > >> [   17.868878] CR2: 000000000000002c CR3: 0000000219b12004 CR4: 00000000003606f0
> > > >> [   17.868879] Call Trace:
> > > >> [   17.868884]  seq_read+0x95/0x470
> > > >> [   17.868887]  ? security_file_permission+0x150/0x160
> > > >> [   17.868889]  vfs_read+0xaa/0x190
> > > >> [   17.868891]  ksys_read+0x67/0xe0
> > > >> [   17.868893]  __x64_sys_read+0x1a/0x20
> > > >> [   17.868896]  do_syscall_64+0x52/0xc0
> > > >> [   17.868898]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > > >> [   17.868900] RIP: 0033:0x7f9d83be91dc
> > > >> [   17.868901] Code: Bad RIP value.
> > > >> [   17.868902] RSP: 002b:00007fff7f5e0250 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > > >> [   17.868903] RAX: ffffffffffffffda RBX: 00005651d262f420 RCX: 00007f9d83be91dc
> > > >> [   17.868904] RDX: 0000000000001000 RSI: 00007fff7f5e0350 RDI: 0000000000000010
> > > >> [   17.868905] RBP: 00007f9d83cc54a0 R08: 0000000000000000 R09: 00005651d26c1830
> > > >> [   17.868906] R10: 00005651d2582010 R11: 0000000000000246 R12: 0000000000001000
> > > >> [   17.868907] R13: 00007fff7f5e0350 R14: 0000000000000d68 R15: 00007f9d83cc48a0
> > > >> [   17.868909] Modules linked in: rfcomm ccm cmac algif_hash algif_skcipher af_alg snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic bnep joydev mei_hdcp wmi_bmof intel_rapl_msr intel_wmi_thunderbolt x86_pkg_temp_thermal intel_powerclamp coretemp nls_iso8859_1 kvm_intel kvm crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel glue_helper crypto_simd cryptd rapl input_leds intel_cstate snd_hda_intel snd_intel_dspcfg rmi_smbus iwlmvm snd_hda_codec serio_raw snd_hwdep mac80211 rmi_core snd_hda_core libarc4 uvcvideo snd_pcm videobuf2_vmalloc btusb videobuf2_memops iwlwifi videobuf2_v4l2 btrtl btbcm videobuf2_common btintel thunderbolt i915 bluetooth mei_me videodev thinkpad_acpi nvram cfg80211 ledtrig_audio mei mc ecdh_generic ecc i2c_algo_bit processor_thermal_device snd_seq_midi drm_kms_helper snd_seq_midi_event intel_soc_dts_iosf syscopyarea sysfillrect snd_rawmidi intel_pch_thermal sysimgblt intel_rapl_common intel_xhci_usb_role_switch fb_sys_fops
>   u
> >  cs
> > >  i_acpi r
> > >  o
> > > > les cec
> > > >> [   17.868935]  typec_ucsi typec nxp_nci_i2c snd_seq nxp_nci wmi nci nfc snd_timer snd_seq_device snd int3403_thermal soundcore int340x_thermal_zone video mac_hid int3400_thermal acpi_pad acpi_thermal_rel sch_fq_codel parport_pc ppdev lp parport drm ip_tables x_tables autofs4 btrfs blake2b_generic libcrc32c xor zstd_compress raid6_pq uas usb_storage psmouse e1000e nvme i2c_i801 i2c_smbus nvme_core intel_lpss_pci intel_lpss idma64 virt_dma pinctrl_sunrisepoint pinctrl_intel
> > > >> [   17.868951] CR2: 000000000000002c
> > > >> [   17.868953] ---[ end trace ee7716fff5dec2fb ]---
> > > >> [   17.868955] RIP: 0010:tpm2_bios_measurements_start+0x38/0x1f0
> > > >> [   17.868957] Code: 55 41 54 53 48 83 ec 30 4c 8b 16 65 48 8b 04 25 28 00 00 00 48 89 45 d0 48 8b 47 70 4c 8b a0 d0 06 00 00 48 8b 88 d8 06 00 00 <41> 8b 5c 24 1c 48 89 4d b0 48 89 d8 48 83 c3 20 4d 85 d2 75 31 4c
> > > >> [   17.868958] RSP: 0018:ffff9da500a9fde0 EFLAGS: 00010282
> > > >> [   17.868959] RAX: ffff917d03dc4000 RBX: 0000000000000000 RCX: 0000000000000010
> > > >> [   17.868960] RDX: 0000000000001000 RSI: ffff917c99b19460 RDI: ffff917c99b19438
> > > >> [   17.868961] RBP: ffff9da500a9fe38 R08: ffffbda4ffa33fc0 R09: ffff917cbfeae4c0
> > > >> [   17.868962] R10: 0000000000000000 R11: 0000000000000002 R12: 0000000000000010
> > > >> [   17.868963] R13: ffff917c99b19438 R14: ffff917c99b19460 R15: ffff917c99b19470
> > > >> [   17.868964] FS:  00007f9d80988b00(0000) GS:ffff917d07400000(0000) knlGS:0000000000000000
> > > >> [   17.868965] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > >> [   17.868966] CR2: 000000000000002c CR3: 0000000219b12004 CR4: 00000000003606f0
> > > > 
> > > > It is possible but initially feels a bit weird:
> > > > 
> > > > -                  sizeof(TCG_SPECID_SIG)) || count > efispecid->num_algs) {
> > > > +                  sizeof(TCG_SPECID_SIG)) ||
> > > > +           !efispecid->num_algs || count != efispecid->num_algs) {
> > > > 
> > > > Assuming that check does not pass because of a more strict constraint,
> > > > __calc_tpm2_event_size() returns 0.
> > > > 
> > > > It is wrapped like this in drivers/char/tpm/eventlog/tpm2.c:
> > > > 
> > > > static size_t calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
> > > > 				   struct tcg_pcr_event *event_header)
> > > > {
> > > > 	return __calc_tpm2_event_size(event, event_header, false);
> > > > }
> > > > 
> > > > I.e. TPM_MEMUNMAP will not get executed because "do_mapping == false".
> > > > tpm2_bios_measurements_start() checks for "size == 0" and returns NULL
> > > > whenever this happens.
> > > > 
> > > > Are you 100% sure that it is exactly this commit that triggers the bug?
> > > 
> > > Yes I am 100% sure. The issue happens all the time, and never happens
> > > if I checkout the previous commit.
> > 
> > I'm seeing this too on my test Ubuntu laptop.  Reverting the patch
> > fixes it, but there's no data.
> 
> For a while I wasn't able to boot the test system with secure boot
> enabled.  The NULL pointer dereference problem occurred during the
> period of time that secure boot was disabled.   Once the secure boot
> issue was resolved, the NULL pointer dereference problem was resolved
> as well, when booting with/without secure boot enabled.

