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Date:   Mon, 19 Jul 2021 11:47:34 +0100
In-Reply-To: <20210719104735.3681732-1-qperret@google.com>
Message-Id: <20210719104735.3681732-14-qperret@google.com>
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References: <20210719104735.3681732-1-qperret@google.com>
Subject: [PATCH 13/14] KVM: arm64: Restrict hyp stage-1 manipulation in
 protected mode
From:   Quentin Perret <qperret@google.com>
To:     maz@kernel.org, james.morse@arm.com, alexandru.elisei@arm.com,
        suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org
Cc:     linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-kernel@vger.kernel.org, ardb@kernel.org, qwandor@google.com,
        tabba@google.com, dbrazdil@google.com, kernel-team@android.com,
        Quentin Perret <qperret@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

The host kernel is currently able to change EL2 stage-1 mappings without
restrictions thanks to the __pkvm_create_mappings() hypercall. But in a
world where the host is no longer part of the TCB, this clearly poses a
problem.

To fix this, introduce a new hypercall to allow the host to share a
range of physical memory with the hypervisor, and remove the
__pkvm_create_mappings() variant. The new hypercall implements
ownership and permission checks before allowing the sharing operation,
and it annotates the shared pages in the hypervisor stage-1 and host
stage-2 page-tables.

Signed-off-by: Quentin Perret <qperret@google.com>
---
 arch/arm64/include/asm/kvm_asm.h              |   2 +-
 arch/arm64/kvm/hyp/include/nvhe/mem_protect.h |   1 +
 arch/arm64/kvm/hyp/include/nvhe/mm.h          |   2 -
 arch/arm64/kvm/hyp/nvhe/hyp-main.c            |  12 +-
 arch/arm64/kvm/hyp/nvhe/mem_protect.c         | 105 ++++++++++++++++++
 arch/arm64/kvm/hyp/nvhe/mm.c                  |   4 +-
 arch/arm64/kvm/mmu.c                          |  14 ++-
 7 files changed, 124 insertions(+), 16 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 9f0bf2109be7..78db818ae2c9 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -59,7 +59,7 @@
 #define __KVM_HOST_SMCCC_FUNC___vgic_v3_save_aprs		13
 #define __KVM_HOST_SMCCC_FUNC___vgic_v3_restore_aprs		14
 #define __KVM_HOST_SMCCC_FUNC___pkvm_init			15
-#define __KVM_HOST_SMCCC_FUNC___pkvm_create_mappings		16
+#define __KVM_HOST_SMCCC_FUNC___pkvm_host_share_hyp		16
 #define __KVM_HOST_SMCCC_FUNC___pkvm_create_private_mapping	17
 #define __KVM_HOST_SMCCC_FUNC___pkvm_cpu_set_vector		18
 #define __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize		19
diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
index b39047463075..f37e4d3b831b 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
@@ -22,6 +22,7 @@ extern struct host_kvm host_kvm;
 
 int __pkvm_prot_finalize(void);
 int __pkvm_mark_hyp(phys_addr_t start, phys_addr_t end);
+int __pkvm_host_share_hyp(phys_addr_t start, phys_addr_t end);
 
 int host_stage2_idmap_locked(u64 start, u64 end, enum kvm_pgtable_prot prot);
 int kvm_host_prepare_stage2(void *pgt_pool_base);
diff --git a/arch/arm64/kvm/hyp/include/nvhe/mm.h b/arch/arm64/kvm/hyp/include/nvhe/mm.h
index c76d7136ed9b..c9a8f535212e 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/mm.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/mm.h
@@ -24,8 +24,6 @@ int hyp_back_vmemmap(phys_addr_t phys, unsigned long size, phys_addr_t back);
 int pkvm_cpu_set_vector(enum arm64_hyp_spectre_vector slot);
 int pkvm_create_mappings(void *from, void *to, enum kvm_pgtable_prot prot);
 int pkvm_create_mappings_locked(void *from, void *to, enum kvm_pgtable_prot prot);
-int __pkvm_create_mappings(unsigned long start, unsigned long size,
-			   unsigned long phys, enum kvm_pgtable_prot prot);
 unsigned long __pkvm_create_private_mapping(phys_addr_t phys, size_t size,
 					    enum kvm_pgtable_prot prot);
 
