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 <Ayush.Jain3@amd.com>
Closes: https://lore.kernel.org/r/20250516112217.GBaCcf6Yoc6LkIIryP@fat_crate.local
Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Ayush Jain <Ayush.Jain3@amd.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 6 insertions, 0 deletions

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);