linux-cve-announce - CVE-2025-68751: s390/fpu: Fix false-positive kmsan report in fpu

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Message-ID: <2026010546-CVE-2025-68751-b3fa@gregkh>
Date: Mon,  5 Jan 2026 10:32:47 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-68751: s390/fpu: Fix false-positive kmsan report in fpu_vstl()

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

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

s390/fpu: Fix false-positive kmsan report in fpu_vstl()

A false-positive kmsan report is detected when running ping command.

An inline assembly instruction 'vstl' can write varied amount of bytes
depending on value of 'index' argument. If 'index' > 0, 'vstl' writes
at least 2 bytes.

clang generates kmsan write helper call depending on inline assembly
constraints. Constraints are evaluated compile-time, but value of
'index' argument is known only at runtime.

clang currently generates call to __msan_instrument_asm_store with 1 byte
as size. Manually call kmsan function to indicate correct amount of bytes
written and fix false-positive report.

This change fixes following kmsan reports:

[   36.563119] =====================================================
[   36.563594] BUG: KMSAN: uninit-value in virtqueue_add+0x35c6/0x7c70
[   36.563852]  virtqueue_add+0x35c6/0x7c70
[   36.564016]  virtqueue_add_outbuf+0xa0/0xb0
[   36.564266]  start_xmit+0x288c/0x4a20
[   36.564460]  dev_hard_start_xmit+0x302/0x900
[   36.564649]  sch_direct_xmit+0x340/0xea0
[   36.564894]  __dev_queue_xmit+0x2e94/0x59b0
[   36.565058]  neigh_resolve_output+0x936/0xb40
[   36.565278]  __neigh_update+0x2f66/0x3a60
[   36.565499]  neigh_update+0x52/0x60
[   36.565683]  arp_process+0x1588/0x2de0
[   36.565916]  NF_HOOK+0x1da/0x240
[   36.566087]  arp_rcv+0x3e4/0x6e0
[   36.566306]  __netif_receive_skb_list_core+0x1374/0x15a0
[   36.566527]  netif_receive_skb_list_internal+0x1116/0x17d0
[   36.566710]  napi_complete_done+0x376/0x740
[   36.566918]  virtnet_poll+0x1bae/0x2910
[   36.567130]  __napi_poll+0xf4/0x830
[   36.567294]  net_rx_action+0x97c/0x1ed0
[   36.567556]  handle_softirqs+0x306/0xe10
[   36.567731]  irq_exit_rcu+0x14c/0x2e0
[   36.567910]  do_io_irq+0xd4/0x120
[   36.568139]  io_int_handler+0xc2/0xe8
[   36.568299]  arch_cpu_idle+0xb0/0xc0
[   36.568540]  arch_cpu_idle+0x76/0xc0
[   36.568726]  default_idle_call+0x40/0x70
[   36.568953]  do_idle+0x1d6/0x390
[   36.569486]  cpu_startup_entry+0x9a/0xb0
[   36.569745]  rest_init+0x1ea/0x290
[   36.570029]  start_kernel+0x95e/0xb90
[   36.570348]  startup_continue+0x2e/0x40
[   36.570703]
[   36.570798] Uninit was created at:
[   36.571002]  kmem_cache_alloc_node_noprof+0x9e8/0x10e0
[   36.571261]  kmalloc_reserve+0x12a/0x470
[   36.571553]  __alloc_skb+0x310/0x860
[   36.571844]  __ip_append_data+0x483e/0x6a30
[   36.572170]  ip_append_data+0x11c/0x1e0
[   36.572477]  raw_sendmsg+0x1c8c/0x2180
[   36.572818]  inet_sendmsg+0xe6/0x190
[   36.573142]  __sys_sendto+0x55e/0x8e0
[   36.573392]  __s390x_sys_socketcall+0x19ae/0x2ba0
[   36.573571]  __do_syscall+0x12e/0x240
[   36.573823]  system_call+0x6e/0x90
[   36.573976]
[   36.574017] Byte 35 of 98 is uninitialized
[   36.574082] Memory access of size 98 starts at 0000000007aa0012
[   36.574218]
[   36.574325] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Tainted: G    B            N  6.17.0-dirty #16 NONE
[   36.574541] Tainted: [B]=BAD_PAGE, [N]=TEST
[   36.574617] Hardware name: IBM 3931 A01 703 (KVM/Linux)
[   36.574755] =====================================================

