[PATCH] tmpfs: implement xattr support for the entire securitynamespace

[Eric Paris]

This patch implements security namespace xattrs for tmpfs filesystems.  The
feodra project, while trying to replace suid apps with file capabilities,
realized that tmpfs, which is used on the build systems, does not support file
capabilities and thus cannot be used to build packages which use file
capabilities.

The xattr interface is a bit, odd.  If a filesystem does not implement any
{get,set,list}xattr functions the VFS will call into some random LSM hooks and
the running LSM can then implement some method for handling xattrs.  SELinux
for example provides a method to support security.selinux but no other
security.* xattrs.

As it stands today when one enables CONFIG_TMPFS_POSIX_ACL tmpfs will have
xattr handler routines specifically to handle acls.  Because of this tmpfs
would loose the VFS/LSM helpers to support the running LSM.  To make up for
that tmpfs had stub functions that did nothing but call into the LSM hooks
which implement the helpers.

This new patch does not use the LSM fallback functions and instead just
implements a native get/set/list xattr feature for the full security.*
namespace like a normal filesystem.  This means that tmpfs can now support
both security.selinux and security.capability, which was not previously
possible.

The basic implementation is that I attach a:

struct shmem_xattr {
	struct list_head list; /* anchored by shmem_inode_info->xattr_list */
	char *name;
	size_t size;
	char value[0];
};

Into the struct shmem_inode_info for each xattr that is set.  This
implementation could easily be turned into 2d array with one dimention being
the xattr prefix and one the xattr suffix.  That could result in an easy
implementation for user.* if we ever want it.  As it stands today though I
assume the prefix is always security.

Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
---

 fs/Kconfig               |    4 ++
 include/linux/shmem_fs.h |    1 
 mm/shmem.c               |  116 ++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 111 insertions(+), 10 deletions(-)

diff --git a/fs/Kconfig b/fs/Kconfig
index f3aa9b0..5e2bfc4 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -132,6 +132,10 @@ config TMPFS_POSIX_ACL
 	  To learn more about Access Control Lists, visit the POSIX ACLs for
 	  Linux website <http://acl.bestbits.at/>.
 
+	  Enablings this option will also enable support for the entire
+	  security.* xattr namespace.  This is to make up for support lost
+	  from the VFS/LSM when a filesystem has any xattr handler.
+
 	  If you don't know what Access Control Lists are, say N.
 
 config HUGETLBFS
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 399be5a..20912d1 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -19,6 +19,7 @@ struct shmem_inode_info {
 	struct page		*i_indirect;	/* top indirect blocks page */
 	swp_entry_t		i_direct[SHMEM_NR_DIRECT]; /* first blocks */
 	struct list_head	swaplist;	/* chain of maybes on swap */
+	struct list_head	xattr_list;	/* list of shmem_xattr */
 	struct inode		vfs_inode;
 };
 
diff --git a/mm/shmem.c b/mm/shmem.c
index 58da7c1..c77634f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -99,6 +99,13 @@ static struct vfsmount *shm_mnt;
 /* Pretend that each entry is of this size in directory's i_size */
 #define BOGO_DIRENT_SIZE 20
 
+struct shmem_xattr {
+	struct list_head list;	/* anchored by shmem_inode_info->xattr_list */
+	char *name;		/* xattr suffix */
+	size_t size;
+	char value[0];
+};
+
 /* Flag allocation requirements to shmem_getpage and shmem_swp_alloc */
 enum sgp_type {
 	SGP_READ,	/* don't exceed i_size, don't allocate page */
@@ -821,6 +828,7 @@ static int shmem_notify_change(struct dentry *dentry, struct iattr *attr)
 static void shmem_evict_inode(struct inode *inode)
 {
 	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct shmem_xattr *xattr, *nxattr;
 
 	if (inode->i_mapping->a_ops == &shmem_aops) {
 		truncate_inode_pages(inode->i_mapping, 0);
@@ -833,6 +841,11 @@ static void shmem_evict_inode(struct inode *inode)
 			mutex_unlock(&shmem_swaplist_mutex);
 		}
 	}
+
+	list_for_each_entry_safe(xattr, nxattr, &info->xattr_list, list) {
+		kfree(xattr->name);
+		kfree(xattr);
+	}
 	BUG_ON(inode->i_blocks);
 	shmem_free_inode(inode->i_sb);
 	end_writeback(inode);
@@ -1595,6 +1608,7 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
 		spin_lock_init(&info->lock);
 		info->flags = flags & VM_NORESERVE;
 		INIT_LIST_HEAD(&info->swaplist);
+		INIT_LIST_HEAD(&info->xattr_list);
 		cache_no_acl(inode);
 
 		switch (mode & S_IFMT) {
@@ -2059,8 +2073,8 @@ static const struct inode_operations shmem_symlink_inode_operations = {
 
