diff --git a/kmod/src/Makefile b/kmod/src/Makefile
index 975ec319..d96c4967 100644
--- a/kmod/src/Makefile
+++ b/kmod/src/Makefile
@@ -8,6 +8,7 @@ CFLAGS_scoutfs_trace.o = -I$(src) # define_trace.h double include
 -include $(src)/Makefile.kernelcompat
  
 scoutfs-y +=			\
+	acl.o			\
 	avl.o			\
 	alloc.o			\
 	block.o			\
diff --git a/kmod/src/acl.c b/kmod/src/acl.c
new file mode 100644
index 00000000..d6ab6a3c
--- /dev/null
+++ b/kmod/src/acl.c
@@ -0,0 +1,355 @@
+/*
+ * Copyright (C) 2022 Versity Software, Inc.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ */
+#include <linux/kernel.h>
+#include <linux/fs.h>
+#include <linux/slab.h>
+#include <linux/xattr.h>
+#include <linux/posix_acl.h>
+#include <linux/posix_acl_xattr.h>
+
+#include "format.h"
+#include "super.h"
+#include "scoutfs_trace.h"
+#include "xattr.h"
+#include "acl.h"
+#include "inode.h"
+#include "trans.h"
+
+/*
+ * POSIX draft ACLs are stored as full xattr items with the entries
+ * encoded as the kernel's posix_acl_xattr_{header,entry} value structs.
+ *
+ * They're accessed and modified via user facing synthetic xattrs, iops
+ * calls from the kernel, during inode mode changes, and during inode
+ * creation.
+ *
+ * ACL access devolves into xattr access which is relatively expensive
+ * so we maintain the cached native form in the vfs inode.  We drop the
+ * cache in lock invalidation which means that cached acl access must
+ * always be performed under cluster locking.
+ */
+
+static int acl_xattr_name_len(int type, char **name, size_t *name_len)
+{
+	int ret = 0;
+
+	switch (type) {
+	case ACL_TYPE_ACCESS:
+		*name = XATTR_NAME_POSIX_ACL_ACCESS;
+		if (name_len)
+			*name_len = sizeof(XATTR_NAME_POSIX_ACL_ACCESS) - 1;
+		break;
+	case ACL_TYPE_DEFAULT:
+		*name = XATTR_NAME_POSIX_ACL_DEFAULT;
+		if (name_len)
+			*name_len = sizeof(XATTR_NAME_POSIX_ACL_DEFAULT) - 1;
+		break;
+	default:
+		ret = -EINVAL;
+		break;
+	}
+
+	return ret;
+}
+
+struct posix_acl *scoutfs_get_acl_locked(struct inode *inode, int type, struct scoutfs_lock *lock)
+{
+	struct posix_acl *acl;
+	char *value = NULL;
+	char *name;
+	int ret;
+
+	if (!IS_POSIXACL(inode))
+		return NULL;
+
+	acl = get_cached_acl(inode, type);
+	if (acl != ACL_NOT_CACHED)
+		return acl;
+
+	ret = acl_xattr_name_len(type, &name, NULL);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	ret = scoutfs_xattr_get_locked(inode, name, NULL, 0, lock);
+	if (ret > 0) {
+		value = kzalloc(ret, GFP_NOFS);
+		if (!value)
+			ret = -ENOMEM;
+		else
+			ret = scoutfs_xattr_get_locked(inode, name, value, ret, lock);
+	}
+	if (ret > 0) {
+		acl = posix_acl_from_xattr(&init_user_ns, value, ret);
+	} else if (ret == -ENODATA || ret == 0) {
+		acl = NULL;
+	} else {
+		acl = ERR_PTR(ret);
+	}
+
+	/* can set null negative cache */
+	if (!IS_ERR(acl))
+		set_cached_acl(inode, type, acl);
+
+	kfree(value);
+
+	return acl;
+}
+
+struct posix_acl *scoutfs_get_acl(struct inode *inode, int type)
+{
+	struct super_block *sb = inode->i_sb;
+	struct scoutfs_lock *lock = NULL;
+	struct posix_acl *acl;
+	int ret;
+
+	if (!IS_POSIXACL(inode))
+		return NULL;
+
+	ret = scoutfs_lock_inode(sb, SCOUTFS_LOCK_READ, 0, inode, &lock);
+	if (ret < 0) {
+		acl = ERR_PTR(ret);
+	} else {
+		acl = scoutfs_get_acl_locked(inode, type, lock);
+		scoutfs_unlock(sb, lock, SCOUTFS_LOCK_READ);
+	}
+
+	return acl;
+}
+
+/*
+ * The caller has acquired the locks and dirtied the inode, they'll
+ * update the inode item if we return 0.
+ */
+int scoutfs_set_acl_locked(struct inode *inode, struct posix_acl *acl, int type,