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 1632f001f4ed..f05ecbd382d0 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -140,14 +140,12 @@ static void handle___pkvm_cpu_set_vector(struct kvm_cpu_context *host_ctxt)
 	cpu_reg(host_ctxt, 1) = pkvm_cpu_set_vector(slot);
 }
 
-static void handle___pkvm_create_mappings(struct kvm_cpu_context *host_ctxt)
+static void handle___pkvm_host_share_hyp(struct kvm_cpu_context *host_ctxt)
 {
-	DECLARE_REG(unsigned long, start, host_ctxt, 1);
-	DECLARE_REG(unsigned long, size, host_ctxt, 2);
-	DECLARE_REG(unsigned long, phys, host_ctxt, 3);
-	DECLARE_REG(enum kvm_pgtable_prot, prot, host_ctxt, 4);
+	DECLARE_REG(phys_addr_t, start, host_ctxt, 1);
+	DECLARE_REG(phys_addr_t, end, host_ctxt, 2);
 
-	cpu_reg(host_ctxt, 1) = __pkvm_create_mappings(start, size, phys, prot);
+	cpu_reg(host_ctxt, 1) = __pkvm_host_share_hyp(start, end);
 }
 
 static void handle___pkvm_create_private_mapping(struct kvm_cpu_context *host_ctxt)
@@ -193,7 +191,7 @@ static const hcall_t host_hcall[] = {
 	HANDLE_FUNC(__vgic_v3_restore_aprs),
 	HANDLE_FUNC(__pkvm_init),
 	HANDLE_FUNC(__pkvm_cpu_set_vector),
-	HANDLE_FUNC(__pkvm_create_mappings),
+	HANDLE_FUNC(__pkvm_host_share_hyp),
 	HANDLE_FUNC(__pkvm_create_private_mapping),
 	HANDLE_FUNC(__pkvm_prot_finalize),
 	HANDLE_FUNC(__pkvm_mark_hyp),
diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 6f28edf58407..20b3cb3fdc67 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -262,6 +262,111 @@ static int host_stage2_idmap(u64 addr)
 	return ret;
 }
 
