Doing multiple squashfs (and other loop?) mounts in parallel breaks

Bug #1836914 reported by John Lenton on 2019-07-17
This bug affects 4 people
Affects Status Importance Assigned to Milestone
linux (Ubuntu)

Bug Description

On a system running a 5.2 kernel, doing a large number of mounts of squashfs filesystems in parallel results in the mounts getting out of sync with their backing devices.

To reproduce,

this breaks people with ~40 snaps (easily achievable given snapd leaves up to 3 previous revisions of snaps mounted at any given time, so a person with ~13 snaps that get updated often will hit this number).

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: linux-image-5.2.0-8-generic 5.2.0-8.9
ProcVersionSignature: Ubuntu 5.2.0-8.9-generic 5.2.0
Uname: Linux 5.2.0-8-generic x86_64
ApportVersion: 2.20.11-0ubuntu5
Architecture: amd64
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CurrentDesktop: ubuntu:GNOME
Date: Wed Jul 17 15:11:15 2019
InstallationDate: Installed on 2019-07-17 (0 days ago)
InstallationMedia: Ubuntu 19.10 "Eoan Ermine" - Alpha amd64 (20190716)
 ens3 no wireless extensions.

 lo no wireless extensions.
Lsusb: Error: command ['lsusb'] failed with exit code 1:
MachineType: QEMU Standard PC (i440FX + PIIX, 1996)
ProcFB: 0 qxldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.2.0-8-generic root=UUID=9618ebbb-955d-4daf-b29a-37162ec7821a ro quiet splash vt.handoff=7
 linux-restricted-modules-5.2.0-8-generic N/A
 linux-backports-modules-5.2.0-8-generic N/A
 linux-firmware 1.180

SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install) 04/01/2014
dmi.bios.vendor: SeaBIOS
dmi.bios.version: Ubuntu-1.8.2-1ubuntu1
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-i440fx-xenial
dmi.modalias: dmi:bvnSeaBIOS:bvrUbuntu-1.8.2-1ubuntu1:bd04/01/2014:svnQEMU:pnStandardPC(i440FX+PIIX,1996):pvrpc-i440fx-xenial:cvnQEMU:ct1:cvrpc-i440fx-xenial: Standard PC (i440FX + PIIX, 1996)
dmi.product.version: pc-i440fx-xenial
dmi.sys.vendor: QEMU

CVE References

John Lenton (chipaca) wrote :
tags: added: rls-ee-incoming
Changed in linux (Ubuntu):
importance: Undecided → Critical
status: New → Confirmed
summary: - Doing multiple squashfs ((and other loop?) mounts in parallel breaks
+ Doing multiple squashfs (and other loop?) mounts in parallel breaks
John Lenton (chipaca) wrote :

I haven't yet tried to reproduce this with an upstream kernel, but given that it's affecting arch users with 5.2 as well, it's fairly likely to be there.

John Lenton (chipaca) wrote :

The mainline builds for i386 and amd64 are failing right now so not sure I can do more.

Jean-Baptiste Lallement (jibel) wrote :

Test script

Kai-Heng Feng (kaihengfeng) wrote :

Commit that causes the regression:
commit 33ec3e53e7b1869d7851e59e126bdb0fe0bd1982
Author: Jan Kara <email address hidden>
Date: Thu May 16 16:01:27 2019 +0200

    loop: Don't change loop device under exclusive opener

    Loop module allows calling LOOP_SET_FD while there are other openers of
    the loop device. Even exclusive ones. This can lead to weird
    consequences such as kernel deadlocks like:

    mount_bdev() lo_ioctl()
          sb_set_blocksize() - sets desired block size B
              __bread_gfp(bdev, block, B)
                - now __getblk_slow() indefinitely loops because B != bdev
                  block size

    Fix the problem by disallowing LOOP_SET_FD ioctl when there are
    exclusive openers of a loop device.

    [Deliberately chosen not to CC stable as a user with priviledges to
    trigger this race has other means of taking the system down and this
    has a potential of breaking some weird userspace setup]

Seems like it solves a race so I'll raise the issue to the commit author.