+			   struct scoutfs_lock *lock, struct list_head *ind_locks)
+{
+	static const struct scoutfs_xattr_prefix_tags tgs = {0,}; /* never scoutfs. prefix */
+	bool set_mode = false;
+	char *value = NULL;
+	umode_t new_mode;
+	size_t name_len;
+	char *name;
+	int size = 0;
+	int ret;
+
+	ret = acl_xattr_name_len(type, &name, &name_len);
+	if (ret < 0)
+		return ret;
+
+	switch (type) {
+	case ACL_TYPE_ACCESS:
+		if (acl) {
+			ret = posix_acl_update_mode(inode, &new_mode, &acl);
+			if (ret < 0)
+				goto out;
+			set_mode = true;
+		}
+		break;
+	case ACL_TYPE_DEFAULT:
+		if (!S_ISDIR(inode->i_mode)) {
+			ret = acl ? -EINVAL : 0;
+			goto out;
+		}
+		break;
+	}
+
+	if (acl) {
+		size = posix_acl_xattr_size(acl->a_count);
+		value = kmalloc(size, GFP_NOFS);
+		if (!value) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		ret = posix_acl_to_xattr(&init_user_ns, acl, value, size);
+		if (ret < 0)
+			goto out;
+	}
+
+	ret = scoutfs_xattr_set_locked(inode, name, name_len, value, size, 0, &tgs,
+				       lock, NULL, ind_locks);
+	if (ret == 0 && set_mode) {
+		inode->i_mode = new_mode;
+		if (!value) {
+			/* can be setting an acl that only affects mode, didn't need xattr */
+			inode_inc_iversion(inode);
+			inode->i_ctime = CURRENT_TIME;
+		}
+	}
+
+out:
+	if (!ret)
+		set_cached_acl(inode, type, acl);
+
+	kfree(value);
+
+	return ret;
+}
+
+int scoutfs_set_acl(struct inode *inode, struct posix_acl *acl, int type)
+{
+	struct super_block *sb = inode->i_sb;
+	struct scoutfs_lock *lock = NULL;
+	LIST_HEAD(ind_locks);
+	int ret;
+
+	ret = scoutfs_lock_inode(sb, SCOUTFS_LOCK_WRITE, SCOUTFS_LKF_REFRESH_INODE, inode, &lock) ?:
+	      scoutfs_inode_index_lock_hold(inode, &ind_locks, false, true);
+	if (ret == 0) {
+		ret = scoutfs_dirty_inode_item(inode, lock) ?:
+		      scoutfs_set_acl_locked(inode, acl, type, lock, &ind_locks);
+		if (ret == 0)
+			scoutfs_update_inode_item(inode, lock, &ind_locks);
+
+		scoutfs_release_trans(sb);
+		scoutfs_inode_index_unlock(sb, &ind_locks);
+	}
+
+	scoutfs_unlock(sb, lock, SCOUTFS_LOCK_WRITE);
+	return ret;
+}
+
+int scoutfs_acl_get_xattr(struct dentry *dentry, const char *name, void *value, size_t size,
+			  int type)
+{
+	struct posix_acl *acl;
+	int ret = 0;
+
+	if (!IS_POSIXACL(dentry->d_inode))
+		return -EOPNOTSUPP;
+
+	acl = scoutfs_get_acl(dentry->d_inode, type);
+	if (IS_ERR(acl))
+		return PTR_ERR(acl);
+	if (acl == NULL)
+		return -ENODATA;
+
+	ret = posix_acl_to_xattr(&init_user_ns, acl, value, size);
+	posix_acl_release(acl);
+
+	return ret;
+}
+
+int scoutfs_acl_set_xattr(struct dentry *dentry, const char *name, const void *value, size_t size,
+			  int flags, int type)
+{
+	struct posix_acl *acl = NULL;
+	int ret;
+
+	if (!inode_owner_or_capable(dentry->d_inode))
+		return -EPERM;
+
+	if (!IS_POSIXACL(dentry->d_inode))
+		return -EOPNOTSUPP;
+
+	if (value) {
+		acl = posix_acl_from_xattr(&init_user_ns, value, size);
+		if (IS_ERR(acl))
+			return PTR_ERR(acl);
+
+		if (acl) {
+			ret = posix_acl_valid(&init_user_ns, acl);
+			if (ret)
+				goto out;
+		}
+	}
+
+	ret = scoutfs_set_acl(dentry->d_inode, acl, type);
+out:
+	posix_acl_release(acl);
+
+	return ret;
+}
+
+/*
+ * Apply the parent's default acl to new inodes access acl and inherit
+ * it as the default for new directories.  The caller holds locks and a
+ * transaction.
+ */
+int scoutfs_init_acl_locked(struct inode *inode, struct inode *dir,
+			    struct scoutfs_lock *lock, struct scoutfs_lock *dir_lock,
