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

[Eric Paris <eparis@parisplace.org>]

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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>

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