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KVM: arm64: Simplify np-guest hypercalls

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When the handling of a guest stage-2 permission fault races with an MMU
notifier, the faulting page might be gone from the guest's stage-2 by
the point we attempt to call (p)kvm_pgtable_stage2_relax_perms(). In the
normal KVM case, this leads to returning -EAGAIN which user_mem_abort()
handles correctly by simply re-entering the guest. However, the pKVM
hypercall implementation has additional logic to check the page state
using __check_host_shared_guest() which gets confused with absence of a
page mapped at the requested IPA and returns -ENOENT, hence breaking
user_mem_abort() and hilarity ensues.

Luckily, several of the hypercalls for managing the stage-2 page-table
of NP guests have no effect on the pKVM ownership tracking (wrprotect,
test_clear_young, mkyoung, and crucially relax_perms), so the extra
state checking logic is in fact not strictly necessary. So, to fix the
discrepancy between standard KVM and pKVM, let's just drop the
superfluous __check_host_shared_guest() logic from those hypercalls and
make the extra state checking a debug assertion dependent on
CONFIG_NVHE_EL2_DEBUG as we already do for other transitions.

Signed-off-by: Quentin Perret <qperret@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250207145438.1333475-3-qperret@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
This commit is contained in:
Quentin Perret 2025-02-07 14:54:38 +00:00 • committed by Marc Zyngier
parent c53fbdb60f
commit eabc7aaef7
1 changed files with 38 additions and 31 deletions
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69
arch/arm64/kvm/hyp/nvhe/mem_protect.c
View file

@ -998,44 +998,57 @@ unlock:
return ret;
}
int __pkvm_host_relax_perms_guest(u64 gfn, struct pkvm_hyp_vcpu *vcpu, enum kvm_pgtable_prot prot)
static void assert_host_shared_guest(struct pkvm_hyp_vm *vm, u64 ipa)
{
struct pkvm_hyp_vm *vm = pkvm_hyp_vcpu_to_hyp_vm(vcpu);
u64 ipa = hyp_pfn_to_phys(gfn);
u64 phys;
int ret;
if (prot & ~KVM_PGTABLE_PROT_RWX)
return -EINVAL;
if (!IS_ENABLED(CONFIG_NVHE_EL2_DEBUG))
return;
host_lock_component();
guest_lock_component(vm);
ret = __check_host_shared_guest(vm, &phys, ipa);
if (!ret)
ret = kvm_pgtable_stage2_relax_perms(&vm->pgt, ipa, prot, 0);
guest_unlock_component(vm);
host_unlock_component();
WARN_ON(ret && ret != -ENOENT);
}
int __pkvm_host_relax_perms_guest(u64 gfn, struct pkvm_hyp_vcpu *vcpu, enum kvm_pgtable_prot prot)
{
struct pkvm_hyp_vm *vm = pkvm_hyp_vcpu_to_hyp_vm(vcpu);
u64 ipa = hyp_pfn_to_phys(gfn);
int ret;
if (pkvm_hyp_vm_is_protected(vm))
return -EPERM;
if (prot & ~KVM_PGTABLE_PROT_RWX)
return -EINVAL;
assert_host_shared_guest(vm, ipa);
guest_lock_component(vm);
ret = kvm_pgtable_stage2_relax_perms(&vm->pgt, ipa, prot, 0);
guest_unlock_component(vm);
return ret;
}
int __pkvm_host_wrprotect_guest(u64 gfn, struct pkvm_hyp_vm *vm)
{
u64 ipa = hyp_pfn_to_phys(gfn);
u64 phys;
int ret;
host_lock_component();
if (pkvm_hyp_vm_is_protected(vm))
return -EPERM;
assert_host_shared_guest(vm, ipa);
guest_lock_component(vm);
ret = __check_host_shared_guest(vm, &phys, ipa);
if (!ret)
ret = kvm_pgtable_stage2_wrprotect(&vm->pgt, ipa, PAGE_SIZE);
ret = kvm_pgtable_stage2_wrprotect(&vm->pgt, ipa, PAGE_SIZE);
guest_unlock_component(vm);
host_unlock_component();
return ret;
}
@ -1043,18 +1056,15 @@ int __pkvm_host_wrprotect_guest(u64 gfn, struct pkvm_hyp_vm *vm)
int __pkvm_host_test_clear_young_guest(u64 gfn, bool mkold, struct pkvm_hyp_vm *vm)
{
u64 ipa = hyp_pfn_to_phys(gfn);
u64 phys;
int ret;
host_lock_component();
if (pkvm_hyp_vm_is_protected(vm))
return -EPERM;
assert_host_shared_guest(vm, ipa);
guest_lock_component(vm);
ret = __check_host_shared_guest(vm, &phys, ipa);
if (!ret)
ret = kvm_pgtable_stage2_test_clear_young(&vm->pgt, ipa, PAGE_SIZE, mkold);
ret = kvm_pgtable_stage2_test_clear_young(&vm->pgt, ipa, PAGE_SIZE, mkold);
guest_unlock_component(vm);
host_unlock_component();
return ret;
}
@ -1063,18 +1073,15 @@ int __pkvm_host_mkyoung_guest(u64 gfn, struct pkvm_hyp_vcpu *vcpu)
{
struct pkvm_hyp_vm *vm = pkvm_hyp_vcpu_to_hyp_vm(vcpu);
u64 ipa = hyp_pfn_to_phys(gfn);
u64 phys;
int ret;
host_lock_component();
if (pkvm_hyp_vm_is_protected(vm))
return -EPERM;
assert_host_shared_guest(vm, ipa);
guest_lock_component(vm);
ret = __check_host_shared_guest(vm, &phys, ipa);
if (!ret)
kvm_pgtable_stage2_mkyoung(&vm->pgt, ipa, 0);
kvm_pgtable_stage2_mkyoung(&vm->pgt, ipa, 0);
guest_unlock_component(vm);
host_unlock_component();
return ret;
}