vfio: Destroy memslot when unmapping the associated VAs

When we want to map a device region into the guest address space, first we
perform an mmap on the device fd. The resulting VMA is a mapping between
host userspace addresses and physical addresses associated with the device.
Next, we create a memslot, which populates the stage 2 table with the
mappings between guest physical addresses and the device physical adresses.

However, when we want to unmap the device from the guest address space, we
only call munmap, which destroys the VMA and the stage 2 mappings, but
doesn't destroy the memslot and kvmtool's internal mem_bank structure
associated with the memslot.

This has been perfectly fine so far, because we only unmap a device region
when we exit kvmtool. This is will change when we add support for
reassignable BARs, and we will have to unmap vfio regions as the guest
kernel writes new addresses in the BARs. This can lead to two possible
problems:

- We refuse to create a valid BAR mapping because of a stale mem_bank
  structure which belonged to a previously unmapped region.

- It is possible that the mmap in vfio_map_region returns the same address
  that was used to create a memslot, but was unmapped by vfio_unmap_region.
  Guest accesses to the device memory will fault because the stage 2
  mappings are missing, and this can lead to performance degradation.

Let's do the right thing and destroy the memslot and the mem_bank struct
associated with it when we unmap a vfio region. Set host_addr to NULL after
the munmap call so we won't try to unmap an address which is currently used
by the process for something else if vfio_unmap_region gets called twice.

Tested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

3 files changed, 99 insertions, 12 deletions

diff --git a/include/kvm/kvm.h b/include/kvm/kvm.h
index 50119a86..9428f57a 100644
--- a/include/kvm/kvm.h
+++ b/include/kvm/kvm.h
@@ -1,6 +1,7 @@
 #ifndef KVM__KVM_H
 #define KVM__KVM_H
 
+#include "kvm/mutex.h"
 #include "kvm/kvm-arch.h"
 #include "kvm/kvm-config.h"
 #include "kvm/util-init.h"
@@ -56,6 +57,7 @@ struct kvm_mem_bank {
 	void			*host_addr;
 	u64			size;
 	enum kvm_mem_type	type;
+	u32			slot;
 };
 
 struct kvm {
@@ -72,6 +74,7 @@ struct kvm {
 	u64			ram_size;
 	void			*ram_start;
 	u64			ram_pagesize;
+	struct mutex		mem_banks_lock;
 	struct list_head	mem_banks;
 
 	bool			nmi_disabled;
@@ -106,6 +109,7 @@ void kvm__irq_line(struct kvm *kvm, int irq, int level);
 void kvm__irq_trigger(struct kvm *kvm, int irq);
 bool kvm__emulate_io(struct kvm_cpu *vcpu, u16 port, void *data, int direction, int size, u32 count);
 bool kvm__emulate_mmio(struct kvm_cpu *vcpu, u64 phys_addr, u8 *data, u32 len, u8 is_write);
+int kvm__destroy_mem(struct kvm *kvm, u64 guest_phys, u64 size, void *userspace_addr);
 int kvm__register_mem(struct kvm *kvm, u64 guest_phys, u64 size, void *userspace_addr,
 		      enum kvm_mem_type type);
 static inline int kvm__register_ram(struct kvm *kvm, u64 guest_phys, u64 size,
diff --git a/kvm.c b/kvm.c
index 57c4ff98..26f6b9bc 100644
--- a/kvm.c
+++ b/kvm.c
@@ -157,6 +157,7 @@ struct kvm *kvm__new(void)
 	if (!kvm)
 		return ERR_PTR(-ENOMEM);
 
+	mutex_init(&kvm->mem_banks_lock);
 	kvm->sys_fd = -1;
 	kvm->vm_fd = -1;
 
@@ -183,20 +184,84 @@ int kvm__exit(struct kvm *kvm)
 }
 core_exit(kvm__exit);
 
