Quoting Eric Paris (eparis@xxxxxxxxxx):
> 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>
Thanks, Eric. I don't see any problems with this. I do wonder whether
it could save quite a bit of space to use hlist_head instead of list_head
if selinux is not enabled?
Acked-by: Serge Hallyn <serge.hallyn@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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