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Fencer Vulns
high 7.8

CVE-2021-47118 json

pid: take a reference when initializing `cad_pid`

published 2024-03-15 2 y ago modified 2026-06-17 4 mo ago also EUVD-2021-33766

cvss
7.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 15%
public exploits
0
none known

Summary

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

pid: take a reference when initializing `cad_pid`

During boot, kernel_init_freeable() initializes `cad_pid` to the init task's struct pid. Later on, we may change `cad_pid` via a sysctl, and when this happens proc_do_cad_pid() will increment the refcount on the new pid via get_pid(), and will decrement the refcount on the old pid via put_pid(). As we never called get_pid() when we initialized `cad_pid`, we decrement a reference we never incremented, can therefore free the init task's struct pid early. As there can be dangling references to the struct pid, we can later encounter a use-after-free (e.g. when delivering signals).

This was spotted when fuzzing v5.13-rc3 with Syzkaller, but seems to have been around since the conversion of `cad_pid` to struct pid in commit 9ec52099e4b8 ("[PATCH] replace cad_pid by a struct pid") from the pre-KASAN stone age of v2.6.19.

Fix this by getting a reference to the init task's struct pid when we assign it to `cad_pid`.

Full KASAN splat below.

================================================================== BUG: KASAN: use-after-free in ns_of_pid include/linux/pid.h:153 [inline] BUG: KASAN: use-after-free in task_active_pid_ns+0xc0/0xc8 kernel/pid.c:509 Read of size 4 at addr ffff23794dda0004 by task syz-executor.0/273

CPU: 1 PID: 273 Comm: syz-executor.0 Not tainted 5.12.0-00001-g9aef892b2d15 #1 Hardware name: linux,dummy-virt (DT) Call trace: ns_of_pid include/linux/pid.h:153 [inline] task_active_pid_ns+0xc0/0xc8 kernel/pid.c:509 do_notify_parent+0x308/0xe60 kernel/signal.c:1950 exit_notify kernel/exit.c:682 [inline] do_exit+0x2334/0x2bd0 kernel/exit.c:845 do_group_exit+0x108/0x2c8 kernel/exit.c:922 get_signal+0x4e4/0x2a88 kernel/signal.c:2781 do_signal arch/arm64/kernel/signal.c:882 [inline] do_notify_resume+0x300/0x970 arch/arm64/kernel/signal.c:936 work_pending+0xc/0x2dc

Allocated by task 0: slab_post_alloc_hook+0x50/0x5c0 mm/slab.h:516 slab_alloc_node mm/slub.c:2907 [inline] slab_alloc mm/slub.c:2915 [inline] kmem_cache_alloc+0x1f4/0x4c0 mm/slub.c:2920 alloc_pid+0xdc/0xc00 kernel/pid.c:180 copy_process+0x2794/0x5e18 kernel/fork.c:2129 kernel_clone+0x194/0x13c8 kernel/fork.c:2500 kernel_thread+0xd4/0x110 kernel/fork.c:2552 rest_init+0x44/0x4a0 init/main.c:687 arch_call_rest_init+0x1c/0x28 start_kernel+0x520/0x554 init/main.c:1064 0x0

Freed by task 270: slab_free_hook mm/slub.c:1562 [inline] slab_free_freelist_hook+0x98/0x260 mm/slub.c:1600 slab_free mm/slub.c:3161 [inline] kmem_cache_free+0x224/0x8e0 mm/slub.c:3177 put_pid.part.4+0xe0/0x1a8 kernel/pid.c:114 put_pid+0x30/0x48 kernel/pid.c:109 proc_do_cad_pid+0x190/0x1b0 kernel/sysctl.c:1401 proc_sys_call_handler+0x338/0x4b0 fs/proc/proc_sysctl.c:591 proc_sys_write+0x34/0x48 fs/proc/proc_sysctl.c:617 call_write_iter include/linux/fs.h:1977 [inline] new_sync_write+0x3ac/0x510 fs/read_write.c:518 vfs_write fs/read_write.c:605 [inline] vfs_write+0x9c4/0x1018 fs/read_write.c:585 ksys_write+0x124/0x240 fs/read_write.c:658 __do_sys_write fs/read_write.c:670 [inline] __se_sys_write fs/read_write.c:667 [inline] __arm64_sys_write+0x78/0xb0 fs/read_write.c:667 __invoke_syscall arch/arm64/kernel/syscall.c:37 [inline] invoke_syscall arch/arm64/kernel/syscall.c:49 [inline] el0_svc_common.constprop.1+0x16c/0x388 arch/arm64/kernel/syscall.c:129 do_el0_svc+0xf8/0x150 arch/arm64/kernel/syscall.c:168 el0_svc+0x28/0x38 arch/arm64/kernel/entry-common.c:416 el0_sync_handler+0x134/0x180 arch/arm64/kernel/entry-common.c:432 el0_sync+0x154/0x180 arch/arm64/kernel/entry.S:701

The buggy address belongs to the object at ffff23794dda0000 which belongs to the cache pid of size 224 The buggy address is located 4 bytes inside of 224-byte region [ff ---truncated---

