Re: [PATCH] tmpfs: implement security.capability xattrs

[Eric Paris <eparis@parisplace.org>]

Bueller?  Bueller?  Any thoughts?  Any problems?

On Tue, Jan 11, 2011 at 4:07 PM, Eric Paris <eparis@redhat.com> wrote:
> This patch implements security.capability xattrs for tmpfs filesystems.  The
> feodra project, while trying to replace suid apps with file capabilities,
> realized that tmpfs, which is used on my build systems, does not support file
> capabilities and thus cannot be used to build packages which use file
> capabilities.  The patch only implements security.capability but there is no
> reason it could not be easily expanded to support *.* xattrs as most of the
> work is already done.  I don't know what other xattrs are in use in the world
> or if they necessarily make sense on tmpfs so I didn't make this
> implementation completely generic.
>
> 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.  Since I only
> allow security.capability obviously this list is only every 0 or 1 entry long.
> I could have been a little simpler, but then the next person having to
> implement an xattr would have to redo everything I did instead of me just
> doing 90% of their work  :)
>
> Signed-off-by: Eric Paris <eparis@redhat.com>
> ---
>
>  include/linux/shmem_fs.h |    8 +++
>  mm/shmem.c               |  112 ++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 116 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 399be5a..6f2ebb8 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -9,6 +9,13 @@
>
>  #define SHMEM_NR_DIRECT 16
>
> +struct shmem_xattr {
> +       struct list_head list; /* anchored by shmem_inode_info->xattr_list */
> +       char *name;
> +       size_t size;
> +       char value[0];
> +};
> +
>  struct shmem_inode_info {
>        spinlock_t              lock;
>        unsigned long           flags;
> @@ -19,6 +26,7 @@ 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 86cd21d..d2bacd6 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -822,6 +822,7 @@ 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);
> @@ -834,6 +835,9 @@ 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);
>        BUG_ON(inode->i_blocks);
>        shmem_free_inode(inode->i_sb);
>        end_writeback(inode);
> @@ -1597,6 +1601,7 @@ 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) {
> @@ -2071,24 +2076,123 @@ 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 *shmem_i;
> +       size_t used;
> +       char *buf = NULL;
> +
> +       used = security_inode_listsecurity(dentry->d_inode, list, list_len);
> +
> +       shmem_i = SHMEM_I(dentry->d_inode);
> +       if (list)
> +               buf = list + used;
> +
> +       spin_lock(&dentry->d_inode->i_lock);
> +       list_for_each_entry(xattr, &shmem_i->xattr_list, list) {
> +               size_t len = XATTR_SECURITY_PREFIX_LEN;
> +               len += strlen(xattr->name) + 1;
> +               if (list_len - (used + len) >= 0 && buf) {
> +                       strncpy(buf, XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN);
> +                       buf += XATTR_SECURITY_PREFIX_LEN;
> +                       strncpy(buf, xattr->name, strlen(xattr->name) + 1);
> +                       buf += strlen(xattr->name) + 1;
> +               }
> +               used += len;
> +       }
> +       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)
>  {
> +       struct shmem_inode_info *shmem_i;
> +       struct shmem_xattr *xattr;
> +       int ret;
> +
>        if (strcmp(name, "") == 0)
>                return -EINVAL;
> -       return xattr_getsecurity(dentry->d_inode, name, buffer, size);
> +
> +       ret = xattr_getsecurity(dentry->d_inode, name, buffer, size);
> +       if (ret != -EOPNOTSUPP)
> +               return ret;
> +
> +       /* if we make this generic this needs to go... */
> +       if (strcmp(name, XATTR_CAPS_SUFFIX))
> +               return -EOPNOTSUPP;
> +
> +       ret = -ENODATA;
> +       shmem_i = SHMEM_I(dentry->d_inode);
> +
> +       spin_lock(&dentry->d_inode->i_lock);
> +       list_for_each_entry(xattr, &shmem_i->xattr_list, list) {
> +               if (!strcmp(name, xattr->name)) {
> +                       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;
>  }
>
>  static int shmem_xattr_security_set(struct dentry *dentry, const char *name,
>                const void *value, size_t size, int flags, int handler_flags)
>  {
> +       int ret;
> +       struct inode *inode = dentry->d_inode;
> +       struct shmem_inode_info *shmem_i = SHMEM_I(inode);
> +       struct shmem_xattr *xattr;
> +       struct shmem_xattr *new_xattr;
> +       size_t len;
> +
>        if (strcmp(name, "") == 0)
>                return -EINVAL;
> -       return security_inode_setsecurity(dentry->d_inode, name, value,
> -                                         size, flags);
> +       ret = security_inode_setsecurity(inode, name, value, size, flags);
> +       if (ret != -EOPNOTSUPP)
> +               return ret;
> +
> +       /*
> +        * We only store fcaps for now, but this could be a lot more generic.
> +        * We could hold the prefix as well as the suffix in the xattr struct
> +        * We would also need to hold a copy of the suffix rather than a
> +        * pointer to XATTR_CAPS_SUFFIX
> +        */
> +       if (strcmp(name, XATTR_CAPS_SUFFIX))
> +               return -EOPNOTSUPP;
> +
> +       /* wrap around? */
> +       len = sizeof(*new_xattr) + size;
> +       if (len <= sizeof(*new_xattr))
> +               return -ENOMEM;
> +
> +       new_xattr = kmalloc(GFP_NOFS, len);
> +       if (!new_xattr)
> +               return -ENOMEM;
> +
> +       new_xattr->name = XATTR_CAPS_SUFFIX;
> +       new_xattr->size = size;
> +       memcpy(new_xattr->value, value, size);
> +
> +       spin_lock(&inode->i_lock);
> +       list_for_each_entry(xattr, &shmem_i->xattr_list, list) {
> +               if (!strcmp(name, xattr->name)) {
> +                       list_replace(&xattr->list, &new_xattr->list);
> +                       goto out;
> +               }
> +       }
> +       list_add(&new_xattr->list, &shmem_i->xattr_list);
> +       xattr = NULL;
> +out:
> +       spin_unlock(&inode->i_lock);
> +       kfree(xattr);
> +       return 0;
>  }
>
>  static const struct xattr_handler shmem_xattr_security_handler = {
>
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