ext4 crypto: reorganize how we store keys in the inode

This is a pretty massive patch which does a number of different things:

1) The per-inode encryption information is now stored in an allocated
   data structure, ext4_crypt_info, instead of directly in the node.
   This reduces the size usage of an in-memory inode when it is not
   using encryption.

2) We drop the ext4_fname_crypto_ctx entirely, and use the per-inode
   encryption structure instead.  This remove an unnecessary memory
   allocation and free for the fname_crypto_ctx as well as allowing us
   to reuse the ctfm in a directory for multiple lookups and file
   creations.

3) We also cache the inode's policy information in the ext4_crypt_info
   structure so we don't have to continually read it out of the
   extended attributes.

4) We now keep the keyring key in the inode's encryption structure
   instead of releasing it after we are done using it to derive the
   per-inode key.  This allows us to test to see if the key has been
   revoked; if it has, we prevent the use of the derived key and free
   it.

5) When an inode is released (or when the derived key is freed), we
   will use memset_explicit() to zero out the derived key, so it's not
   left hanging around in memory.  This implies that when a user logs
   out, it is important to first revoke the key, and then unlink it,
   and then finally, to use "echo 3 > /proc/sys/vm/drop_caches" to
   release any decrypted pages and dcache entries from the system
   caches.

6) All this, and we also shrink the number of lines of code by around
   100.  :-)

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 5 insertions, 10 deletions

diff --git a/fs/ext4/symlink.c b/fs/ext4/symlink.c
index ca65d45c7f8d..32870881188e 100644
--- a/fs/ext4/symlink.c
+++ b/fs/ext4/symlink.c
@@ -29,7 +29,6 @@ static void *ext4_follow_link(struct dentry *dentry, struct nameidata *nd)
 	char *caddr, *paddr = NULL;
 	struct ext4_str cstr, pstr;
 	struct inode *inode = d_inode(dentry);
-	struct ext4_fname_crypto_ctx *ctx = NULL;
 	struct ext4_encrypted_symlink_data *sd;
 	loff_t size = min_t(loff_t, i_size_read(inode), PAGE_SIZE - 1);
 	int res;
@@ -38,19 +37,17 @@ static void *ext4_follow_link(struct dentry *dentry, struct nameidata *nd)
 	if (!ext4_encrypted_inode(inode))
 		return page_follow_link_light(dentry, nd);
 
-	ctx = ext4_get_fname_crypto_ctx(inode, inode->i_sb->s_blocksize);
-	if (IS_ERR(ctx))
-		return ctx;
+	res = ext4_setup_fname_crypto(inode);
+	if (res)
+		return ERR_PTR(res);
 
 	if (ext4_inode_is_fast_symlink(inode)) {
 		caddr = (char *) EXT4_I(inode)->i_data;
 		max_size = sizeof(EXT4_I(inode)->i_data);
 	} else {
 		cpage = read_mapping_page(inode->i_mapping, 0, NULL);
-		if (IS_ERR(cpage)) {
-			ext4_put_fname_crypto_ctx(&ctx);
+		if (IS_ERR(cpage))
 			return cpage;
-		}
 		caddr = kmap(cpage);
 		caddr[size] = 0;
 	}
@@ -75,21 +72,19 @@ static void *ext4_follow_link(struct dentry *dentry, struct nameidata *nd)
 	}
 	pstr.name = paddr;
 	pstr.len = plen;
-	res = _ext4_fname_disk_to_usr(ctx, NULL, &cstr, &pstr);
+	res = _ext4_fname_disk_to_usr(inode, NULL, &cstr, &pstr);
 	if (res < 0)
 		goto errout;
 	/* Null-terminate the name */
 	if (res <= plen)
 		paddr[res] = '\0';
 	nd_set_link(nd, paddr);
-	ext4_put_fname_crypto_ctx(&ctx);
 	if (cpage) {
 		kunmap(cpage);
 		page_cache_release(cpage);
 	}
 	return NULL;
 errout:
-	ext4_put_fname_crypto_ctx(&ctx);
 	if (cpage) {
 		kunmap(cpage);
 		page_cache_release(cpage);