Commit 2c58d548 authored by Eric Biggers's avatar Eric Biggers Committed by Theodore Ts'o
Browse files

fscrypt: cache decrypted symlink target in ->i_link 



Path lookups that traverse encrypted symlink(s) are very slow because
each encrypted symlink needs to be decrypted each time it's followed.
This also involves dropping out of rcu-walk mode.

Make encrypted symlinks faster by caching the decrypted symlink target
in ->i_link.  The first call to fscrypt_get_symlink() sets it.  Then,
the existing VFS path lookup code uses the non-NULL ->i_link to take the
fast path where ->get_link() isn't called, and lookups in rcu-walk mode
remain in rcu-walk mode.

Also set ->i_link immediately when a new encrypted symlink is created.

To safely free the symlink target after an RCU grace period has elapsed,
introduce a new function fscrypt_free_inode(), and make the relevant
filesystems call it just before actually freeing the inode.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 4c4f7c19

fs/crypto/hooks.c

+33 −7
Original line number Diff line number Diff line
@@ -189,11 +189,9 @@ int __fscrypt_encrypt_symlink(struct inode *inode, const char *target, 
	sd->len = cpu_to_le16(ciphertext_len);



	err = fname_encrypt(inode, &iname, sd->encrypted_path, ciphertext_len);

	if (err) {

		if (!disk_link->name)

			kfree(sd);

		return err;

	}

	if (err)

		goto err_free_sd;



	/*

	 * Null-terminating the ciphertext doesn't make sense, but we still

	 * count the null terminator in the length, so we might as well
@@ -201,9 +199,20 @@ int __fscrypt_encrypt_symlink(struct inode *inode, const char *target, 
	 */

	sd->encrypted_path[ciphertext_len] = '\0';



	/* Cache the plaintext symlink target for later use by get_link() */

	err = -ENOMEM;

	inode->i_link = kmemdup(target, len + 1, GFP_NOFS);

	if (!inode->i_link)

		goto err_free_sd;



	if (!disk_link->name)

		disk_link->name = (unsigned char *)sd;

	return 0;



err_free_sd:

	if (!disk_link->name)

		kfree(sd);

	return err;

}

EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
@@ -212,7 +221,7 @@ EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink); 
 * @inode: the symlink inode

 * @caddr: the on-disk contents of the symlink

 * @max_size: size of @caddr buffer

 * @done: if successful, will be set up to free the returned target

 * @done: if successful, will be set up to free the returned target if needed

 *

 * If the symlink's encryption key is available, we decrypt its target.

 * Otherwise, we encode its target for presentation.
@@ -227,12 +236,18 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr, 
{

	const struct fscrypt_symlink_data *sd;

	struct fscrypt_str cstr, pstr;

	bool has_key;

	int err;



	/* This is for encrypted symlinks only */

	if (WARN_ON(!IS_ENCRYPTED(inode)))

		return ERR_PTR(-EINVAL);



	/* If the decrypted target is already cached, just return it. */

	pstr.name = READ_ONCE(inode->i_link);

	if (pstr.name)

		return pstr.name;



	/*

	 * Try to set up the symlink's encryption key, but we can continue

	 * regardless of whether the key is available or not.
@@ -240,6 +255,7 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr, 
	err = fscrypt_get_encryption_info(inode);

	if (err)

		return ERR_PTR(err);

	has_key = fscrypt_has_encryption_key(inode);



	/*

	 * For historical reasons, encrypted symlink targets are prefixed with
@@ -271,7 +287,17 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr, 
		goto err_kfree;



	pstr.name[pstr.len] = '\0';



	/*

	 * Cache decrypted symlink targets in i_link for later use.  Don't cache

	 * symlink targets encoded without the key, since those become outdated

	 * once the key is added.  This pairs with the READ_ONCE() above and in

	 * the VFS path lookup code.

	 */

	if (!has_key ||

	    cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)

		set_delayed_call(done, kfree_link, pstr.name);



	return pstr.name;



err_kfree:

fs/crypto/keyinfo.c

+21 −0
Original line number Diff line number Diff line
@@ -584,9 +584,30 @@ out: 
}

EXPORT_SYMBOL(fscrypt_get_encryption_info);



/**

 * fscrypt_put_encryption_info - free most of an inode's fscrypt data

 *

 * Free the inode's fscrypt_info.  Filesystems must call this when the inode is

 * being evicted.  An RCU grace period need not have elapsed yet.

 */

void fscrypt_put_encryption_info(struct inode *inode)

{

	put_crypt_info(inode->i_crypt_info);

	inode->i_crypt_info = NULL;

}

EXPORT_SYMBOL(fscrypt_put_encryption_info);



/**

 * fscrypt_free_inode - free an inode's fscrypt data requiring RCU delay

 *

 * Free the inode's cached decrypted symlink target, if any.  Filesystems must

 * call this after an RCU grace period, just before they free the inode.

 */

void fscrypt_free_inode(struct inode *inode)

{

	if (IS_ENCRYPTED(inode) && S_ISLNK(inode->i_mode)) {

		kfree(inode->i_link);

		inode->i_link = NULL;

	}

}

EXPORT_SYMBOL(fscrypt_free_inode);

fs/ext4/super.c

+3 −0
Original line number Diff line number Diff line
@@ -1110,6 +1110,9 @@ static int ext4_drop_inode(struct inode *inode) 
static void ext4_i_callback(struct rcu_head *head)

{

	struct inode *inode = container_of(head, struct inode, i_rcu);



	fscrypt_free_inode(inode);



	kmem_cache_free(ext4_inode_cachep, EXT4_I(inode));

}

fs/f2fs/super.c

+3 −0
Original line number Diff line number Diff line
@@ -1003,6 +1003,9 @@ static void f2fs_dirty_inode(struct inode *inode, int flags) 
static void f2fs_i_callback(struct rcu_head *head)

{

	struct inode *inode = container_of(head, struct inode, i_rcu);



	fscrypt_free_inode(inode);



	kmem_cache_free(f2fs_inode_cachep, F2FS_I(inode));

}

fs/ubifs/super.c

+3 −0
Original line number Diff line number Diff line
@@ -276,7 +276,10 @@ static void ubifs_i_callback(struct rcu_head *head) 
{

	struct inode *inode = container_of(head, struct inode, i_rcu);

	struct ubifs_inode *ui = ubifs_inode(inode);



	kfree(ui->data);

	fscrypt_free_inode(inode);



	kmem_cache_free(ubifs_inode_slab, ui);

}

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