s_iflags overlap prevents unprivileged overlayfs mounts

Bug #1851677 reported by Seth Forshee on 2019-11-07
8
This bug affects 1 person
Affects Status Importance Assigned to Milestone
linux (Ubuntu)
Medium
Seth Forshee
Disco
Medium
Seth Forshee
Eoan
Medium
Seth Forshee

Bug Description

SRU Justification

Impact: SB_I_NOSUID was added by a sauce patch, and over time it has come to occpy the same bit in s_iflags as SB_I_USERNS_VISIBLE without being noticed. overlayfs will set SB_I_NOSUID when any lower mount is nosuid. When this happens for a user namespace mount, mount_too_revealing() will perform additional, unnecessary checks which may block mounting when it should be allowed.

Fix: Relocate SB_I_NOSUID to the top of s_iflags to remove the conflict and to make future conflicts less likely.

Test Case: Attached.

Regression Potential: These flags are internal to the kernel, so moving SB_I_NOSUID is safe. It's possible that the overlapping flags caused other unintended behaviors, and if so these will also change and could result in user-visible changes. However, any such behaviors would also be incorrect.

---

As reported by Jann Horn on bug 1850994, unprivileged overlayfs mounting can fail in disco/eoan due to SB_I_NOSUID and SB_I_USERNS_VISIBLE. overlayfs sets SB_I_NOSUID in some cases, which causes mount_too_revealing() to see SB_I_USERNS_VISIBLE as being set. This leads to performing additional, unnecessary checks which may fail.

CVE References

Seth Forshee (sforshee) on 2019-11-07
description: updated
Seth Forshee (sforshee) wrote :

Attaching test script to reproduce the bug, adapted from the reproducer for bug 1850994. Requires the bindfs package.

description: updated
Changed in linux (Ubuntu Disco):
assignee: nobody → Seth Forshee (sforshee)
importance: Undecided → Medium
status: New → In Progress
Changed in linux (Ubuntu Eoan):
assignee: nobody → Seth Forshee (sforshee)
importance: Undecided → Medium
status: New → In Progress
Seth Forshee (sforshee) on 2019-11-07
description: updated
Seth Forshee (sforshee) on 2019-11-07
Changed in linux (Ubuntu):
status: In Progress → Fix Committed
Changed in linux (Ubuntu Disco):
status: In Progress → Fix Committed
Changed in linux (Ubuntu Eoan):
status: In Progress → Fix Committed

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-eoan' to 'verification-done-eoan'. If the problem still exists, change the tag 'verification-needed-eoan' to 'verification-failed-eoan'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

tags: added: verification-needed-eoan

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-disco' to 'verification-done-disco'. If the problem still exists, change the tag 'verification-needed-disco' to 'verification-failed-disco'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

tags: added: verification-needed-disco
Khaled El Mously (kmously) wrote :

I ran the test.sh script on the 5.3.0-23.25 kernel (the kernel in -updates) and the 5.3.0-24.26 kernel (the one in -proposed). The -23 kernel gave me a "/tmp/mnt: permission denied" error whereas the -24 kernel did not give this error.

Does this mean this bug is verified?

According to my understanding, the bug was the denial of the mounting performed by the testcase, and no errors returned means that the bug is fixed.

I have also verified this fix with Disco (5.0.0-37-generic), so I will mark both series as verified.

Thanks.

tags: added: verification-done-disco verification-done-eoan
removed: verification-needed-disco verification-needed-eoan
Launchpad Janitor (janitor) wrote :
Download full text (19.3 KiB)

This bug was fixed in the package linux - 5.0.0-37.40

---------------
linux (5.0.0-37.40) disco; urgency=medium

  * disco/linux: 5.0.0-37.40 -proposed tracker (LP: #1852253)

  * System hangs at early boot (LP: #1851216)
    - x86/timer: Skip PIT initialization on modern chipsets

  * drm/i915: Add support for another CMP-H PCH (LP: #1848491)
    - drm/i915/cml: Add second PCH ID for CMP

  * Some EFI systems fail to boot in efi_init() when booted via maas
    (LP: #1851810)
    - efi: efi_get_memory_map -- increase map headroom

  * seccomp: fix SECCOMP_USER_NOTIF_FLAG_CONTINUE test (LP: #1849281)
    - SAUCE: seccomp: avoid overflow in implicit constant conversion
    - SAUCE: seccomp: rework define for SECCOMP_USER_NOTIF_FLAG_CONTINUE
    - SAUCE: seccomp: fix SECCOMP_USER_NOTIF_FLAG_CONTINUE test

  * dkms artifacts may expire from the pool (LP: #1850958)
    - [Packaging] dkms -- try launchpad librarian for pool downloads
    - [Packaging] dkms -- dkms-build quieten wget verbiage