I noticed that Kai had disabled secure boot, as well. I found a Thinkpad
with the same BIOS revision as Kai's Thinkpad (N27ET38W - 1.24) and
tested an unpatched v5.9 kernel running on an up-to-date Ubuntu 20.04.1
userspace. The main difference is that he has a Thinkpad T580 and I was
using a Thinkpad P52s. I was unable to reproduce the crash either by
running fwupdtpmevlog or cat'ing /sys/kernel/security/tpm0/binary_bios_measurements.

I noticed that Kai had cleared his secure boot allowed signature
database (db) so I also tried that but was still unsuccessful in
reproducing the crash.

Of course, I also tested with secure boot enabled and also after
toggling every secure boot and TPM related option in the BIOS. Still no
luck.

Kenneth mentioned elsewhere[1] that he experienced the crash with
Ubuntu's 5.8.0-18 kernel so I also tried that but still no luck.

Everyone that has reported this crash[1][2][3][4] seems to have been
running with an Ubuntu userspace. I do see that Ubuntu recently fixed a
shim bug[5] on Sept 24. Ubuntu auto-installs security updates by default
but this was not a security update so perhaps folks that were initially
affected by this crash hadn't yet applied the update. It looks unrelated
to me but it might explain why I can't reproduce this crash now, how the
crash no longer affects Mimi, and why nobody else is complaining about
the crash in recent weeks.

Kai, could you please try once more with v5.9 and ensuring that your
Ubuntu system is fully updated? If you're able to reproduce it with
v5.9, please send me your kernel config and your raw
/sys/kernel/security/tpm0/binary_bios_measurements file contents after
rebooting into a working kernel. I really appreciate it!

Tyler

[1] https://lore.kernel.org/linux-integrity/alpine.DEB.2.23.453.2009121522400.7398@xps-7390/
[2] https://lore.kernel.org/lkml/E1FDCCCB-CA51-4AEE-AC83-9CDE995EAE52@xxxxxxxxxxxxx/
[3] https://lore.kernel.org/lkml/846fe4da67d05f57fba33e38c9a6e394e657adc3.camel@xxxxxxxxxxxxx/
[4] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1898998
[5] https://bugs.launchpad.net/ubuntu/+source/shim/+bug/1864223

> 
> Mimi
>