CVE-2026-43073

What the bug is

This function was a masterclass in bad naming, for various historical reasons. It claimed to be a non-cached user copy. It is literally _neither_ of those things. It's a specialty memory copy routine that uses non-temporal stores for the destination (but not the source), and that does exception handling for both source and destination accesses. Also note that while it works for unaligned targets, any unaligned parts (whether at beginning or end) will not use non-temporal stores, since only words and quadwords can be non-temporal on x86. The exception handling means that it _can_ be used for user space accesses, but not on its own - it needs all the normal "start user space access" logic around it. But typically the user space access would be the source, not the non-temporal destination. That was the original intention of this, where the destination was some fragile persistent memory target that needed non-temporal stores in order to catch machine check exceptions synchronously and deal with them gracefully. Thus that non-descriptive name: one use case was to copy from user space into a non-cached kernel buffer. However, the existing users are a mix of that intended use-case, and a couple of random drivers that just did this as a performance tweak. Some of those random drivers then actively misused the user copying version (with STAC/CLAC and all) to do kernel copies without ever even caring about the exception handling, _just_ for the non-temporal destination. Rename it as a first small step to actually make it halfway sane, and change the prototype to be more normal: it doesn't take a user pointer unless the caller has done the proper conversion, and the argument size is the full size_t (it still won't actually copy more than 4GB in one go, but there's also no reason to silently truncate the size argument in the caller). Finally, use this now sanely named function in the NTB code, which mis-used a user copy version (with STAC/CLAC and all) of this interface despite it not actually being a user copy at all.

Where it lives

The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.

arch/x86/include/asm/uaccess_64.h
arch/x86/lib/copy_user_uncached_64.S
arch/x86/lib/usercopy_64.c
drivers/infiniband/sw/rdmavt/qp.c
drivers/ntb/ntb_transport.c
tools/objtool/check.c

What this is

X86 ARCHITECTURE (32-BIT AND 64-BIT)MAINTAINERS section matched on arch/x86/ · linux-kernel@vger.kernel.org · Maintained

Kernel module

copy_user_uncached_64

Check it on the machine

grep -wE 'CONFIG_INFINIBAND|CONFIG_INFINIBAND_RDMAVT' /boot/config-$(uname -r)

Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.

lsmod | grep -w copy_user_uncached_64

Prints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.

Show whether Debian builds this, per release

Debian builds this, but not on every flavour. At least one release below builds the code on some of its kernel flavours and not on others, so the answer depends on the architecture and flavour installed.

• bookwormMnot built on amd64/cloud-amd64, arm64/arm64, arm64/cloud-arm64, arm64/rt/arm64, armhf/armmp, armhf/armmp-lpae, armhf/rt/armmp, i386/686, i386/686-pae, i386/rt/686-pae, ppc64el/powerpc64le
• trixie, forky, sidy

M

a module where it is built at all; some flavours do not build it

y

built into the kernel image on every flavour - only a patch removes it

Build switches

CONFIG_INFINIBAND, CONFIG_INFINIBAND_RDMAVT

Reads as

all on means the file is compiled; any one off means it is not.

From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages

Signals

Published

2026-05-05 (5 months ago)source

CVSS

no vector published, unrated

Attack vector

not published

EPSS

0.16%, higher than 5.1% of all CVEssource

CISA KEV

not listedthe KEV catalogue

Debian urgency

not yet assignedsource

Subsystem

arch/x86

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
4.2	6.12.83	14b9194db4a2
4.2	6.18.24	c6d4e0599e7e
4.2	6.19.14	d993e1723aa2
4.2	7.0.1	efea91ad1729
4.2	7.1	d187a86de793