Seth Forshee (sforshee) wrote :

I confirmed the issue and that reverting the patch fixed it. I've pushed a revert as a temporary fix, Kai-Heng please keep me posted on the activity upstream.

Brad Figg (brad-figg) on 2019-07-24
tags: added: cscc
John Lenton (chipaca) wrote :

Can somebody build and test a kernel with Jan's patch, from
https://<email address hidden>/

Kai-Heng Feng (kaihengfeng) wrote :

A test kernel can be found here:

tags: added: patch
Launchpad Janitor (janitor) wrote :
Download full text (37.9 KiB)

This bug was fixed in the package linux - 5.2.0-10.11

linux (5.2.0-10.11) eoan; urgency=medium

  * eoan/linux: 5.2.0-10.11 -proposed tracker (LP: #1838113)

  * Packaging resync (LP: #1786013)
    - [Packaging] resync git-ubuntu-log

  * Eoan update: v5.2.4 upstream stable release (LP: #1838428)
    - bnx2x: Prevent load reordering in tx completion processing
    - caif-hsi: fix possible deadlock in cfhsi_exit_module()
    - hv_netvsc: Fix extra rcu_read_unlock in netvsc_recv_callback()
    - igmp: fix memory leak in igmpv3_del_delrec()
    - ipv4: don't set IPv6 only flags to IPv4 addresses
    - ipv6: rt6_check should return NULL if 'from' is NULL
    - ipv6: Unlink sibling route in case of failure
    - net: bcmgenet: use promisc for unsupported filters
    - net: dsa: mv88e6xxx: wait after reset deactivation
    - net: make skb_dst_force return true when dst is refcounted
    - net: neigh: fix multiple neigh timer scheduling
    - net: openvswitch: fix csum updates for MPLS actions
    - net: phy: sfp: hwmon: Fix scaling of RX power
    - net_sched: unset TCQ_F_CAN_BYPASS when adding filters
    - net: stmmac: Re-work the queue selection for TSO packets
    - net/tls: make sure offload also gets the keys wiped
    - nfc: fix potential illegal memory access
    - r8169: fix issue with confused RX unit after PHY power-down on RTL8411b
    - rxrpc: Fix send on a connected, but unbound socket
    - sctp: fix error handling on stream scheduler initialization
    - sctp: not bind the socket in sctp_connect
    - sky2: Disable MSI on ASUS P6T
    - tcp: be more careful in tcp_fragment()
    - tcp: fix tcp_set_congestion_control() use from bpf hook
    - tcp: Reset bytes_acked and bytes_received when disconnecting
    - vrf: make sure skb->data contains ip header to make routing
    - net/mlx5e: IPoIB, Add error path in mlx5_rdma_setup_rn
    - net: bridge: mcast: fix stale nsrcs pointer in igmp3/mld2 report handling
    - net: bridge: mcast: fix stale ipv6 hdr pointer when handling v6 query
    - net: bridge: don't cache ether dest pointer on input
    - net: bridge: stp: don't cache eth dest pointer before skb pull
    - macsec: fix use-after-free of skb during RX
    - macsec: fix checksumming after decryption
    - netrom: fix a memory leak in nr_rx_frame()
    - netrom: hold sock when setting skb->destructor
    - selftests: txring_overwrite: fix incorrect test of mmap() return value
    - net/tls: fix poll ignoring partially copied records
    - net/tls: reject offload of TLS 1.3
    - net/mlx5e: Fix port tunnel GRE entropy control
    - net/mlx5e: Rx, Fix checksum calculation for new hardware
    - net/mlx5e: Fix return value from timeout recover function
    - net/mlx5e: Fix error flow in tx reporter diagnose
    - bnxt_en: Fix VNIC accounting when enabling aRFS on 57500 chips.
    - mlxsw: spectrum_dcb: Configure DSCP map as the last rule is removed
    - net/mlx5: E-Switch, Fix default encap mode
    - mlxsw: spectrum: Do not process learned records with a dummy FID
    - dma-buf: balance refcount inbalance
    - dma-buf: Discard old fence_excl on retrying get_fences_rcu for realloc
    - Revert "gpio/spi: Fix spi-gpio...

Changed in linux (Ubuntu):
status: Confirmed → Fix Released
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