linux-cve-announce - CVE-2025-21632: x86/fpu: Ensure shadow stack is active before "getting" registers

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Message-ID: <2025011942-CVE-2025-21632-9fde@gregkh>
Date: Sun, 19 Jan 2025 11:17:41 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2025-21632: x86/fpu: Ensure shadow stack is active before "getting" registers

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

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

x86/fpu: Ensure shadow stack is active before "getting" registers

The x86 shadow stack support has its own set of registers. Those registers
are XSAVE-managed, but they are "supervisor state components" which means
that userspace can not touch them with XSAVE/XRSTOR.  It also means that
they are not accessible from the existing ptrace ABI for XSAVE state.
Thus, there is a new ptrace get/set interface for it.

The regset code that ptrace uses provides an ->active() handler in
addition to the get/set ones. For shadow stack this ->active() handler
verifies that shadow stack is enabled via the ARCH_SHSTK_SHSTK bit in the
thread struct. The ->active() handler is checked from some call sites of
the regset get/set handlers, but not the ptrace ones. This was not
understood when shadow stack support was put in place.

As a result, both the set/get handlers can be called with
XFEATURE_CET_USER in its init state, which would cause get_xsave_addr() to
return NULL and trigger a WARN_ON(). The ssp_set() handler luckily has an
ssp_active() check to avoid surprising the kernel with shadow stack
behavior when the kernel is not ready for it (ARCH_SHSTK_SHSTK==0). That
check just happened to avoid the warning.

But the ->get() side wasn't so lucky. It can be called with shadow stacks
disabled, triggering the warning in practice, as reported by Christina
Schimpe:

WARNING: CPU: 5 PID: 1773 at arch/x86/kernel/fpu/regset.c:198 ssp_get+0x89/0xa0
[...]
Call Trace:
<TASK>
? show_regs+0x6e/0x80
? ssp_get+0x89/0xa0
? __warn+0x91/0x150
? ssp_get+0x89/0xa0
? report_bug+0x19d/0x1b0
? handle_bug+0x46/0x80
? exc_invalid_op+0x1d/0x80
? asm_exc_invalid_op+0x1f/0x30
? __pfx_ssp_get+0x10/0x10
? ssp_get+0x89/0xa0
? ssp_get+0x52/0xa0
__regset_get+0xad/0xf0
copy_regset_to_user+0x52/0xc0
ptrace_regset+0x119/0x140
ptrace_request+0x13c/0x850
? wait_task_inactive+0x142/0x1d0
? do_syscall_64+0x6d/0x90
arch_ptrace+0x102/0x300
[...]

Ensure that shadow stacks are active in a thread before looking them up
in the XSAVE buffer. Since ARCH_SHSTK_SHSTK and user_ssp[SHSTK_EN] are
set at the same time, the active check ensures that there will be
something to find in the XSAVE buffer.

[ dhansen: changelog/subject tweaks ]

The Linux kernel CVE team has assigned CVE-2025-21632 to this issue.

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

	Issue introduced in 6.6 with commit 2fab02b25ae7cf5f714ab456b03d9a3fe5ae98c9 and fixed in 6.6.72 with commit 0a3a872214188e4268d31581ed0cd44508e038cf
	Issue introduced in 6.6 with commit 2fab02b25ae7cf5f714ab456b03d9a3fe5ae98c9 and fixed in 6.12.10 with commit 6bfe1fc22f462bec87422cdcbec4d7a2f43ff01d
	Issue introduced in 6.6 with commit 2fab02b25ae7cf5f714ab456b03d9a3fe5ae98c9 and fixed in 6.13-rc7 with commit a9d9c33132d49329ada647e4514d210d15e31d81

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-2025-21632
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/fpu/regset.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/0a3a872214188e4268d31581ed0cd44508e038cf
	https://git.kernel.org/stable/c/6bfe1fc22f462bec87422cdcbec4d7a2f43ff01d
	https://git.kernel.org/stable/c/a9d9c33132d49329ada647e4514d210d15e31d81