linux-cve-announce - CVE-2024-56761: x86/fred: Clear WFE in missing-ENDBRANCH #CPs

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Message-ID: <2025010654-CVE-2024-56761-0be0@gregkh>
Date: Mon,  6 Jan 2025 17:20:55 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-56761: x86/fred: Clear WFE in missing-ENDBRANCH #CPs

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

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

x86/fred: Clear WFE in missing-ENDBRANCH #CPs

An indirect branch instruction sets the CPU indirect branch tracker
(IBT) into WAIT_FOR_ENDBRANCH (WFE) state and WFE stays asserted
across the instruction boundary.  When the decoder finds an
inappropriate instruction while WFE is set ENDBR, the CPU raises a #CP
fault.

For the "kernel IBT no ENDBR" selftest where #CPs are deliberately
triggered, the WFE state of the interrupted context needs to be
cleared to let execution continue.  Otherwise when the CPU resumes
from the instruction that just caused the previous #CP, another
missing-ENDBRANCH #CP is raised and the CPU enters a dead loop.

This is not a problem with IDT because it doesn't preserve WFE and
IRET doesn't set WFE.  But FRED provides space on the entry stack
(in an expanded CS area) to save and restore the WFE state, thus the
WFE state is no longer clobbered, so software must clear it.

Clear WFE to avoid dead looping in ibt_clear_fred_wfe() and the
!ibt_fatal code path when execution is allowed to continue.

Clobbering WFE in any other circumstance is a security-relevant bug.

[ dhansen: changelog rewording ]

The Linux kernel CVE team has assigned CVE-2024-56761 to this issue.

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

	Issue introduced in 6.6 with commit a5f6c2ace9974adf92ce65dacca8126d90adabfe and fixed in 6.12.8 with commit b939f108e86b76119428a6fa4e92491e09ac7867
	Issue introduced in 6.6 with commit a5f6c2ace9974adf92ce65dacca8126d90adabfe and fixed in 6.13-rc5 with commit dc81e556f2a017d681251ace21bf06c126d5a192

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-2024-56761
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/kernel/cet.c

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/b939f108e86b76119428a6fa4e92491e09ac7867
	https://git.kernel.org/stable/c/dc81e556f2a017d681251ace21bf06c126d5a192