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x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

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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: 11d7956d52 ("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>
This commit is contained in:
Eric Biggers 2025-05-18 12:32:12 -07:00 committed by Herbert Xu
parent 61fc01f8f7
commit 2297554f01
4 changed files with 31 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
arch/x86/include/asm/fpu/api.h
View File

@ -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 */

34
arch/x86/kernel/fpu/core.c
View File

@ -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();
}

3
arch/x86/kernel/fpu/init.c
View File

@ -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)

6
arch/x86/kernel/smpboot.c
View File

@ -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);