[PATCH v9 01/41] KVM: guest_memfd: Introduce per-gmem attributes, use to guard user mappings

[Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org>]

From: Sean Christopherson <seanjc@google.com>

Start plumbing in guest_memfd support for in-place private<=>shared
conversions by tracking attributes via a maple tree.  KVM currently tracks
private vs. shared attributes on a per-VM basis, which made sense when a
guest_memfd _only_ supported private memory, but tracking per-VM simply
can't work for in-place conversions as the shared/private status of a given
page needs to be per-gmem_inode, not per-VM.

Use the filemap invalidation lock to protect the maple tree, as taking the
lock for read when faulting in memory (for userspace or the guest) isn't
expected to result in meaningful contention, and using a separate lock
would add significant complexity (avoiding deadlock is quite difficult).

Co-developed-by: Vishal Annapurve <vannapurve@google.com>
Signed-off-by: Vishal Annapurve <vannapurve@google.com>
Co-developed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Shivank Garg <shivankg@amd.com>
Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 virt/kvm/guest_memfd.c | 133 ++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 116 insertions(+), 17 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 0bca42a0b346b..3d6547c5ba9a6 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -4,6 +4,7 @@
 #include <linux/falloc.h>
 #include <linux/fs.h>
 #include <linux/kvm_host.h>
+#include <linux/maple_tree.h>
 #include <linux/mempolicy.h>
 #include <linux/pseudo_fs.h>
 #include <linux/pagemap.h>
@@ -33,6 +34,13 @@ struct gmem_inode {
 	struct list_head gmem_file_list;
 
 	u64 flags;
+	/*
+	 * Every index in this inode, whether memory is populated or
+	 * not, is tracked in attributes. The entire range of indices,
+	 * corresponding to the size of this inode, is represented in
+	 * this maple tree.
+	 */
+	struct maple_tree attributes;
 };
 
 static __always_inline struct gmem_inode *GMEM_I(struct inode *inode)
@@ -60,9 +68,25 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
 	return gfn - slot->base_gfn + slot->gmem.pgoff;
 }
 
+static u64 kvm_gmem_interpret_entry(struct inode *inode, void *entry)
+{
+	if (WARN_ON_ONCE(!entry)) {
+		bool initially_shared = GMEM_I(inode)->flags &
+					GUEST_MEMFD_FLAG_INIT_SHARED;
+
+		return initially_shared ? 0 : KVM_MEMORY_ATTRIBUTE_PRIVATE;
+	}
+
+	return xa_to_value(entry);
+}
+
 static bool kvm_gmem_is_private_mem(struct inode *inode, pgoff_t index)
 {
-	return !(GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED);
+	struct maple_tree *mt = &GMEM_I(inode)->attributes;
+	void *entry = mtree_load(mt, index);
+
+	return kvm_gmem_interpret_entry(inode, entry) &
+	       KVM_MEMORY_ATTRIBUTE_PRIVATE;
 }
 
 static bool kvm_gmem_is_shared_mem(struct inode *inode, pgoff_t index)
@@ -363,10 +387,13 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
 	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
 		return VM_FAULT_SIGBUS;
 
-	if (!kvm_gmem_is_shared_mem(inode, vmf->pgoff))
-		return VM_FAULT_SIGBUS;
+	filemap_invalidate_lock_shared(inode->i_mapping);
+	if (kvm_gmem_is_shared_mem(inode, vmf->pgoff))
+		folio = kvm_gmem_get_folio(inode, vmf->pgoff);
+	else
+		folio = ERR_PTR(-EACCES);
+	filemap_invalidate_unlock_shared(inode->i_mapping);
 
-	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
 	if (IS_ERR(folio)) {
 		if (PTR_ERR(folio) == -EAGAIN)
 			return VM_FAULT_RETRY;
@@ -519,6 +546,51 @@ bool __weak kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
 	return true;
 }
 