+int kvm__destroy_mem(struct kvm *kvm, u64 guest_phys, u64 size,
+		     void *userspace_addr)
+{
+	struct kvm_userspace_memory_region mem;
+	struct kvm_mem_bank *bank;
+	int ret;
+
+	mutex_lock(&kvm->mem_banks_lock);
+	list_for_each_entry(bank, &kvm->mem_banks, list)
+		if (bank->guest_phys_addr == guest_phys &&
+		    bank->size == size && bank->host_addr == userspace_addr)
+			break;
+
+	if (&bank->list == &kvm->mem_banks) {
+		pr_err("Region [%llx-%llx] not found", guest_phys,
+		       guest_phys + size - 1);
+		ret = -EINVAL;
+		goto out;
+	}
+
+	if (bank->type == KVM_MEM_TYPE_RESERVED) {
+		pr_err("Cannot delete reserved region [%llx-%llx]",
+		       guest_phys, guest_phys + size - 1);
+		ret = -EINVAL;
+		goto out;
+	}
+
+	mem = (struct kvm_userspace_memory_region) {
+		.slot			= bank->slot,
+		.guest_phys_addr	= guest_phys,
+		.memory_size		= 0,
+		.userspace_addr		= (unsigned long)userspace_addr,
+	};
+
+	ret = ioctl(kvm->vm_fd, KVM_SET_USER_MEMORY_REGION, &mem);
+	if (ret < 0) {
+		ret = -errno;
+		goto out;
+	}
+
+	list_del(&bank->list);
+	free(bank);
+	kvm->mem_slots--;
+	ret = 0;
+
+out:
+	mutex_unlock(&kvm->mem_banks_lock);
+	return ret;
+}
+
 int kvm__register_mem(struct kvm *kvm, u64 guest_phys, u64 size,
 		      void *userspace_addr, enum kvm_mem_type type)
 {
 	struct kvm_userspace_memory_region mem;
 	struct kvm_mem_bank *merged = NULL;
 	struct kvm_mem_bank *bank;
+	struct list_head *prev_entry;
+	u32 slot;
 	int ret;
 
-	/* Check for overlap */
+	mutex_lock(&kvm->mem_banks_lock);
+	/* Check for overlap and find first empty slot. */
+	slot = 0;
+	prev_entry = &kvm->mem_banks;
 	list_for_each_entry(bank, &kvm->mem_banks, list) {
 		u64 bank_end = bank->guest_phys_addr + bank->size - 1;
 		u64 end = guest_phys + size - 1;
-		if (guest_phys > bank_end || end < bank->guest_phys_addr)
+		if (guest_phys > bank_end || end < bank->guest_phys_addr) {
+			/*
+			 * Keep the banks sorted ascending by slot, so it's
+			 * easier for us to find a free slot.
+			 */
+			if (bank->slot == slot) {
+				slot++;
+				prev_entry = &bank->list;
+			}
 			continue;
+		}
 
 		/* Merge overlapping reserved regions */
 		if (bank->type == KVM_MEM_TYPE_RESERVED &&
@@ -226,38 +291,50 @@ int kvm__register_mem(struct kvm *kvm, u64 guest_phys, u64 size,
 		       kvm_mem_type_to_string(bank->type), bank->guest_phys_addr,
 		       bank->guest_phys_addr + bank->size - 1);
 
-		return -EINVAL;
+		ret = -EINVAL;
+		goto out;
 	}
 
-	if (merged)
-		return 0;
+	if (merged) {
+		ret = 0;
+		goto out;
+	}
 
 	bank = malloc(sizeof(*bank));
-	if (!bank)
-		return -ENOMEM;
+	if (!bank) {
+		ret = -ENOMEM;
+		goto out;
+	}
 
 	INIT_LIST_HEAD(&bank->list);
 	bank->guest_phys_addr		= guest_phys;
 	bank->host_addr			= userspace_addr;
 	bank->size			= size;
 	bank->type			= type;
+	bank->slot			= slot;
 
 	if (type != KVM_MEM_TYPE_RESERVED) {
 		mem = (struct kvm_userspace_memory_region) {
-			.slot			= kvm->mem_slots++,
+			.slot			= slot,
 			.guest_phys_addr	= guest_phys,
 			.memory_size		= size,
 			.userspace_addr		= (unsigned long)userspace_addr,
 		};
 
 		ret = ioctl(kvm->vm_fd, KVM_SET_USER_MEMORY_REGION, &mem);
-		if (ret < 0)
-			return -errno;
+		if (ret < 0) {
+			ret = -errno;
+			goto out;
+		}
 	}
 
-	list_add(&bank->list, &kvm->mem_banks);
+	list_add(&bank->list, prev_entry);
+	kvm->mem_slots++;
+	ret = 0;
 
-	return 0;
+out:
+	mutex_unlock(&kvm->mem_banks_lock);
+	return ret;
 }
 
 void *guest_flat_to_host(struct kvm *kvm, u64 offset)
diff --git a/vfio/core.c b/vfio/core.c
index 0ed1e6fe..b80ebf3b 100644
--- a/vfio/core.c
+++ b/vfio/core.c
@@ -256,8 +256,14 @@ int vfio_map_region(struct kvm *kvm, struct vfio_device *vdev,
 
 void vfio_unmap_region(struct kvm *kvm, struct vfio_region *region)
 {
+	u64 map_size;
+
 	if (region->host_addr) {
+		map_size = ALIGN(region->info.size, PAGE_SIZE);
+		kvm__destroy_mem(kvm, region->guest_phys_addr, map_size,
+				 region->host_addr);
 		munmap(region->host_addr, region->info.size);
+		region->host_addr = NULL;
 	} else if (region->is_ioport) {
 		ioport__unregister(kvm, region->port_base);
 	} else {