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    SubjectRe: [PATCH] tmpfs: implement security.capability xattrs
    From
    I know there exist thoughts on this patch somewhere on the internets.
    Let 'em rip! I can handle it!

    -Eric

    On Thu, Feb 17, 2011 at 4:27 PM, Eric Paris <eparis@parisplace.org> wrote:
    > 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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