 #ifdef CONFIG_TMPFS_POSIX_ACL
 /*
- * Superblocks without xattr inode operations will get security.* xattr
- * support from the VFS "for free". As soon as we have any other xattrs
+ * Superblocks without xattr inode operations may get some security.* xattr
+ * support from the LSM "for free". As soon as we have any other xattrs
  * like ACLs, we also need to implement the security.* handlers at
  * filesystem level, though.
  */
@@ -2069,24 +2083,106 @@ static size_t shmem_xattr_security_list(struct dentry *dentry, char *list,
 					size_t list_len, const char *name,
 					size_t name_len, int handler_flags)
 {
-	return security_inode_listsecurity(dentry->d_inode, list, list_len);
+	struct shmem_xattr *xattr;
+	struct shmem_inode_info *info;
+	size_t used = 0;
+
+	info = SHMEM_I(dentry->d_inode);
+
+	spin_lock(&dentry->d_inode->i_lock);
+	list_for_each_entry(xattr, &info->xattr_list, list) {
+		used += XATTR_SECURITY_PREFIX_LEN;
+		used += strlen(xattr->name) + 1;
+
+		if (list) {
+			if (list_len < used) {
+				used = -ERANGE;
+				break;
+			}
+			strncpy(list, XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN);
+			list += XATTR_SECURITY_PREFIX_LEN;
+			strncpy(list, xattr->name, strlen(xattr->name) + 1);
+			list += strlen(xattr->name) + 1;
+		}
+	}
+	spin_unlock(&dentry->d_inode->i_lock);
+
+	return used;
 }
 
 static int shmem_xattr_security_get(struct dentry *dentry, const char *name,
 		void *buffer, size_t size, int handler_flags)
 {
-	if (strcmp(name, "") == 0)
-		return -EINVAL;
-	return xattr_getsecurity(dentry->d_inode, name, buffer, size);
+	struct shmem_inode_info *info;
+	struct shmem_xattr *xattr;
+	int ret = -ENODATA;
+
+	info = SHMEM_I(dentry->d_inode);
+
+	spin_lock(&dentry->d_inode->i_lock);
+	list_for_each_entry(xattr, &info->xattr_list, list) {
+		if (strcmp(name, xattr->name))
+			continue;
+
+		ret = xattr->size;
+		if (buffer) {
+			if (size < xattr->size)
+				ret = -ERANGE;
+			else
+				memcpy(buffer, xattr->value, xattr->size);
+		}
+		break;
+	}
+	spin_unlock(&dentry->d_inode->i_lock);
+	return ret;
 }
 
+/*
+ * We only handle security.* but we could potentially store the prefix
+ * as well as the suffix in struct shmem_xattr and support *.*
+ */
 static int shmem_xattr_security_set(struct dentry *dentry, const char *name,
 		const void *value, size_t size, int flags, int handler_flags)
 {
-	if (strcmp(name, "") == 0)
-		return -EINVAL;
-	return security_inode_setsecurity(dentry->d_inode, name, value,
-					  size, flags);
+	struct inode *inode = dentry->d_inode;
+	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct shmem_xattr *xattr;
+	struct shmem_xattr *new_xattr;
+	size_t len;
+
+	/* wrap around? */
+	len = sizeof(*new_xattr) + size;
+	if (len <= sizeof(*new_xattr))
+		return -ENOMEM;
+
+	new_xattr = kmalloc(len, GFP_NOFS);
+	if (!new_xattr)
+		return -ENOMEM;
+
+	new_xattr->name = kstrdup(name, GFP_NOFS);
+	if (!new_xattr->name) {
+		kfree(new_xattr);
+		return -ENOMEM;
+	}
+
+	new_xattr->size = size;
+	memcpy(new_xattr->value, value, size);
+
+	spin_lock(&inode->i_lock);
+	list_for_each_entry(xattr, &info->xattr_list, list) {
+		if (!strcmp(name, xattr->name)) {
+			list_replace(&xattr->list, &new_xattr->list);
+			goto out;
+		}
+	}
+	list_add(&new_xattr->list, &info->xattr_list);
+	xattr = NULL;
+out:
+	spin_unlock(&inode->i_lock);
+	if (xattr)
+		kfree(xattr->name);
+	kfree(xattr);
+	return 0;
 }
 
 static const struct xattr_handler shmem_xattr_security_handler = {

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