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

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Message-ID: <20200928140623.GA69515@linux.intel.com>
Date:   Mon, 28 Sep 2020 17:06:23 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     tyhicks@...ux.microsoft.com, linux-integrity@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [Regression] "tpm: Require that all digests are present in
 TCG_PCR_EVENT2 structures" causes null pointer dereference

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 ucsi_acpi roles 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?

/Jarkko