From: Eric Biggers Date: Sun, 18 May 2025 19:32:12 +0000 (-0700) Subject: x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining X-Git-Tag: block-6.16-20250606~34^2 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=2297554f01df6d3d4e98a3915c183ce3e491740a;p=linux-block.git x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining 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 Closes: https://lore.kernel.org/r/20250516112217.GBaCcf6Yoc6LkIIryP@fat_crate.local Signed-off-by: Eric Biggers Tested-by: Ayush Jain Reviewed-by: Thomas Gleixner Signed-off-by: Herbert Xu --- diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h index f42de5f05e7e..3ebeee2644de 100644 --- a/arch/x86/include/asm/fpu/api.h +++ b/arch/x86/include/asm/fpu/api.h @@ -126,6 +126,7 @@ 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 */ diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c index 91d6341f281f..399f43aa78d5 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/arch/x86/kernel/fpu/core.c @@ -43,8 +43,11 @@ struct fpu_state_config fpu_user_cfg __ro_after_init; */ 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: @@ -61,15 +64,18 @@ bool irq_fpu_usable(void) 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: @@ -431,9 +437,10 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask) 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)) { @@ -453,9 +460,10 @@ 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(); } diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c index 998a08f17e33..1975c37c3668 100644 --- a/arch/x86/kernel/fpu/init.c +++ b/arch/x86/kernel/fpu/init.c @@ -51,6 +51,9 @@ 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) diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index d6cf1e23c2a3..2901f5cfd825 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1188,6 +1188,12 @@ void cpu_disable_common(void) 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);