Re: [PATCH] KVM: arm64: Fix debug checking for np-guests using huge mappings

[Vincent Donnefort <vdonnefort@google.com>]

Thanks for the fix!

On Fri, Aug 15, 2025 at 05:26:55PM +0100, Ben Horgan wrote:
> When running with transparent huge pages and CONFIG_NVHE_EL2_DEBUG then
> the debug checking in assert_host_shared_guest() fails on the launch of an
> np-guest. This WARN_ON() causes a panic and generates the stack below.
> 
> In __pkvm_host_relax_perms_guest() the debug checking assumes the mapping
> is a single page but it may be a block map. Update the checking so that
> the size is not checked and just assumes the correct size.
> 
> While we're here make the same fix in __pkvm_host_mkyoung_guest().
> 
>   Info: # lkvm run -k /share/arch/arm64/boot/Image -m 704 -c 8 --name guest-128
>   Info: Removed ghost socket file "/.lkvm//guest-128.sock".
> [ 1406.521757] kvm [141]: nVHE hyp BUG at: arch/arm64/kvm/hyp/nvhe/mem_protect.c:1088!
> [ 1406.521804] kvm [141]: nVHE call trace:
> [ 1406.521828] kvm [141]:  [<ffff8000811676b4>] __kvm_nvhe_hyp_panic+0xb4/0xe8
> [ 1406.521946] kvm [141]:  [<ffff80008116d12c>] __kvm_nvhe_assert_host_shared_guest+0xb0/0x10c
> [ 1406.522049] kvm [141]:  [<ffff80008116f068>] __kvm_nvhe___pkvm_host_relax_perms_guest+0x48/0x104
> [ 1406.522157] kvm [141]:  [<ffff800081169df8>] __kvm_nvhe_handle___pkvm_host_relax_perms_guest+0x64/0x7c
> [ 1406.522250] kvm [141]:  [<ffff800081169f0c>] __kvm_nvhe_handle_trap+0x8c/0x1a8
> [ 1406.522333] kvm [141]:  [<ffff8000811680fc>] __kvm_nvhe___skip_pauth_save+0x4/0x4
> [ 1406.522454] kvm [141]: ---[ end nVHE call trace ]---
> [ 1406.522477] kvm [141]: Hyp Offset: 0xfffece8013600000
> [ 1406.522554] Kernel panic - not syncing: HYP panic:
> [ 1406.522554] PS:834003c9 PC:0000b1806db6d170 ESR:00000000f2000800
> [ 1406.522554] FAR:ffff8000804be420 HPFAR:0000000000804be0 PAR:0000000000000000
> [ 1406.522554] VCPU:0000000000000000
> [ 1406.523337] CPU: 3 UID: 0 PID: 141 Comm: kvm-vcpu-0 Not tainted 6.16.0-rc7 #97 PREEMPT
> [ 1406.523485] Hardware name: FVP Base RevC (DT)
> [ 1406.523566] Call trace:
> [ 1406.523629]  show_stack+0x18/0x24 (C)
> [ 1406.523753]  dump_stack_lvl+0xd4/0x108
> [ 1406.523899]  dump_stack+0x18/0x24
> [ 1406.524040]  panic+0x3d8/0x448
> [ 1406.524184]  nvhe_hyp_panic_handler+0x10c/0x23c
> [ 1406.524325]  kvm_handle_guest_abort+0x68c/0x109c
> [ 1406.524500]  handle_exit+0x60/0x17c
> [ 1406.524630]  kvm_arch_vcpu_ioctl_run+0x2e0/0x8c0
> [ 1406.524794]  kvm_vcpu_ioctl+0x1a8/0x9cc
> [ 1406.524919]  __arm64_sys_ioctl+0xac/0x104
> [ 1406.525067]  invoke_syscall+0x48/0x10c
> [ 1406.525189]  el0_svc_common.constprop.0+0x40/0xe0
> [ 1406.525322]  do_el0_svc+0x1c/0x28
> [ 1406.525441]  el0_svc+0x38/0x120
> [ 1406.525588]  el0t_64_sync_handler+0x10c/0x138
> [ 1406.525750]  el0t_64_sync+0x1ac/0x1b0
> [ 1406.525876] SMP: stopping secondary CPUs
> [ 1406.525965] Kernel Offset: disabled
> [ 1406.526032] CPU features: 0x0000,00000080,8e134ca1,9446773f
> [ 1406.526130] Memory Limit: none
> [ 1406.959099] ---[ end Kernel panic - not syncing: HYP panic:
> [ 1406.959099] PS:834003c9 PC:0000b1806db6d170 ESR:00000000f2000800
> [ 1406.959099] FAR:ffff8000804be420 HPFAR:0000000000804be0 PAR:0000000000000000
> [ 1406.959099] VCPU:0000000000000000 ]
> 
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> Fixes: db14091d8f75 ("KVM: arm64: Stage-2 huge mappings for np-guests")

Not sure if it really matters but it's more about fixing f28f1d02f4ea (KVM: arm64: Add a range
to __pkvm_host_unshare_guest()) which introduced the check size !=
kvm_granule_size(). Even though this is noop until db14091d8f75

> Cc: Vincent Donnefort <vdonnefort@google.com>
> Cc: Quentin Perret <qperret@google.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: stable@vger.kernel.org

Reviewed-by: Vincent Donnefort <vdonnefort@google.com> 

> 
> ---
> 
> This addresses the bug I raised here:
> https://lore.kernel.org/linux-arm-kernel/17b526ff-b824-4c24-8ac0-6821e5cc8900@arm.com/
> 
> ---
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index 8957734d6183..ddc8beb55eee 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -1010,9 +1010,12 @@ static int __check_host_shared_guest(struct pkvm_hyp_vm *vm, u64 *__phys, u64 ip
>  		return ret;
>  	if (!kvm_pte_valid(pte))
>  		return -ENOENT;
> -	if (kvm_granule_size(level) != size)
> +	if (size && kvm_granule_size(level) != size)
>  		return -E2BIG;
>  
> +	if (!size)
> +		size = kvm_granule_size(level);
> +
>  	state = guest_get_page_state(pte, ipa);
>  	if (state != PKVM_PAGE_SHARED_BORROWED)
>  		return -EPERM;
> @@ -1100,7 +1103,7 @@ int __pkvm_host_relax_perms_guest(u64 gfn, struct pkvm_hyp_vcpu *vcpu, enum kvm_
>  	if (prot & ~KVM_PGTABLE_PROT_RWX)
>  		return -EINVAL;
>  
> -	assert_host_shared_guest(vm, ipa, PAGE_SIZE);
> +	assert_host_shared_guest(vm, ipa, 0);
>  	guest_lock_component(vm);
>  	ret = kvm_pgtable_stage2_relax_perms(&vm->pgt, ipa, prot, 0);
>  	guest_unlock_component(vm);
> @@ -1156,7 +1159,7 @@ int __pkvm_host_mkyoung_guest(u64 gfn, struct pkvm_hyp_vcpu *vcpu)
>  	if (pkvm_hyp_vm_is_protected(vm))
>  		return -EPERM;
>  
> -	assert_host_shared_guest(vm, ipa, PAGE_SIZE);
> +	assert_host_shared_guest(vm, ipa, 0);
>  	guest_lock_component(vm);
>  	kvm_pgtable_stage2_mkyoung(&vm->pgt, ipa, 0);
>  	guest_unlock_component(vm);
> -- 
> 2.43.0
>