Re: [PATCH] tmpfs: implement security.capability xattrs
From: Eric Paris
Date: Wed Mar 02 2011 - 14:30:09 EST
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@xxxxxxxxxxxxxx> wrote:
> Bueller? Bueller? Any thoughts? Any problems?
>
> On Tue, Jan 11, 2011 at 4:07 PM, Eric Paris <eparis@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>>
>> 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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