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Message-ID: <3c612c2f-026e-4bc2-bbb4-3e9097acc6f5@amd.com>
Date: Mon, 19 May 2025 09:48:35 +0530
From: "Jain, Ayush" <ayushjai@....com>
To: Eric Biggers <ebiggers@...nel.org>, "x86@...nel.org" <x86@...nel.org>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>,
"Jain, Ayush" <Ayush.Jain3@....com>, Herbert Xu
<herbert@...dor.apana.org.au>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH] x86/fpu: Fix irq_fpu_usable() to return false during CPU
onlining
Hello Eric,
I have tested this on AMD EPYC 8534P 64-Core Processor
and this patch fixes reported issue.
Tested-by: Ayush Jain <Ayush.Jain3@....com>
Thanks,
Ayush
On 5/19/2025 1:02 AM, Eric Biggers wrote:
> 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
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
> arch/x86/include/asm/fpu/api.h | 1 +
> arch/x86/kernel/fpu/core.c | 34 +++++++++++++++++++++-------------
> arch/x86/kernel/fpu/init.c | 3 +++
> arch/x86/kernel/smpboot.c | 6 ++++++
> 4 files changed, 31 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
> index 8e6848f55dcdb..cd6f194a912bf 100644
> --- a/arch/x86/include/asm/fpu/api.h
> +++ b/arch/x86/include/asm/fpu/api.h
> @@ -124,10 +124,11 @@ extern void fpstate_init_soft(struct swregs_state *soft);
> #else
> static inline void fpstate_init_soft(struct swregs_state *soft) {}
> #endif
>
> /* State tracking */
> +DECLARE_PER_CPU(bool, kernel_fpu_allowed);
> DECLARE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
>
> /* Process cleanup */
> #ifdef CONFIG_X86_64
> extern void fpstate_free(struct fpu *fpu);
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 948b4f5fad99c..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, in_kernel_fpu);
> +/*
> + * 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(in_kernel_fpu)) {
> - 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
> @@ -437,13 +443,14 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
> {
> if (!irqs_disabled())
> fpregs_lock();
>
> WARN_ON_FPU(!irq_fpu_usable());
> - WARN_ON_FPU(this_cpu_read(in_kernel_fpu));
>
> - this_cpu_write(in_kernel_fpu, true);
> + /* Toggle kernel_fpu_allowed to false: */
> + WARN_ON_FPU(!this_cpu_read(kernel_fpu_allowed));
> + this_cpu_write(kernel_fpu_allowed, false);
>
> if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
> !test_thread_flag(TIF_NEED_FPU_LOAD)) {
> set_thread_flag(TIF_NEED_FPU_LOAD);
> save_fpregs_to_fpstate(x86_task_fpu(current));
> @@ -459,13 +466,14 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
> }
> EXPORT_SYMBOL_GPL(kernel_fpu_begin_mask);
>
> void kernel_fpu_end(void)
> {
> - WARN_ON_FPU(!this_cpu_read(in_kernel_fpu));
> + /* Toggle kernel_fpu_allowed back to true: */
> + WARN_ON_FPU(this_cpu_read(kernel_fpu_allowed));
> + this_cpu_write(kernel_fpu_allowed, true);
>
> - this_cpu_write(in_kernel_fpu, false);
> if (!irqs_disabled())
> fpregs_unlock();
> }
> EXPORT_SYMBOL_GPL(kernel_fpu_end);
>
> diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
> index 6bb3e35c40e24..99db41bf9fa6b 100644
> --- a/arch/x86/kernel/fpu/init.c
> +++ b/arch/x86/kernel/fpu/init.c
> @@ -49,10 +49,13 @@ 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: */
> + this_cpu_write(kernel_fpu_allowed, true);
> }
>
> static bool __init fpu__probe_without_cpuid(void)
> {
> unsigned long cr0;
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index e266c4edea17e..58ede3fa6a75b 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:
> + */
> + this_cpu_write(kernel_fpu_allowed, false);
> +
> /* 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();
>
> base-commit: 8566fc3b96539e3235909d6bdda198e1282beaed
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