Subject: CVE-2024-26941: drm/dp: Fix divide-by-zero regression on DP MST unplug with nouveau

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

drm/dp: Fix divide-by-zero regression on DP MST unplug with nouveau

Fix a regression when using nouveau and unplugging a StarTech MSTDP122DP
DisplayPort 1.2 MST hub (the same regression does not appear when using
a Cable Matters DisplayPort 1.4 MST hub). Trace:

 divide error: 0000 [#1] PREEMPT SMP PTI
 CPU: 7 PID: 2962 Comm: Xorg Not tainted 6.8.0-rc3+ #744
 Hardware name: Razer Blade/DANA_MB, BIOS 01.01 08/31/2018
 RIP: 0010:drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
 Code: c6 b8 01 00 00 00 75 61 01 c6 41 0f af f3 41 0f af f1 c1 e1 04 48 63 c7 31 d2 89 ff 48 8b 5d f8 c9 48 0f af f1 48 8d 44 06 ff <48> f7 f7 31 d2 31 c9 31 f6 31 ff 45 31 c0 45 31 c9 45 31 d2 45 31
 RSP: 0018:ffffb2c5c211fa30 EFLAGS: 00010206
 RAX: ffffffffffffffff RBX: 0000000000000000 RCX: 0000000000f59b00
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
 RBP: ffffb2c5c211fa48 R08: 0000000000000001 R09: 0000000000000020
 R10: 0000000000000004 R11: 0000000000000000 R12: 0000000000023b4a
 R13: ffff91d37d165800 R14: ffff91d36fac6d80 R15: ffff91d34a764010
 FS:  00007f4a1ca3fa80(0000) GS:ffff91d6edbc0000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000559491d49000 CR3: 000000011d180002 CR4: 00000000003706f0
 Call Trace:
  <TASK>
  ? show_regs+0x6d/0x80
  ? die+0x37/0xa0
  ? do_trap+0xd4/0xf0
  ? do_error_trap+0x71/0xb0
  ? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
  ? exc_divide_error+0x3a/0x70
  ? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
  ? asm_exc_divide_error+0x1b/0x20
  ? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
  ? drm_dp_calc_pbn_mode+0x2e/0x70 [drm_display_helper]
  nv50_msto_atomic_check+0xda/0x120 [nouveau]
  drm_atomic_helper_check_modeset+0xa87/0xdf0 [drm_kms_helper]
  drm_atomic_helper_check+0x19/0xa0 [drm_kms_helper]
  nv50_disp_atomic_check+0x13f/0x2f0 [nouveau]
  drm_atomic_check_only+0x668/0xb20 [drm]
  ? drm_connector_list_iter_next+0x86/0xc0 [drm]
  drm_atomic_commit+0x58/0xd0 [drm]
  ? __pfx___drm_printfn_info+0x10/0x10 [drm]
  drm_atomic_connector_commit_dpms+0xd7/0x100 [drm]
  drm_mode_obj_set_property_ioctl+0x1c5/0x450 [drm]
  ? __pfx_drm_connector_property_set_ioctl+0x10/0x10 [drm]
  drm_connector_property_set_ioctl+0x3b/0x60 [drm]
  drm_ioctl_kernel+0xb9/0x120 [drm]
  drm_ioctl+0x2d0/0x550 [drm]
  ? __pfx_drm_connector_property_set_ioctl+0x10/0x10 [drm]
  nouveau_drm_ioctl+0x61/0xc0 [nouveau]
  __x64_sys_ioctl+0xa0/0xf0
  do_syscall_64+0x76/0x140
  ? do_syscall_64+0x85/0x140
  ? do_syscall_64+0x85/0x140
  entry_SYSCALL_64_after_hwframe+0x6e/0x76
 RIP: 0033:0x7f4a1cd1a94f
 Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
 RSP: 002b:00007ffd2f1df520 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
 RAX: ffffffffffffffda RBX: 00007ffd2f1df5b0 RCX: 00007f4a1cd1a94f
 RDX: 00007ffd2f1df5b0 RSI: 00000000c01064ab RDI: 000000000000000f
 RBP: 00000000c01064ab R08: 000056347932deb8 R09: 000056347a7d99c0
 R10: 0000000000000000 R11: 0000000000000246 R12: 000056347938a220
 R13: 000000000000000f R14: 0000563479d9f3f0 R15: 0000000000000000
  </TASK>
 Modules linked in: rfcomm xt_conntrack nft_chain_nat xt_MASQUERADE nf_nat nf_conntrack_netlink nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 xfrm_user xfrm_algo xt_addrtype nft_compat nf_tables nfnetlink br_netfilter bridge stp llc ccm cmac algif_hash overlay algif_skcipher af_alg bnep binfmt_misc snd_sof_pci_intel_cnl snd_sof_intel_hda_common snd_soc_hdac_hda snd_sof_pci snd_sof_xtensa_dsp snd_sof_intel_hda snd_sof snd_sof_utils snd_soc_acpi_intel_match snd_soc_acpi snd_soc_core snd_compress snd_sof_intel_hda_mlink snd_hda_ext_core iwlmvm intel_rapl_msr intel_rapl_common intel_tcc_cooling x86_pkg_temp_thermal intel_powerclamp mac80211 coretemp kvm_intel snd_hda_codec_hdmi kvm snd_hda_codec_realtek snd_hda_codec_generic uvcvideo libarc4 snd_hda_intel snd_intel_dspcfg snd_hda_codec iwlwifi videobuf2_vmalloc videobuf2_memops uvc irqbypass btusb videobuf2_v4l2 snd_seq_midi crct10dif_pclmul hid_multitouch crc32_pclmul snd_seq_midi_event btrtl snd_hwdep videodev polyval_clmulni polyval_generic snd_rawmidi
  ghash_clmulni_intel aesni_intel btintel crypto_simd snd_hda_core cryptd snd_seq btbcm ee1004 8250_dw videobuf2_common btmtk rapl nls_iso8859_1 mei_hdcp thunderbolt bluetooth intel_cstate wmi_bmof intel_wmi_thunderbolt cfg80211 snd_pcm mc snd_seq_device i2c_i801 r8169 ecdh_generic snd_timer i2c_smbus ecc snd mei_me intel_lpss_pci mei ahci intel_lpss soundcore realtek libahci idma64 intel_pch_thermal i2c_hid_acpi i2c_hid acpi_pad sch_fq_codel msr parport_pc ppdev lp parport efi_pstore ip_tables x_tables autofs4 dm_crypt raid10 raid456 libcrc32c async_raid6_recov async_memcpy async_pq async_xor xor async_tx raid6_pq raid1 raid0 joydev input_leds hid_generic usbhid hid nouveau i915 drm_ttm_helper gpu_sched drm_gpuvm drm_exec i2c_algo_bit drm_buddy ttm drm_display_helper drm_kms_helper cec rc_core drm nvme nvme_core mxm_wmi xhci_pci xhci_pci_renesas video wmi pinctrl_cannonlake mac_hid
 ---[ end trace 0000000000000000 ]---

Fix this by avoiding the divide if bpp is 0.

The Linux kernel CVE team has assigned CVE-2024-26941 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.8 with commit c1d6a22b7219 and fixed in 6.8.3 with commit 828862071a6c
	Issue introduced in 6.8 with commit c1d6a22b7219 and fixed in 6.9 with commit 9cbd1dae8427

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26941
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	drivers/gpu/drm/display/drm_dp_helper.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/828862071a6ca0c52655e6e62ac7abfef3e5c578
	https://git.kernel.org/stable/c/9cbd1dae842737bfafa4b10a87909fa209dde250