medium 4.4

CVE-2021-47034 json

powerpc/64s: Fix pte update for kernel memory on radix

published 2024-02-28 2 y ago modified 2026-06-17 4 mo ago also EUVD-2021-33682

cvss: 4.4; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.2%; EPSS 0.2% chance of exploitation in 30 days, percentile 12%
public exploits: 0; none known

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

When adding a PTE a ptesync is needed to order the update of the PTE with subsequent accesses otherwise a spurious fault may be raised.

radix__set_pte_at() does not do this for performance gains. For non-kernel memory this is not an issue as any faults of this kind are corrected by the page fault handler. For kernel memory these faults are not handled. The current solution is that there is a ptesync in flush_cache_vmap() which should be called when mapping from the vmalloc region.

However, map_kernel_page() does not call flush_cache_vmap(). This is troublesome in particular for code patching with Strict RWX on radix. In do_patch_instruction() the page frame that contains the instruction to be patched is mapped and then immediately patched. With no ordering or synchronization between setting up the PTE and writing to the page it is possible for faults.

As the code patching is done using __put_user_asm_goto() the resulting fault is obscured - but using a normal store instead it can be seen:

BUG: Unable to handle kernel data access on write at 0xc008000008f24a3c Faulting instruction address: 0xc00000000008bd74 Oops: Kernel access of bad area, sig: 11 [#1] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA PowerNV Modules linked in: nop_module(PO+) [last unloaded: nop_module] CPU: 4 PID: 757 Comm: sh Tainted: P O 5.10.0-rc5-01361-ge3c1b78c8440-dirty #43 NIP: c00000000008bd74 LR: c00000000008bd50 CTR: c000000000025810 REGS: c000000016f634a0 TRAP: 0300 Tainted: P O (5.10.0-rc5-01361-ge3c1b78c8440-dirty) MSR: 9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE> CR: 44002884 XER: 00000000 CFAR: c00000000007c68c DAR: c008000008f24a3c DSISR: 42000000 IRQMASK: 1

This results in the kind of issue reported here: https://lore.kernel.org/linuxppc-dev/15AC5B0E-A221-4B8C-9039-FA96B8EF7C88@lca.pw/

Chris Riedl suggested a reliable way to reproduce the issue: $ mount -t debugfs none /sys/kernel/debug $ (while true; do echo function > /sys/kernel/debug/tracing/current_tracer ; echo nop > /sys/kernel/debug/tracing/current_tracer ; done) &

Turning ftrace on and off does a large amount of code patching which in usually less then 5min will crash giving a trace like:

ftrace-powerpc: (____ptrval____): replaced (4b473b11) != old (60000000) ------------[ ftrace bug ]------------ ftrace failed to modify [<c000000000bf8e5c>] napi_busy_loop+0xc/0x390 actual: 11:3b:47:4b Setting ftrace call site to call ftrace function ftrace record flags: 80000001 (1) expected tramp: c00000000006c96c ------------[ cut here ]------------ WARNING: CPU: 4 PID: 809 at kernel/trace/ftrace.c:2065 ftrace_bug+0x28c/0x2e8 Modules linked in: nop_module(PO-) [last unloaded: nop_module] CPU: 4 PID: 809 Comm: sh Tainted: P O 5.10.0-rc5-01360-gf878ccaf250a #1 NIP: c00000000024f334 LR: c00000000024f330 CTR: c0000000001a5af0 REGS: c000000004c8b760 TRAP: 0700 Tainted: P O (5.10.0-rc5-01360-gf878ccaf250a) MSR: 900000000282b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 28008848 XER: 20040000 CFAR: c0000000001a9c98 IRQMASK: 0 GPR00: c00000000024f330 c000000004c8b9f0 c000000002770600 0000000000000022 GPR04: 00000000ffff7fff c000000004c8b6d0 0000000000000027 c0000007fe9bcdd8 GPR08: 0000000000000023 ffffffffffffffd8 0000000000000027 c000000002613118 GPR12: 0000000000008000 c0000007fffdca00 0000000000000000 0000000000000000 GPR16: 0000000023ec37c5 0000000000000000 0000000000000000 0000000000000008 GPR20: c000000004c8bc90 c0000000027a2d20 c000000004c8bcd0 c000000002612fe8 GPR24: 0000000000000038 0000000000000030 0000000000000028 0000000000000020 GPR28: c000000000ff1b68 c000000000bf8e5c c00000000312f700 c000000000fbb9b0 NIP ftrace_bug+0x28c/0x2e8 LR ftrace_bug+0x288/0x2e8 Call T ---truncated---

Patch / fix

fixed in 1 distros 3 links 6

Linux Linux 4.19.191, 5.4.119, 5.10.37, 5.11.21, 5.12.4, 5.13

debian ubuntu suse

linux Debian 14, Debian 13, Debian 12 5.10.38-1

debian advisories DEBIAN-CVE-2021-47034

linux Ubuntu 20.04 LTS 5.4.0-77.86
linux-aws Ubuntu 20.04 LTS 5.4.0-1051.53
linux-gcp Ubuntu 20.04 LTS 5.4.0-1046.49
linux-kvm Ubuntu 20.04 LTS 5.4.0-1041.42
linux-azure Ubuntu 20.04 LTS 5.4.0-1051.53
linux-gkeop Ubuntu 20.04 LTS 5.4.0-1018.19
linux-raspi Ubuntu 20.04 LTS 5.4.0-1038.41
linux-oracle Ubuntu 20.04 LTS 5.4.0-1048.52

ubuntu advisories UBUNTU-CVE-2021-47034

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

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-2021-47034 updated 2026-11-06 cvss 4.4

kernel: powerpc/64s: Fix pte update for kernel memory on radix

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 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more 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 (2024-02-28)

Exploit prediction

0.22% chance of exploitation in the next 30 days, higher than 12% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CISA ADP	3.1	4.4	CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
CNA	3.1	4.4	CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H

Affected products

Linux Linux

Classification

Technology feeds

Linux kernel rss critical rss

References

Sources

CVE List CVE-2021-47034
NVD CVE-2021-47034

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.

Summary