  * update ENA driver to version 2.1.0 (LP: #1850175)
    - net: ena: fix swapped parameters when calling
      ena_com_indirect_table_fill_entry
    - net: ena: fix: Free napi resources when ena_up() fails
    - net: ena: fix incorrect test of supported hash function
    - net: ena: fix return value of ena_com_config_llq_info()
    - net: ena: improve latency by disabling adaptive interrupt moderation by
      default
    - net: ena: fix ena_com_fill_hash_function() implementation
    - net: ena: add handling of llq max tx burst size
    - net: ena: ethtool: add extra properties retrieval via get_priv_flags
    - net: ena: replace free_tx/rx_ids union with single free_ids field in
      ena_ring
    - net: ena: arrange ena_probe() function variables in reverse christmas tree
    - net: ena: add newline at the end of pr_err prints
    - net: ena: documentation: update ena.txt
    - net: ena: allow automatic fallback to polling mode
    - net: ena: add support for changing max_header_size in LLQ mode
    - net: ena: optimise calculations for CQ doorbell
    - net: ena: add good checksum counter
    - net: ena: use dev_info_once instead of static variable
    - net: ena: add MAX_QUEUES_EXT get feature admin command
    - net: ena: enable negotiating larger Rx ring size
    - net: ena: make ethtool show correct current and max queue sizes
    - net: ena: allow queue allocation backoff when low on memory
    - net: ena: add ethtool function for changing io queue sizes
    - net: ena: remove inline keyword from functions in *.c
    - net: ena: update driver version from 2.0.3 to 2.1.0
    - net: ena: Fix bug where ring allocation backoff stopped too late
    - Revert "net: ena: ethtool: add extra properties retrieval via
      get_priv_flags"
    - net: ena: don't wake up tx queue when down
    - net: ena: clean up indentation issue

  * Add Intel Comet Lake ethernet support (LP: #1848555)
    - SAUCE: e1000e: Add support for Comet Lake

  * Intel Wireless AC 3168 on Eoan complaints FW error in SYNC CMD
    GEO_TX_POWER_LIMIT (LP: #1846016)
    - iwlwifi: exclude GEO SAR support for 3168

  * tsc marked unstable after entered PC10 on Intel CoffeeLake (LP: #1840239...

Changed in linux (Ubuntu Disco):
status: Fix Committed → Fix Released
Launchpad Janitor (janitor) wrote :
Download full text (33.2 KiB)

This bug was fixed in the package linux - 5.3.0-24.26

---------------
linux (5.3.0-24.26) eoan; urgency=medium

  * eoan/linux: 5.3.0-24.26 -proposed tracker (LP: #1852232)

  * Eoan update: 5.3.9 upstream stable release (LP: #1851550)
    - io_uring: fix up O_NONBLOCK handling for sockets
    - dm snapshot: introduce account_start_copy() and account_end_copy()
    - dm snapshot: rework COW throttling to fix deadlock
    - Btrfs: fix inode cache block reserve leak on failure to allocate data space
    - btrfs: qgroup: Always free PREALLOC META reserve in
      btrfs_delalloc_release_extents()
    - iio: adc: meson_saradc: Fix memory allocation order
    - iio: fix center temperature of bmc150-accel-core
    - libsubcmd: Make _FORTIFY_SOURCE defines dependent on the feature
    - perf tests: Avoid raising SEGV using an obvious NULL dereference
    - perf map: Fix overlapped map handling
    - perf script brstackinsn: Fix recovery from LBR/binary mismatch
    - perf jevents: Fix period for Intel fixed counters
    - perf tools: Propagate get_cpuid() error
    - perf annotate: Propagate perf_env__arch() error
    - perf annotate: Fix the signedness of failure returns
    - perf annotate: Propagate the symbol__annotate() error return
    - perf annotate: Fix arch specific ->init() failure errors
    - perf annotate: Return appropriate error code for allocation failures
    - perf annotate: Don't return -1 for error when doing BPF disassembly
    - staging: rtl8188eu: fix null dereference when kzalloc fails
    - RDMA/siw: Fix serialization issue in write_space()
    - RDMA/hfi1: Prevent memory leak in sdma_init
    - RDMA/iw_cxgb4: fix SRQ access from dump_qp()
    - RDMA/iwcm: Fix a lock inversion issue
    - HID: hyperv: Use in-place iterator API in the channel callback
    - kselftest: exclude failed TARGETS from runlist
    - selftests/kselftest/runner.sh: Add 45 second timeout per test
    - nfs: Fix nfsi->nrequests count error on nfs_inode_remove_request
    - arm64: cpufeature: Effectively expose FRINT capability to userspace
    - arm64: Fix incorrect irqflag restore for priority masking for compat
    - arm64: ftrace: Ensure synchronisation in PLT setup for Neoverse-N1 #1542419
    - tty: serial: owl: Fix the link time qualifier of 'owl_uart_exit()'
    - tty: serial: rda: Fix the link time qualifier of 'rda_uart_exit()'
    - serial/sifive: select SERIAL_EARLYCON
    - tty: n_hdlc: fix build on SPARC
    - misc: fastrpc: prevent memory leak in fastrpc_dma_buf_attach
    - RDMA/core: Fix an error handling path in 'res_get_common_doit()'
    - RDMA/cm: Fix memory leak in cm_add/remove_one
    - RDMA/nldev: Reshuffle the code to avoid need to rebind QP in error path
    - RDMA/mlx5: Do not allow rereg of a ODP MR
    - RDMA/mlx5: Order num_pending_prefetch properly with synchronize_srcu
    - RDMA/mlx5: Add missing synchronize_srcu() for MW cases
    - gpio: max77620: Use correct unit for debounce times
    - fs: cifs: mute -Wunused-const-variable message
    - arm64: vdso32: Fix broken compat vDSO build warnings
    - arm64: vdso32: Detect binutils support for dmb ishld
    - serial: mctrl_gpio: Check for NULL pointer
    - serial: 8250_...

Changed in linux (Ubuntu Eoan):
status: Fix Committed → Fix Released
Changed in linux (Ubuntu):
status: Fix Committed → Fix Released
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