+			    struct list_head *ind_locks)
+{
+	struct posix_acl *acl = NULL;
+	int ret = 0;
+
+	if (!S_ISLNK(inode->i_mode)) {
+		if (IS_POSIXACL(dir)) {
+			acl = scoutfs_get_acl_locked(dir, ACL_TYPE_DEFAULT, dir_lock);
+			if (IS_ERR(acl))
+				return PTR_ERR(acl);
+		}
+
+		if (!acl)
+			inode->i_mode &= ~current_umask();
+	}
+
+	if (IS_POSIXACL(dir) && acl) {
+		if (S_ISDIR(inode->i_mode)) {
+			ret = scoutfs_set_acl_locked(inode, acl, ACL_TYPE_DEFAULT,
+						     lock, ind_locks);
+			if (ret)
+				goto out;
+		}
+		ret = posix_acl_create(&acl, GFP_NOFS, &inode->i_mode);
+		if (ret < 0)
+			return ret;
+		if (ret > 0)
+			ret = scoutfs_set_acl_locked(inode, acl, ACL_TYPE_ACCESS,
+						     lock, ind_locks);
+	} else {
+		cache_no_acl(inode);
+	}
+out:
+	posix_acl_release(acl);
+	return ret;
+}
+
+/*
+ * Update the access ACL based on a newly set mode.  If we return an
+ * error then the xattr wasn't changed.
+ *
+ * Annoyingly, setattr_copy has logic that transforms the final set mode
+ * that we want to use to update the acl.   But we don't want to modify
+ * the other inode fields while discovering the resulting mode.  We're
+ * relying on acl_chmod not caring about the transformation (currently
+ * just clears sgid).  It would be better if we could get the resulting
+ * mode to give to acl_chmod without modifying the other inode fields.
+ *
+ * The caller has the inode mutex, a cluster lock, transaction, and will
+ * update the inode item if we return success.
+ */
+int scoutfs_acl_chmod_locked(struct inode *inode, struct iattr *attr,
+			     struct scoutfs_lock *lock, struct list_head *ind_locks)
+{
+	struct posix_acl *acl;
+	int ret = 0;
+
+	if (!IS_POSIXACL(inode) || !(attr->ia_valid & ATTR_MODE))
+		return 0;
+
+	if (S_ISLNK(inode->i_mode))
+		return -EOPNOTSUPP;
+
+	acl = scoutfs_get_acl_locked(inode, ACL_TYPE_ACCESS, lock);
+	if (IS_ERR_OR_NULL(acl))
+		return PTR_ERR(acl);
+
+	ret = posix_acl_chmod(&acl, GFP_KERNEL, attr->ia_mode);
+	if (ret)
+		return ret;
+
+	ret = scoutfs_set_acl_locked(inode, acl, ACL_TYPE_ACCESS, lock, ind_locks);
+	posix_acl_release(acl);
+	return ret;
+}
diff --git a/kmod/src/acl.h b/kmod/src/acl.h
new file mode 100644
index 00000000..a9235eb5
--- /dev/null
+++ b/kmod/src/acl.h
@@ -0,0 +1,18 @@
+#ifndef _SCOUTFS_ACL_H_
+#define _SCOUTFS_ACL_H_
+
+struct posix_acl *scoutfs_get_acl(struct inode *inode, int type);
+struct posix_acl *scoutfs_get_acl_locked(struct inode *inode, int type, struct scoutfs_lock *lock);
+int scoutfs_set_acl(struct inode *inode, struct posix_acl *acl, int type);
+int scoutfs_set_acl_locked(struct inode *inode, struct posix_acl *acl, int type,
+			   struct scoutfs_lock *lock, struct list_head *ind_locks);
+int scoutfs_acl_get_xattr(struct dentry *dentry, const char *name, void *value, size_t size,
+			  int type);
+int scoutfs_acl_set_xattr(struct dentry *dentry, const char *name, const void *value, size_t size,
+			  int flags, int type);
+int scoutfs_acl_chmod_locked(struct inode *inode, struct iattr *attr,
+			     struct scoutfs_lock *lock, struct list_head *ind_locks);
+int scoutfs_init_acl_locked(struct inode *inode, struct inode *dir,
+			    struct scoutfs_lock *lock, struct scoutfs_lock *dir_lock,
+			    struct list_head *ind_locks);
+#endif
diff --git a/kmod/src/dir.c b/kmod/src/dir.c
index 92b9eb29..e4b83256 100644
--- a/kmod/src/dir.c
+++ b/kmod/src/dir.c
@@ -32,6 +32,7 @@
 #include "hash.h"
 #include "omap.h"
 #include "forest.h"
+#include "acl.h"
 #include "counters.h"
 #include "scoutfs_trace.h"
 