[   63.532541] =====================================================
[   63.533639] BUG: KMSAN: uninit-value in virtqueue_add+0x35c6/0x7c70
[   63.533989]  virtqueue_add+0x35c6/0x7c70
[   63.534940]  virtqueue_add_outbuf+0xa0/0xb0
[   63.535861]  start_xmit+0x288c/0x4a20
[   63.536708]  dev_hard_start_xmit+0x302/0x900
[   63.537020]  sch_direct_xmit+0x340/0xea0
[   63.537997]  __dev_queue_xmit+0x2e94/0x59b0
[   63.538819]  neigh_resolve_output+0x936/0xb40
[   63.539793]  ip_finish_output2+0x1ee2/0x2200
[   63.540784]  __ip_finish_output+0x272/0x7a0
[   63.541765]  ip_finish_output+0x4e/0x5e0
[   63.542791]  ip_output+0x166/0x410
[   63.543771]  ip_push_pending_frames+0x1a2/0x470
[   63.544753]  raw_sendmsg+0x1f06/0x2180
[   63.545033]  inet_sendmsg+0xe6/0x190
[   63.546006]  __sys_sendto+0x55e/0x8e0
[   63.546859]  __s390x_sys_socketcall+0x19ae/0x2ba0
[   63.547730]  __do_syscall+0x12e/0x240
[   63.548019]  system_call+0x6e/0x90
[   63.548989]
[   63.549779] Uninit was created at:
[   63.550691]  kmem_cache_alloc_node_noprof+0x9e8/0x10e0
[   63.550975]  kmalloc_reserve+0x12a/0x470
[   63.551969]  __alloc_skb+0x310/0x860
[   63.552949]  __ip_append_data+0x483e/0x6a30
[   63.553902]  ip_append_data+0x11c/0x1e0
[   63.554912]  raw_sendmsg+0x1c8c/0x2180
[   63.556719]  inet_sendmsg+0xe6/0x190
[   63.557534]  __sys_sendto+0x55e/0x8e0
[   63.557875]  __s390x_sys_socketcall+0x19ae/0x2ba0
[   63.558869]  __do_syscall+0x12e/0x240
[   63.559832]  system_call+0x6e/0x90
[   63.560780]
[   63.560972] Byte 35 of 98 is uninitialized
[   63.561741] Memory access of size 98 starts at 0000000005704312
[   63.561950]
[   63.562824] CPU: 3 UID: 0 PID: 192 Comm: ping Tainted: G    B            N  6.17.0-dirty #16 NONE
[   63.563868] Tainted: [B]=BAD_PAGE, [N]=TEST
[   63.564751] Hardware name: IBM 3931 A01 703 (KVM/Linux)
[   63.564986] =====================================================

The Linux kernel CVE team has assigned CVE-2025-68751 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.9 with commit dcd3e1de9d17dc43dfed87a9fc814b9dec508043 and fixed in 6.17.13 with commit 946357a538bb47740635c25520924351d2d91544
	Issue introduced in 6.9 with commit dcd3e1de9d17dc43dfed87a9fc814b9dec508043 and fixed in 6.18.2 with commit 13dcd6308cb8f67134ee5d5d762b2a66363c695b
	Issue introduced in 6.9 with commit dcd3e1de9d17dc43dfed87a9fc814b9dec508043 and fixed in 6.19-rc1 with commit 14e4e4175b64dd9216b522f6ece8af6997d063b2

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-68751
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/s390/include/asm/fpu-insn.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/946357a538bb47740635c25520924351d2d91544
	https://git.kernel.org/stable/c/13dcd6308cb8f67134ee5d5d762b2a66363c695b
	https://git.kernel.org/stable/c/14e4e4175b64dd9216b522f6ece8af6997d063b2