Re: [PATCH v3 2/2] KVM: arm64: Don't acquire RCU read lock for exclusive table walks

[Will Deacon]

On Wed, Nov 16, 2022 at 04:56:55PM +0000, Oliver Upton wrote:
> Marek reported a BUG resulting from the recent parallel faults changes,
> as the hyp stage-1 map walker attempted to allocate table memory while
> holding the RCU read lock:
> 
>   BUG: sleeping function called from invalid context at
>   include/linux/sched/mm.h:274
>   in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
>   preempt_count: 0, expected: 0
>   RCU nest depth: 1, expected: 0
>   2 locks held by swapper/0/1:
>     #0: ffff80000a8a44d0 (kvm_hyp_pgd_mutex){+.+.}-{3:3}, at:
>   __create_hyp_mappings+0x80/0xc4
>     #1: ffff80000a927720 (rcu_read_lock){....}-{1:2}, at:
>   kvm_pgtable_walk+0x0/0x1f4
>   CPU: 2 PID: 1 Comm: swapper/0 Not tainted 6.1.0-rc3+ #5918
>   Hardware name: Raspberry Pi 3 Model B (DT)
>   Call trace:
>     dump_backtrace.part.0+0xe4/0xf0
>     show_stack+0x18/0x40
>     dump_stack_lvl+0x8c/0xb8
>     dump_stack+0x18/0x34
>     __might_resched+0x178/0x220
>     __might_sleep+0x48/0xa0
>     prepare_alloc_pages+0x178/0x1a0
>     __alloc_pages+0x9c/0x109c
>     alloc_page_interleave+0x1c/0xc4
>     alloc_pages+0xec/0x160
>     get_zeroed_page+0x1c/0x44
>     kvm_hyp_zalloc_page+0x14/0x20
>     hyp_map_walker+0xd4/0x134
>     kvm_pgtable_visitor_cb.isra.0+0x38/0x5c
>     __kvm_pgtable_walk+0x1a4/0x220
>     kvm_pgtable_walk+0x104/0x1f4
>     kvm_pgtable_hyp_map+0x80/0xc4
>     __create_hyp_mappings+0x9c/0xc4
>     kvm_mmu_init+0x144/0x1cc
>     kvm_arch_init+0xe4/0xef4
>     kvm_init+0x3c/0x3d0
>     arm_init+0x20/0x30
>     do_one_initcall+0x74/0x400
>     kernel_init_freeable+0x2e0/0x350
>     kernel_init+0x24/0x130
>     ret_from_fork+0x10/0x20
> 
> Since the hyp stage-1 table walkers are serialized by kvm_hyp_pgd_mutex,
> RCU protection really doesn't add anything. Don't acquire the RCU read
> lock for an exclusive walk. While at it, add a warning which codifies
> the lack of support for shared walks in the hypervisor code.
> 
> Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h | 22 ++++++++++++++++------
>  arch/arm64/kvm/hyp/pgtable.c         |  4 ++--
>  2 files changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index f23af693e3c5..a07fc5e35a8c 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -229,8 +229,16 @@ static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walke
>  	return pteref;
>  }
>  
> -static inline void kvm_pgtable_walk_begin(void) {}
> -static inline void kvm_pgtable_walk_end(void) {}
> +static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
> +{
> +	/*
> +	 * Due to the lack of RCU (or a similar protection scheme), only
> +	 * non-shared table walkers are allowed in the hypervisor.
> +	 */
> +	WARN_ON(walker->flags & KVM_PGTABLE_WALK_SHARED);
> +}

I think it would be better to propagate the error to the caller rather
than WARN here. Since you're rejigging things anyway, can you have this
function return int?

Will