@@ -765,7 +766,8 @@ retry:
 	if (ret)
 		goto out_unlock;
 
-	ret = scoutfs_new_inode(sb, dir, mode, rdev, ino, *inode_lock, &inode);
+	ret = scoutfs_new_inode(sb, dir, mode, rdev, ino, *inode_lock, &inode) ?:
+	      scoutfs_init_acl_locked(inode, dir, *inode_lock, *dir_lock, ind_locks);
 	if (ret < 0)
 		goto out;
 
@@ -1246,6 +1248,7 @@ const struct inode_operations scoutfs_symlink_iops = {
 	.getxattr	= generic_getxattr,
 	.listxattr	= scoutfs_listxattr,
 	.removexattr	= generic_removexattr,
+	.get_acl	= scoutfs_get_acl,
 };
 
 /*
@@ -1982,6 +1985,7 @@ const struct inode_operations_wrapper scoutfs_dir_iops = {
 	.getxattr	= generic_getxattr,
 	.listxattr	= scoutfs_listxattr,
 	.removexattr	= generic_removexattr,
+	.get_acl	= scoutfs_get_acl,
 	.symlink	= scoutfs_symlink,
 	.permission	= scoutfs_permission,
 	},
diff --git a/kmod/src/inode.c b/kmod/src/inode.c
index 9b8beede..02456fde 100644
--- a/kmod/src/inode.c
+++ b/kmod/src/inode.c
@@ -36,6 +36,7 @@
 #include "omap.h"
 #include "forest.h"
 #include "btree.h"
+#include "acl.h"
 
 /*
  * XXX
@@ -140,6 +141,7 @@ static const struct inode_operations scoutfs_file_iops = {
 	.getxattr	= generic_getxattr,
 	.listxattr	= scoutfs_listxattr,
 	.removexattr	= generic_removexattr,
+	.get_acl	= scoutfs_get_acl,
 	.fiemap		= scoutfs_data_fiemap,
 };
 
