linux-cve-announce - CVE-2024-50102: x86: fix user address masking non-canonical speculation issue

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Message-ID: <2024110553-CVE-2024-50102-3d1e@gregkh>
Date: Tue,  5 Nov 2024 18:10:51 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-50102: x86: fix user address masking non-canonical speculation issue

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

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

x86: fix user address masking non-canonical speculation issue

It turns out that AMD has a "Meltdown Lite(tm)" issue with non-canonical
accesses in kernel space.  And so using just the high bit to decide
whether an access is in user space or kernel space ends up with the good
old "leak speculative data" if you have the right gadget using the
result:

  CVE-2020-12965 “Transient Execution of Non-Canonical Accesses“

Now, the kernel surrounds the access with a STAC/CLAC pair, and those
instructions end up serializing execution on older Zen architectures,
which closes the speculation window.

But that was true only up until Zen 5, which renames the AC bit [1].
That improves performance of STAC/CLAC a lot, but also means that the
speculation window is now open.

Note that this affects not just the new address masking, but also the
regular valid_user_address() check used by access_ok(), and the asm
version of the sign bit check in the get_user() helpers.

It does not affect put_user() or clear_user() variants, since there's no
speculative result to be used in a gadget for those operations.

The Linux kernel CVE team has assigned CVE-2024-50102 to this issue.

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

	Issue introduced in 6.4 with commit 6014bc27561f and fixed in 6.11.6 with commit 291313693677
	Issue introduced in 6.4 with commit 6014bc27561f and fixed in 6.12-rc5 with commit 86e6b1547b3d

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-2024-50102
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/x86/include/asm/uaccess_64.h
	arch/x86/kernel/cpu/common.c
	arch/x86/kernel/vmlinux.lds.S
	arch/x86/lib/getuser.S

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/291313693677a345d4f50aae3c68e28b469f601e
	https://git.kernel.org/stable/c/86e6b1547b3d013bc392adf775b89318441403c2