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Message-ID: <20250520152938.21881-3-ebiggers@kernel.org>
Date: Tue, 20 May 2025 08:29:38 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
linux-crypto@...r.kernel.org,
linux-pm@...r.kernel.org,
Borislav Petkov <bp@...en8.de>,
Thomas Gleixner <tglx@...utronix.de>,
Ayush Jain <Ayush.Jain3@....com>,
Herbert Xu <herbert@...dor.apana.org.au>,
Ard Biesheuvel <ardb@...nel.org>
Subject: [PATCH v3 2/2] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining
From: Eric Biggers <ebiggers@...gle.com>
irq_fpu_usable() incorrectly returned true before the FPU is
initialized. The x86 CPU onlining code can call sha256() to checksum
AMD microcode images, before the FPU is initialized. Since sha256()
recently gained a kernel-mode FPU optimized code path, a crash occurred
in kernel_fpu_begin_mask() during hotplug CPU onlining.
(The crash did not occur during boot-time CPU onlining, since the
optimized sha256() code is not enabled until subsys_initcalls run.)
Fix this by making irq_fpu_usable() return false before fpu__init_cpu()
has run. To do this without adding any additional overhead to
irq_fpu_usable(), replace the existing per-CPU bool in_kernel_fpu with
kernel_fpu_allowed which tracks both initialization and usage rather
than just usage. The initial state is false; FPU initialization sets it
to true; kernel-mode FPU sections toggle it to false and then back to
true; and CPU offlining restores it to the initial state of false.
Fixes: 11d7956d526f ("crypto: x86/sha256 - implement library instead of shash")
Reported-by: Ayush Jain <Ayush.Jain3@....com>
Closes: https://lore.kernel.org/r/20250516112217.GBaCcf6Yoc6LkIIryP@fat_crate.local
Tested-by: Ayush Jain <Ayush.Jain3@....com>
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
arch/x86/include/asm/fpu/api.h | 1 +
arch/x86/kernel/fpu/core.c | 24 +++++++++++++++---------
arch/x86/kernel/fpu/init.c | 13 +++++++++++++
arch/x86/kernel/fpu/internal.h | 2 ++
arch/x86/kernel/smpboot.c | 6 ++++++
5 files changed, 37 insertions(+), 9 deletions(-)
diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index 8e6848f55dcdb..2983acd95f5de 100644
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -116,10 +116,11 @@ extern void fpu_reset_from_exception_fixup(void);
/* Boot, hotplug and resume */
extern void fpu__init_cpu(void);
extern void fpu__init_system(void);
extern void fpu__init_check_bugs(void);
extern void fpu__resume_cpu(void);
+extern void fpu__disable_cpu(void);
#ifdef CONFIG_MATH_EMULATION
extern void fpstate_init_soft(struct swregs_state *soft);
#else
static inline void fpstate_init_soft(struct swregs_state *soft) {}
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 6495259a23962..ea138583dd92a 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -42,12 +42,15 @@ struct fpu_state_config fpu_user_cfg __ro_after_init;
* Represents the initial FPU state. It's mostly (but not completely) zeroes,
* depending on the FPU hardware format:
*/
struct fpstate init_fpstate __ro_after_init;
-/* Track in-kernel FPU usage */
-static DEFINE_PER_CPU(bool, kernel_fpu_allowed) = true;
+/*
+ * Track FPU initialization and kernel-mode usage. 'true' means the FPU is
+ * initialized and is not currently being used by the kernel:
+ */
+DEFINE_PER_CPU(bool, kernel_fpu_allowed);
/*
* Track which context is using the FPU on the CPU:
*/
DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
@@ -70,19 +73,22 @@ bool irq_fpu_usable(void)
{
if (WARN_ON_ONCE(in_nmi()))
return false;
/*
- * In kernel FPU usage already active? This detects any explicitly
- * nested usage in task or softirq context, which is unsupported. It
- * also detects attempted usage in a hardirq that has interrupted a
- * kernel-mode FPU section.
+ * Return false in the following cases:
+ *
+ * - FPU is not yet initialized. This can happen only when the call is
+ * coming from CPU onlining, for example for microcode checksumming.
+ * - The kernel is already using the FPU, either because of explicit
+ * nesting (which should never be done), or because of implicit
+ * nesting when a hardirq interrupted a kernel-mode FPU section.
+ *
+ * The single boolean check below handles both cases:
*/
- if (!this_cpu_read(kernel_fpu_allowed)) {
- WARN_ON_FPU(!in_hardirq());
+ if (!this_cpu_read(kernel_fpu_allowed))
return false;
- }
/*
* When not in NMI or hard interrupt context, FPU can be used in:
*
* - Task context except from within fpregs_lock()'ed critical
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 6bb3e35c40e24..c581a3e452dfd 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -49,10 +49,23 @@ static void fpu__init_cpu_generic(void)
*/
void fpu__init_cpu(void)
{
fpu__init_cpu_generic();
fpu__init_cpu_xstate();
+
+ /* Start allowing kernel-mode FPU: */
+ WARN_ON_FPU(this_cpu_read(kernel_fpu_allowed));
+ this_cpu_write(kernel_fpu_allowed, true);
+}
+
+/*
+ * Stop allowing kernel-mode FPU. Called when a CPU is brought offline:
+ */
+void fpu__disable_cpu(void)
+{
+ WARN_ON_FPU(!this_cpu_read(kernel_fpu_allowed));
+ this_cpu_write(kernel_fpu_allowed, false);
}
static bool __init fpu__probe_without_cpuid(void)
{
unsigned long cr0;
diff --git a/arch/x86/kernel/fpu/internal.h b/arch/x86/kernel/fpu/internal.h
index 975de070c9c98..9782152d609c7 100644
--- a/arch/x86/kernel/fpu/internal.h
+++ b/arch/x86/kernel/fpu/internal.h
@@ -2,10 +2,12 @@
#ifndef __X86_KERNEL_FPU_INTERNAL_H
#define __X86_KERNEL_FPU_INTERNAL_H
extern struct fpstate init_fpstate;
+DECLARE_PER_CPU(bool, kernel_fpu_allowed);
+
/* CPU feature check wrappers */
static __always_inline __pure bool use_xsave(void)
{
return cpu_feature_enabled(X86_FEATURE_XSAVE);
}
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index d7d61b3de2bf6..cf42a7632dd49 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1186,10 +1186,16 @@ void cpu_disable_common(void)
{
int cpu = smp_processor_id();
remove_siblinginfo(cpu);
+ /*
+ * Stop allowing kernel-mode FPU. This is needed so that if the CPU is
+ * brought online again, the initial state is not allowed:
+ */
+ fpu__disable_cpu();
+
/* It's now safe to remove this processor from the online map */
lock_vector_lock();
remove_cpu_from_maps(cpu);
unlock_vector_lock();
fixup_irqs();
--
2.49.0
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