[PATCH v3 8/9] KVM: arm/arm64: Preserve Exec permission across R/W permission faults

Marc Zyngier
2017-10-23T16:11:21Z

So far, we loose the Exec property whenever we take permission
faults, as we always reconstruct the PTE/PMD from scratch. This
can be counter productive as we can end-up with the following
fault sequence:

X -> RO -> ROX -> RW -> RWX

Instead, we can lookup the existing PTE/PMD and clear the XN bit in the
new entry if it was already cleared in the old one, leadig to a much
nicer fault sequence:

X -> ROX -> RWX

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
---
arch/arm/include/asm/kvm_mmu.h | 10 ++++++++++
arch/arm64/include/asm/kvm_mmu.h | 10 ++++++++++
virt/kvm/arm/mmu.c | 27 +++++++++++++++++++++++++++
3 files changed, 47 insertions(+)

diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
index 4d7a54cbb3ab..aab64fe52146 100644
--- a/arch/arm/include/asm/kvm_mmu.h
+++ b/arch/arm/include/asm/kvm_mmu.h
@@ -107,6 +107,11 @@ static inline bool kvm_s2pte_readonly(pte_t *pte)
return (pte_val(*pte) & L_PTE_S2_RDWR) == L_PTE_S2_RDONLY;
}

+static inline bool kvm_s2pte_exec(pte_t *pte)
+{
+ return !(pte_val(*pte) & L_PTE_XN);
+}
+
static inline void kvm_set_s2pmd_readonly(pmd_t *pmd)
{
pmd_val(*pmd) = (pmd_val(*pmd) & ~L_PMD_S2_RDWR) | L_PMD_S2_RDONLY;
@@ -117,6 +122,11 @@ static inline bool kvm_s2pmd_readonly(pmd_t *pmd)
return (pmd_val(*pmd) & L_PMD_S2_RDWR) == L_PMD_S2_RDONLY;
}

+static inline bool kvm_s2pmd_exec(pmd_t *pmd)
+{
+ return !(pmd_val(*pmd) & PMD_SECT_XN);
+}
+
static inline bool kvm_page_empty(void *ptr)
{
struct page *ptr_page = virt_to_page(ptr);
diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index 1e1b20cb348f..126abefffe7f 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -203,6 +203,11 @@ static inline bool kvm_s2pte_readonly(pte_t *pte)
return (pte_val(*pte) & PTE_S2_RDWR) == PTE_S2_RDONLY;
}

+static inline bool kvm_s2pte_exec(pte_t *pte)
+{
+ return !(pte_val(*pte) & PTE_S2_XN);
+}
+
static inline void kvm_set_s2pmd_readonly(pmd_t *pmd)
{
kvm_set_s2pte_readonly((pte_t *)pmd);
@@ -213,6 +218,11 @@ static inline bool kvm_s2pmd_readonly(pmd_t *pmd)
return kvm_s2pte_readonly((pte_t *)pmd);
}

+static inline bool kvm_s2pmd_exec(pmd_t *pmd)
+{
+ return !(pmd_val(*pmd) & PMD_S2_XN);
+}
+
static inline bool kvm_page_empty(void *ptr)
{
struct page *ptr_page = virt_to_page(ptr);
diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index f956efbd933d..b83b5a8442bb 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -926,6 +926,25 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
return 0;
}

+static bool stage2_is_exec(struct kvm *kvm, phys_addr_t addr)
+{
+ pmd_t *pmdp;
+ pte_t *ptep;
+
+ pmdp = stage2_get_pmd(kvm, NULL, addr);
+ if (!pmdp || pmd_none(*pmdp) || !pmd_present(*pmdp))
+ return false;
+
+ if (pmd_thp_or_huge(*pmdp))
+ return kvm_s2pmd_exec(pmdp);
+
+ ptep = pte_offset_kernel(pmdp, addr);
+ if (!ptep || pte_none(*ptep) || !pte_present(*ptep))
+ return false;
+
+ return kvm_s2pte_exec(ptep);
+}
+
static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
phys_addr_t addr, const pte_t *new_pte,
unsigned long flags)
@@ -1407,6 +1426,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
if (exec_fault) {
new_pmd = kvm_s2pmd_mkexec(new_pmd);
invalidate_icache_guest_page(vcpu, pfn, PMD_SIZE);
+ } else if (fault_status == FSC_PERM) {
+ /* Preserve execute if XN was already cleared */
+ if (stage2_is_exec(kvm, fault_ipa))
+ new_pmd = kvm_s2pmd_mkexec(new_pmd);
}

ret = stage2_set_pmd_huge(kvm, memcache, fault_ipa, &new_pmd);
@@ -1425,6 +1448,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
if (exec_fault) {
new_pte = kvm_s2pte_mkexec(new_pte);
invalidate_icache_guest_page(vcpu, pfn, PAGE_SIZE);
+ } else if (fault_status == FSC_PERM) {
+ /* Preserve execute if XN was already cleared */
+ if (stage2_is_exec(kvm, fault_ipa))
+ new_pte = kvm_s2pte_mkexec(new_pte);
}

ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte, flags);
--
2.11.0