[STABLE 6.6.y] [PATCH] KVM: arm64: Fix softirq masking in FPSIMD register saving sequence

[Ard Biesheuvel]

On Fri, 3 Oct 2025 at 20:41, Will Deacon <will at kernel.org> wrote:
>
> Stable commit 28b82be094e2 ("KVM: arm64: Fix kernel BUG() due to bad
> backport of FPSIMD/SVE/SME fix") fixed a kernel BUG() caused by a bad
> backport of upstream commit fbc7e61195e2 ("KVM: arm64: Unconditionally
> save+flush host FPSIMD/SVE/SME state") by ensuring that softirqs are
> disabled/enabled across the fpsimd register save operation.
>
> Unfortunately, although this fixes the original issue, it can now lead
> to deadlock when re-enabling softirqs causes pending softirqs to be
> handled with locks already held:
>
>  | BUG: spinlock recursion on CPU#7, CPU 3/KVM/57616
>  |  lock: 0xffff3045ef850240, .magic: dead4ead, .owner: CPU 3/KVM/57616, .owner_cpu: 7
>  | CPU: 7 PID: 57616 Comm: CPU 3/KVM Tainted: G           O       6.1.152 #1
>  | Hardware name: SoftIron SoftIron Platform Mainboard/SoftIron Platform Mainboard, BIOS 1.31 May 11 2023
>  | Call trace:
>  |  dump_backtrace+0xe4/0x110
>  |  show_stack+0x20/0x30
>  |  dump_stack_lvl+0x6c/0x88
>  |  dump_stack+0x18/0x34
>  |  spin_dump+0x98/0xac
>  |  do_raw_spin_lock+0x70/0x128
>  |  _raw_spin_lock+0x18/0x28
>  |  raw_spin_rq_lock_nested+0x18/0x28
>  |  update_blocked_averages+0x70/0x550
>  |  run_rebalance_domains+0x50/0x70
>  |  handle_softirqs+0x198/0x328
>  |  __do_softirq+0x1c/0x28
>  |  ____do_softirq+0x18/0x28
>  |  call_on_irq_stack+0x30/0x48
>  |  do_softirq_own_stack+0x24/0x30
>  |  do_softirq+0x74/0x90
>  |  __local_bh_enable_ip+0x64/0x80
>  |  fpsimd_save_and_flush_cpu_state+0x5c/0x68
>  |  kvm_arch_vcpu_put_fp+0x4c/0x88
>  |  kvm_arch_vcpu_put+0x28/0x88
>  |  kvm_sched_out+0x38/0x58
>  |  __schedule+0x55c/0x6c8
>  |  schedule+0x60/0xa8
>
> Take a tiny step towards the upstream fix in 9b19700e623f ("arm64:
> fpsimd: Drop unneeded 'busy' flag") by additionally disabling hardirqs
> while saving the fpsimd registers.
>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Cc: Lee Jones <lee at kernel.org>
> Cc: Sasha Levin <sashal at kernel.org>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> Cc: <stable at vger.kernel.org> # 6.6.y
> Fixes: 28b82be094e2 ("KVM: arm64: Fix kernel BUG() due to bad backport of FPSIMD/SVE/SME fix")
> Reported-by: Kenneth Van Alstyne <kvanals at kvanals.org>
> Link: https://lore.kernel.org/r/010001999bae0958-4d80d25d-8dda-4006-a6b9-798f3e774f6c-000000@email.amazonses.com
> Signed-off-by: Will Deacon <will at kernel.org>

Acked-by: Ard Biesheuvel <ardb at kernel.org>