+static int kvm_gmem_init_inode(struct inode *inode, loff_t size, u64 flags)
+{
+	struct gmem_inode *gi = GMEM_I(inode);
+	MA_STATE(mas, &gi->attributes, 0, (size >> PAGE_SHIFT) - 1);
+	u64 attrs;
+	int r;
+
+	inode->i_op = &kvm_gmem_iops;
+	inode->i_mapping->a_ops = &kvm_gmem_aops;
+	inode->i_mode |= S_IFREG;
+	inode->i_size = size;
+	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
+
+	/*
+	 * guest_memfd memory is neither migratable nor swappable: set
+	 * inaccessible to gate off both.
+	 */
+	mapping_set_inaccessible(inode->i_mapping);
+	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
+
+	gi->flags = flags;
+
+	mt_set_external_lock(&gi->attributes,
+			     &inode->i_mapping->invalidate_lock);
+
+	/*
+	 * Store default attributes for the entire gmem instance. Ensuring every
+	 * index is represented in the maple tree at all times simplifies the
+	 * conversion and merging logic.
+	 */
+	attrs = gi->flags & GUEST_MEMFD_FLAG_INIT_SHARED ? 0 : KVM_MEMORY_ATTRIBUTE_PRIVATE;
+
+	/*
+	 * Acquire the invalidation lock purely to make lockdep happy.  The
+	 * maple tree library expects all stores to be protected via the lock,
+	 * and the library can't know when the tree is reachable only by the
+	 * caller, as is the case here.
+	 */
+	filemap_invalidate_lock(inode->i_mapping);
+	r = mas_store_gfp(&mas, xa_mk_value(attrs), GFP_KERNEL);
+	filemap_invalidate_unlock(inode->i_mapping);
+
+	return r;
+}
+
 static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 {
 	static const char *name = "[kvm-gmem]";
@@ -549,16 +621,9 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 		goto err_fops;
 	}
 
-	inode->i_op = &kvm_gmem_iops;
-	inode->i_mapping->a_ops = &kvm_gmem_aops;
-	inode->i_mode |= S_IFREG;
-	inode->i_size = size;
-	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
-	mapping_set_inaccessible(inode->i_mapping);
-	/* Unmovable mappings are supposed to be marked unevictable as well. */
-	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
-
-	GMEM_I(inode)->flags = flags;
+	err = kvm_gmem_init_inode(inode, size, flags);
+	if (err)
+		goto err_inode;
 
 	file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR, &kvm_gmem_fops);
 	if (IS_ERR(file)) {
@@ -763,9 +828,13 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	if (!file)
 		return -EFAULT;
 
+	filemap_invalidate_lock_shared(file_inode(file)->i_mapping);
+
 	folio = __kvm_gmem_get_pfn(file, slot, index, pfn, max_order);
-	if (IS_ERR(folio))
-		return PTR_ERR(folio);
+	if (IS_ERR(folio)) {
+		r = PTR_ERR(folio);
+		goto out;
+	}
 
 	if (!folio_test_uptodate(folio)) {
 		clear_highpage(folio_page(folio, 0));
@@ -785,6 +854,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	else
 		folio_put(folio);
 
+out:
+	filemap_invalidate_unlock_shared(file_inode(file)->i_mapping);
 	return r;
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_pfn);
@@ -916,6 +987,15 @@ static struct inode *kvm_gmem_alloc_inode(struct super_block *sb)
 
 	mpol_shared_policy_init(&gi->policy, NULL);
 
+	/*
+	 * Memory attributes are protected by the filemap invalidation lock, but
+	 * the lock structure isn't available at this time.  Immediately mark
+	 * maple tree as using external locking so that accessing the tree
+	 * before it's fully initialized results in NULL pointer dereferences
+	 * and not more subtle bugs.
+	 */
+	mt_init_flags(&gi->attributes, MT_FLAGS_LOCK_EXTERN | MT_FLAGS_USE_RCU);
+
 	gi->flags = 0;
 	INIT_LIST_HEAD(&gi->gmem_file_list);
 	return &gi->vfs_inode;
@@ -923,7 +1003,26 @@ static struct inode *kvm_gmem_alloc_inode(struct super_block *sb)
 
 static void kvm_gmem_destroy_inode(struct inode *inode)
 {
-	mpol_free_shared_policy(&GMEM_I(inode)->policy);
+	struct gmem_inode *gi = GMEM_I(inode);
+
+	mpol_free_shared_policy(&gi->policy);
+
+	/*
+	 * Note!  Checking for an empty tree is functionally necessary
+	 * to avoid explosions if the tree hasn't been fully
+	 * initialized, i.e. if the inode is being destroyed before
+	 * guest_memfd can set the external lock, lockdep would find
+	 * that the tree's internal ma_lock was not held.
+	 */
+	if (!mtree_empty(&gi->attributes)) {
+		/*
+		 * Acquire the invalidation lock purely to make lockdep happy,
+		 * the inode is unreachable at this point.
+		 */
+		filemap_invalidate_lock(inode->i_mapping);
+		__mt_destroy(&gi->attributes);
+		filemap_invalidate_unlock(inode->i_mapping);
+	}
 }
 
 static void kvm_gmem_free_inode(struct inode *inode)

-- 
2.55.0.508.g3f0d502094-goog