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 Cc: stable@vger.kernel.org # 5.15.y Fixes: 23249dade24e ("KVM: arm64: Fix kernel BUG() due to bad backport of FPSIMD/SVE/SME fix") Reported-by: Kenneth Van Alstyne kvanals@kvanals.org Link: https://lore.kernel.org/r/010001999bae0958-4d80d25d-8dda-4006-a6b9-798f3e774... Signed-off-by: Will Deacon will@kernel.org --- arch/arm64/kernel/fpsimd.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index fc51cdd5aaa7..db1ed940a6dc 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -1300,13 +1300,17 @@ static void fpsimd_flush_cpu_state(void) */ void fpsimd_save_and_flush_cpu_state(void) { + unsigned long flags; + if (!system_supports_fpsimd()) return; WARN_ON(preemptible()); - get_cpu_fpsimd_context(); + local_irq_save(flags); + __get_cpu_fpsimd_context(); fpsimd_save(); fpsimd_flush_cpu_state(); - put_cpu_fpsimd_context(); + __put_cpu_fpsimd_context(); + local_irq_restore(flags); }

#ifdef CONFIG_KERNEL_MODE_NEON