Re: [STABLE 5.15.y] [PATCH] KVM: arm64: Fix softirq masking in FPSIMD register saving sequence

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Oct 03, 2025 at 07:43:19PM +0100, Will Deacon wrote:
> On Fri, Oct 03, 2025 at 07:39:17PM +0100, Will Deacon wrote:
> > Stable commit 23249dade24e ("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@kernel.org>
> > Cc: Lee Jones <lee@kernel.org>
> > Cc: Sasha Levin <sashal@kernel.org>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org
> 
> Sorry, Greg, I lost a stray '>' here ^^^  and so you didn't end up on CC
> for this one.

No worries, I see the stuff going to stable@vger.kernel.org as well :)

Now all queued up, thanks for these.

greg k-h