[Bug 1991975] Re: dev file system is mounted without nosuid or noexec

[Dimitri John Ledkov]

initramfs-tools also mounts /dev with nosuid, without noexec

> mount -t devtmpfs -o nosuid,mode=0755 udev /dev

I believe all of these should be the same, thus kernel can mount /dev
with nosuid, but should not mount it with noexec.

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Title:
  dev file system is mounted without nosuid or noexec

Status in linux package in Ubuntu:
  Confirmed
Status in systemd package in Ubuntu:
  New
Status in linux source package in Focal:
  In Progress
Status in systemd source package in Focal:
  Invalid
Status in linux source package in Jammy:
  In Progress
Status in systemd source package in Jammy:
  Invalid

Bug description:
  [ SRU TEMPLATE ]
  [ Impact ]

   * nosuid, and noexec bits are not set on /dev
   * This has the potential for nefarious actors to use this as an avenue for attack. see https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1450960 for more discussion around this.
   * It is not best security practice.

  [ Test Plan ]

     1.Boot a Canonical Supplied EC2 instance
     2.Check the mount options for /dev.
     3.You will notice the lack of nosuid and noexec on /dev.

  [ Where problems could occur ]

   * As of 2022/10/06, I need to test this, but don't know how to build
  -aws flavored ubuntu kernels. Instructions welcome.  I'm holding off
  on adding SRU tags until I can actually get this tested.

   * If this is applied to non initramfs-less kernels it could potentially cause a regression for very old hardware that does nefarious things with memory.  For a larger discussion about that see:
  https://lore.kernel.org/lkml/YcMfDOyrg647RCmd@debian-BULLSEYE-live-builder-AMD64/T/

   * Low risk if a driver depends on /dev allowing suid or exec this
  might prevent boot.  That being said, all kernels that have been
  booting with an initramfs have been getting nosuid, and noexec set so
  hopefully we can consider that risk fairly well tested.

  [ Other Info ]

   * Patch is accepted into 5.17, and will drop out quickly
   * Any server booting with an initramfs already has nosuid, and noexec set, so hopefully

  <<<<<<< ORIGINAL TEXT >>>>>>>>>>>>

  This is similar to
  https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1450960 but new.

  I discovered that my ec2 instances based off of Canonical supplied AMI
  ami-0a23d90349664c6ee *(us-east-2), have dev mounted mounted without
  the nosuid option.

  https://us-east-2.console.aws.amazon.com/ec2/home?region=us-
  east-2#Images:visibility=public-images;imageId=ami-0a23d90349664c6ee

  My usb installed 20.04.4 home machine does not have this problem, but
  it has been installed for quite some time.  My 22.04 laptop machine
  also does not have this issue.

  Reproduce.
  Start an ec2 instance based off of ami-0a23d90349664c6ee.
  $ mount | grep devtmpfs
  nosuid is not found in the options list.

  I've checked the initrd, and /etc/init.d/udev script and all places I
  know of where dev gets mounted set nosuid, so it's non-obvious what
  boot code-path is being taken that results in nosuid missing.

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: udev 245.4-4ubuntu3.18
  ProcVersionSignature: Ubuntu 5.15.0-1020.24~20.04.1-aws 5.15.53
  Uname: Linux 5.15.0-1020-aws x86_64
  ApportVersion: 2.20.11-0ubuntu27.24
  Architecture: amd64
  CasperMD5CheckResult: skip
  CustomUdevRuleFiles: 60-cdrom_id.rules 70-snap.snapd.rules
  Date: Thu Oct  6 17:39:42 2022
  Ec2AMI: ami-0a23d90349664c6ee
  Ec2AMIManifest: (unknown)
  Ec2AvailabilityZone: us-east-2c
  Ec2InstanceType: t2.medium
  Ec2Kernel: unavailable
  Ec2Ramdisk: unavailable
  Lsusb: Error: command ['lsusb'] failed with exit code 1:
  Lsusb-t:

  Lsusb-v: Error: command ['lsusb', '-v'] failed with exit code 1:
  MachineType: Xen HVM domU
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   LANG=C.UTF-8
   SHELL=/bin/bash
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.15.0-1020-aws root=PARTUUID=5bb90437-9efc-421d-aa94-c512c3b666a3 ro console=tty1 console=ttyS0 nvme_core.io_timeout=4294967295 panic=-1
  SourcePackage: systemd
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 08/24/2006
  dmi.bios.release: 4.2
  dmi.bios.vendor: Xen
  dmi.bios.version: 4.2.amazon
  dmi.chassis.type: 1
  dmi.chassis.vendor: Xen
  dmi.modalias: dmi:bvnXen:bvr4.2.amazon:bd08/24/2006:br4.2:svnXen:pnHVMdomU:pvr4.2.amazon:cvnXen:ct1:cvr:sku:
  dmi.product.name: HVM domU
  dmi.product.version: 4.2.amazon
  dmi.sys.vendor: Xen

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