@@ -150,6 +152,7 @@ static const struct inode_operations scoutfs_special_iops = {
 	.getxattr	= generic_getxattr,
 	.listxattr	= scoutfs_listxattr,
 	.removexattr	= generic_removexattr,
+	.get_acl	= scoutfs_get_acl,
 };
 
 /*
@@ -507,10 +510,15 @@ retry:
 	if (ret)
 		goto out;
 
+	ret = scoutfs_acl_chmod_locked(inode, attr, lock, &ind_locks);
+	if (ret < 0)
+		goto release;
+
 	setattr_copy(inode, attr);
 	inode_inc_iversion(inode);
 	scoutfs_update_inode_item(inode, lock, &ind_locks);
 
+release:
 	scoutfs_release_trans(sb);
 	scoutfs_inode_index_unlock(sb, &ind_locks);
 out:
diff --git a/kmod/src/lock.c b/kmod/src/lock.c
index 9275e3a4..db682063 100644
--- a/kmod/src/lock.c
+++ b/kmod/src/lock.c
@@ -18,6 +18,7 @@
 #include <linux/mm.h>
 #include <linux/sort.h>
 #include <linux/ctype.h>
+#include <linux/posix_acl.h>
 
 #include "super.h"
 #include "lock.h"
@@ -156,6 +157,8 @@ static void invalidate_inode(struct super_block *sb, u64 ino)
 		if (!linfo->unmounting)
 			d_prune_aliases(inode);
 
+		forget_all_cached_acls(inode);
+
 		si->drop_invalidated = true;
 		if (scoutfs_lock_is_covered(sb, &si->ino_lock_cov) && inode->i_nlink > 0) {
 			iput(inode);
diff --git a/kmod/src/options.c b/kmod/src/options.c
index a447a931..9ff18c9d 100644
--- a/kmod/src/options.c
+++ b/kmod/src/options.c
@@ -29,14 +29,18 @@
 #include "inode.h"
 
 enum {
+	Opt_acl,
 	Opt_metadev_path,
+	Opt_noacl,
 	Opt_orphan_scan_delay_ms,
 	Opt_quorum_slot_nr,
 	Opt_err,
 };
 
 static const match_table_t tokens = {
+	{Opt_acl, "acl"},
 	{Opt_metadev_path, "metadev_path=%s"},
+	{Opt_noacl, "noacl"},
 	{Opt_orphan_scan_delay_ms, "orphan_scan_delay_ms=%s"},
 	{Opt_quorum_slot_nr, "quorum_slot_nr=%s"},
 	{Opt_err, NULL}
@@ -134,12 +138,20 @@ static int parse_options(struct super_block *sb, char *options, struct scoutfs_m
 		token = match_token(p, tokens, args);
 		switch (token) {
 
+		case Opt_acl:
+			sb->s_flags |= MS_POSIXACL;
+			break;
+
 		case Opt_metadev_path:
 			ret = parse_bdev_path(sb, &args[0], &opts->metadev_path);
 			if (ret < 0)
 				return ret;
 			break;
 
+		case Opt_noacl:
+			sb->s_flags &= ~MS_POSIXACL;
+			break;
+
 		case Opt_orphan_scan_delay_ms:
 			if (opts->orphan_scan_delay_ms != -1) {
 				scoutfs_err(sb, "multiple orphan_scan_delay_ms options provided, only provide one.");
@@ -250,10 +262,15 @@ int scoutfs_options_show(struct seq_file *seq, struct dentry *root)
 {
 	struct super_block *sb = root->d_sb;
 	struct scoutfs_mount_options opts;
+	const bool is_acl = !!(sb->s_flags & MS_POSIXACL);
 
 	scoutfs_options_read(sb, &opts);
 
