Re: [PATCH] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

[Thomas Gleixner <tglx@linutronix.de>]

On Sun, May 18 2025 at 12:32, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.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@amd.com>
> Closes: https://lore.kernel.org/r/20250516112217.GBaCcf6Yoc6LkIIryP@fat_crate.local
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>