Patch / fix

  • Linux Linux 4.4.272, 4.9.272, 4.14.236, 4.19.194, 5.4.125, 5.10.43, 5.12.10, 5.13
  • linux Debian 14, Debian 13, Debian 12 5.10.46-1

debian advisories DEBIAN-CVE-2021-47118

  • linux Ubuntu 18.04 LTS 4.15.0-154.161
  • linux Ubuntu 20.04 LTS 5.4.0-81.91
  • linux-aws Ubuntu 18.04 LTS 4.15.0-1110.117
  • linux-aws Ubuntu 20.04 LTS 5.4.0-1055.58
  • linux-gcp Ubuntu 20.04 LTS 5.4.0-1051.55
  • linux-kvm Ubuntu 18.04 LTS 4.15.0-1098.100
  • linux-kvm Ubuntu 20.04 LTS 5.4.0-1045.47
  • linux-azure Ubuntu 20.04 LTS 5.4.0-1056.58

ubuntu advisories UBUNTU-CVE-2021-47118

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • kernel-rt RHEL 8 4.18.0-553.5.1.rt7.346.el8_10
  • kernel-rt-kvm RHEL 8 4.18.0-553.5.1.rt7.346.el8_10
  • kernel-rt-core RHEL 8 4.18.0-553.5.1.rt7.346.el8_10
  • kernel-rt-modules RHEL 8 4.18.0-553.5.1.rt7.346.el8_10
  • kernel-rt-debug-kvm RHEL 8 4.18.0-553.5.1.rt7.346.el8_10
  • kernel-rt-debug-core RHEL 8 4.18.0-553.5.1.rt7.346.el8_10
  • kernel-rt-debug-modules RHEL 8 4.18.0-553.5.1.rt7.346.el8_10
  • kernel-rt-modules-extra RHEL 8 4.18.0-553.5.1.rt7.346.el8_10

red hat advisories RHSA-2024:3627RHSA-2024:3618

  • kernel-syms SLES for SAP 12 SP5, SLES 12 SP5 4.12.14-122.216.1
  • kernel-source SLES for SAP 12 SP5, SLES 12 SP5 4.12.14-122.216.1
  • kernel-default SLES for SAP 12 SP5, SLES 12 SP5, SLE Workstation Extension 12 SP5 +2 more 4.12.14-122.216.1
  • kernel-obs-build SLE Software Development Kit 12 SP5 4.12.14-122.216.1
  • kernel-rt SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-syms-rt SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-rt_debug SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-source-rt SLE Real Time 12 SP5 4.12.14-10.182.1

suse advisories SUSE-SU-2024:1870-1SUSE-SU-2024:1646-1SUSE-SU-2024:1643-1SUSE-SU-2024:1489-1SUSE-SU-2024:1465-1SUSE-SU-2024:1454-1

  • kernel AlmaLinux 8 4.18.0-553.5.1.el8_10
  • perf AlmaLinux 8 4.18.0-553.5.1.el8_10
  • bpftool AlmaLinux 8 4.18.0-553.5.1.el8_10
  • kernel-core AlmaLinux 8 4.18.0-553.5.1.el8_10
  • kernel-tools AlmaLinux 8 4.18.0-553.5.1.el8_10
  • python3-perf AlmaLinux 8 4.18.0-553.5.1.el8_10
  • kernel-modules AlmaLinux 8 4.18.0-553.5.1.el8_10
  • kernel-zfcpdump AlmaLinux 8 4.18.0-553.5.1.el8_10

almalinux advisories ALSA-2024:3618ALSA-2024:3627

  • kernel-rt Rocky Linux 8 4.18.0-553.5.1.rt7.346.el8_10
  • kernel Rocky Linux 8 4.18.0-553.5.1.el8_10

rocky linux advisories RLSA-2024:3627RLSA-2024:3618

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-47118 updated cvss 4.1

kernel: pid: take a reference when initializing `cad_pid`

  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux BaseOS (v. 8) perf 4.18.0-553.5.1.el8_10, kernel 4.18.0-553.5.1.el8_10, bpftool 4.18.0-553.5.1.el8_10, kernel-core 4.18.0-553.5.1.el8_10, kernel-tools 4.18.0-553.5.1.el8_10, python3-perf 4.18.0-553.5.1.el8_10, +15 more Red Hat Enterprise Linux CRB (v. 8) kernel-debuginfo-common-x86_64 4.18.0-553.5.1.el8_10, kernel-debuginfo-common-aarch64 4.18.0-553.5.1.el8_10, kernel-debuginfo-common-ppc64le 4.18.0-553.5.1.el8_10
    fixed: vendor fix RHSA-2024:3618
  • Red Hat Enterprise Linux NFV (v. 8) kernel-rt 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-kvm 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-core 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-modules 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-debug-kvm 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-debug-core 4.18.0-553.5.1.rt7.346.el8_10, +4 more Red Hat Enterprise Linux RT (v. 8) kernel-rt 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-core 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-modules 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-debug-core 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-debug-modules 4.18.0-553.5.1.rt7.346.el8_10, kernel-rt-modules-extra 4.18.0-553.5.1.rt7.346.el8_10, +2 more
    fixed: vendor fix RHSA-2024:3627
  • Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
2 y ago

Exploit prediction

0.25% chance of exploitation in the next 30 days, higher than 15% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 7.8 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Affected products

  • Linux Linux

Classification

Weakness
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.