+	if (is_acl)
+		seq_puts(seq, ",acl");
 	seq_printf(seq, ",metadev_path=%s", opts.metadev_path);
+	if (!is_acl)
+		seq_puts(seq, ",noacl");
 	seq_printf(seq, ",orphan_scan_delay_ms=%u", opts.orphan_scan_delay_ms);
 	if (opts.quorum_slot_nr >= 0)
 		seq_printf(seq, ",quorum_slot_nr=%d", opts.quorum_slot_nr);
diff --git a/kmod/src/super.c b/kmod/src/super.c
index f786d23f..0e7fb3e8 100644
--- a/kmod/src/super.c
+++ b/kmod/src/super.c
@@ -485,7 +485,7 @@ static int scoutfs_fill_super(struct super_block *sb, void *data, int silent)
 	sb->s_op = &scoutfs_super_ops;
 	sb->s_export_op = &scoutfs_export_ops;
 	sb->s_xattr = scoutfs_xattr_handlers;
-	sb->s_flags |= MS_I_VERSION;
+	sb->s_flags |= MS_I_VERSION | MS_POSIXACL;
 
 	/* btree blocks use long lived bh->b_data refs */
 	mapping_set_gfp_mask(sb->s_bdev->bd_inode->i_mapping, GFP_NOFS);
diff --git a/kmod/src/xattr.c b/kmod/src/xattr.c
index 04a9dc0b..abdf8e0b 100644
--- a/kmod/src/xattr.c
+++ b/kmod/src/xattr.c
@@ -15,6 +15,7 @@
 #include <linux/dcache.h>
 #include <linux/xattr.h>
 #include <linux/crc32c.h>
+#include <linux/posix_acl.h>
 
 #include "format.h"
 #include "inode.h"
@@ -26,6 +27,7 @@
 #include "xattr.h"
 #include "lock.h"
 #include "hash.h"
+#include "acl.h"
 #include "scoutfs_trace.h"
 
 /*
@@ -902,11 +904,27 @@ static const struct xattr_handler scoutfs_xattr_security_handler = {
 	.set = scoutfs_xattr_set_handler,
 };
 
+static const struct xattr_handler scoutfs_xattr_acl_access_handler = {
+	.prefix = XATTR_NAME_POSIX_ACL_ACCESS,
+	.flags  = ACL_TYPE_ACCESS,
+	.get    = scoutfs_acl_get_xattr,
+	.set    = scoutfs_acl_set_xattr,
+};
+
+static const struct xattr_handler scoutfs_xattr_acl_default_handler = {
+	.prefix = XATTR_NAME_POSIX_ACL_DEFAULT,
+	.flags  = ACL_TYPE_DEFAULT,
+	.get    = scoutfs_acl_get_xattr,
+	.set    = scoutfs_acl_set_xattr,
+};
+
 const struct xattr_handler *scoutfs_xattr_handlers[] = {
 	&scoutfs_xattr_user_handler,
 	&scoutfs_xattr_scoutfs_handler,
 	&scoutfs_xattr_trusted_handler,
 	&scoutfs_xattr_security_handler,
+	&scoutfs_xattr_acl_access_handler,
+	&scoutfs_xattr_acl_default_handler,
 	NULL
 };
 
diff --git a/utils/man/scoutfs.5 b/utils/man/scoutfs.5
index f6cbe193..8b20483b 100644
--- a/utils/man/scoutfs.5
+++ b/utils/man/scoutfs.5
@@ -15,12 +15,27 @@ general mount options described in the
 .BR mount (8)
 manual page.
 .TP
+.B acl
+The acl mount option enables support for POSIX Access Control Lists
+as detailed in
+.BR acl (5) .
+Support for POSIX ACLs is the default.
+.TP
 .B metadev_path=<device>
 The metadev_path option specifies the path to the block device that
 contains the filesystem's metadata.
 .sp
 This option is required.
 .TP
+.B noacl
+The noacl mount option disables the default support for POSIX Access
+Control Lists.  Any existing system.posix_acl_default and
+system.posix_acl_access extended attributes remain in inodes.   They
+will appear in listings from
+.BR listxattr (5)
+but specific retrieval or reomval operations will fail.  They will be
+used for enforcement again if ACL support is later enabled.
+.TP
 .B orphan_scan_delay_ms=<number>
 This option sets the average expected delay, in milliseconds, between
 each mount's scan of the global orphaned inode list.  Jitter is added to