+static int hyp_range_is_shared_walker(u64 addr, u64 end, u32 level,
+				      kvm_pte_t *ptep,
+				      enum kvm_pgtable_walk_flags flag,
+				      void * const arg)
+{
+	enum kvm_pgtable_prot prot;
+	kvm_pte_t pte = *ptep;
+
+	if (!kvm_pte_valid(pte))
+		return -EPERM;
+
+	prot = kvm_pgtable_hyp_pte_prot(pte);
+	if (!prot)
+		return -EPERM;
+
+	/* Check that the page has been shared with the hypervisor before */
+	if (prot != (PAGE_HYP | KVM_PGTABLE_STATE_SHARED | KVM_PGTABLE_STATE_BORROWED))
+		return -EPERM;
+
+	return 0;
+}
+
+static int hyp_range_is_shared(phys_addr_t start, phys_addr_t end)
+{
+	struct kvm_pgtable_walker walker = {
+		.cb = hyp_range_is_shared_walker,
+		.flags = KVM_PGTABLE_WALK_LEAF,
+	};
+
+	return kvm_pgtable_walk(&pkvm_pgtable, (u64)__hyp_va(start),
+				end - start, &walker);
+}
+
+static int check_host_share_hyp_walker(u64 addr, u64 end, u32 level,
+				       kvm_pte_t *ptep,
+				       enum kvm_pgtable_walk_flags flag,
+				       void * const arg)
+{
+	enum kvm_pgtable_prot prot;
+	kvm_pte_t pte = *ptep;
+
+	/* If invalid, only allow to share pristine pages */
+	if (!kvm_pte_valid(pte))
+		return pte ? -EPERM : 0;
+
+	prot = kvm_pgtable_stage2_pte_prot(pte);
+	if (!prot)
+		return -EPERM;
+
+	/* Cannot share a page that is not owned */
+	if (prot & KVM_PGTABLE_STATE_BORROWED)
+		return -EPERM;
+
+	/* Cannot share a page with restricted access */
+	if ((prot & KVM_PGTABLE_PROT_RWX) ^ KVM_PGTABLE_PROT_RWX)
+		return -EPERM;
+
+	/* Allow double-sharing (requires cross-checking the hyp stage-1) */
+	if (prot & KVM_PGTABLE_STATE_SHARED)
+		return hyp_range_is_shared(addr, addr + 1);
+
+	return 0;
+}
+
+static int check_host_share_hyp(phys_addr_t start, phys_addr_t end)
+{
+	struct kvm_pgtable_walker walker = {
+		.cb = check_host_share_hyp_walker,
+		.flags = KVM_PGTABLE_WALK_LEAF,
+	};
+
+	return kvm_pgtable_walk(&host_kvm.pgt, start, end - start, &walker);
+}
+
+int __pkvm_host_share_hyp(phys_addr_t start, phys_addr_t end)
+{
+	enum kvm_pgtable_prot prot;
+	int ret;
+
+	if (!range_is_memory(start, end))
+		return -EINVAL;
+
+	hyp_spin_lock(&host_kvm.lock);
+	hyp_spin_lock(&pkvm_pgd_lock);
+
+	ret = check_host_share_hyp(start, end);
+	if (ret)
+		goto unlock;
+
+	prot = KVM_PGTABLE_PROT_RWX | KVM_PGTABLE_STATE_SHARED;
+	ret = host_stage2_idmap_locked(start, end, prot);
+	if (ret && ret != -EAGAIN)
+		goto unlock;
+
+	prot = PAGE_HYP | KVM_PGTABLE_STATE_SHARED | KVM_PGTABLE_STATE_BORROWED;
+	ret = pkvm_create_mappings_locked(__hyp_va(start), __hyp_va(end), prot);
+	/* XXX - undo host stage-2 changes if ret != 0 */
+
+unlock:
+	hyp_spin_unlock(&pkvm_pgd_lock);
+	hyp_spin_unlock(&host_kvm.lock);
+
+	return ret;
+}
+
 int __pkvm_mark_hyp(phys_addr_t start, phys_addr_t end)
 {
 	int ret;
diff --git a/arch/arm64/kvm/hyp/nvhe/mm.c b/arch/arm64/kvm/hyp/nvhe/mm.c
index dde22e2a322a..95f6c34a38ec 100644
--- a/arch/arm64/kvm/hyp/nvhe/mm.c
+++ b/arch/arm64/kvm/hyp/nvhe/mm.c
@@ -23,8 +23,8 @@ u64 __io_map_base;
 struct memblock_region hyp_memory[HYP_MEMBLOCK_REGIONS];
 unsigned int hyp_memblock_nr;
 
-int __pkvm_create_mappings(unsigned long start, unsigned long size,
-			  unsigned long phys, enum kvm_pgtable_prot prot)
+static int __pkvm_create_mappings(unsigned long start, unsigned long size,
+				  unsigned long phys, enum kvm_pgtable_prot prot)
 {
 	int err;
 
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 0625bf2353c2..2158d1e00acd 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -259,10 +259,8 @@ static int __create_hyp_mappings(unsigned long start, unsigned long size,
 {
 	int err;
 
-	if (!kvm_host_owns_hyp_mappings()) {
-		return kvm_call_hyp_nvhe(__pkvm_create_mappings,
-					 start, size, phys, prot);
-	}
+	if (WARN_ON(!kvm_host_owns_hyp_mappings()))
+		return -EINVAL;
 
 	mutex_lock(&kvm_hyp_pgd_mutex);
 	err = kvm_pgtable_hyp_map(hyp_pgtable, start, size, phys, prot);
@@ -302,6 +300,14 @@ int create_hyp_mappings(void *from, void *to, enum kvm_pgtable_prot prot)
 	if (is_kernel_in_hyp_mode())
 		return 0;
 
+	if (!kvm_host_owns_hyp_mappings()) {
+		if (WARN_ON(prot != PAGE_HYP))
+			return -EPERM;
+		return kvm_call_hyp_nvhe(__pkvm_host_share_hyp,
+					 kvm_kaddr_to_phys(from),
+					 kvm_kaddr_to_phys(to));
+	}
+
 	start = start & PAGE_MASK;
 	end = PAGE_ALIGN(end);
 
-- 
2.32.0.402.g57bb445576-goog