Re: [PATCH] fscrypt: remove broken support for detecting keyring key revocation

[Michael Halcrow]

On Tue, Feb 21, 2017 at 03:07:11PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
> 
> Filesystem encryption ostensibly supported revoking a keyring key
> that had been used to "unlock" encrypted files, causing those files
> to become "locked" again. This was, however, buggy for several
> reasons, the most severe of which was that when key revocation
> happened to be detected for an inode, its fscrypt_info was
> immediately freed, even while other threads could be using it for
> encryption or decryption concurrently.  This could be exploited to
> crash the kernel or worse.

Removing the attempt at that functionality seems like the right
approach.

> This patch fixes the use-after-free by removing the code which
> detects the keyring key having been revoked, invalidated, or
> expired.  Instead, an encrypted inode that is "unlocked" now simply
> remains unlocked until it is evicted from memory.  Note that this is
> no worse than the case for block device-level encryption,
> e.g. dm-crypt, and it still remains possible for a privileged user
> to evict unused pages, inodes, and dentries by running 'sync; echo 3
> > /proc/sys/vm/drop_caches', or by simply unmounting the filesystem.
> In fact, one of those actions was already needed anyway for key
> revocation to work even somewhat sanely.  This change is not
> expected to break any applications.

I don't see any problem with this reasoning.

> In the future I'd like to implement a real API for fscrypt key
> revocation that interacts sanely with ongoing filesystem operations ---
> waiting for existing operations to complete and blocking new operations,
> and invalidating and sanitizing key material and plaintext from the VFS
> caches.  But this is a hard problem, and for now this bug must be fixed.
> 
> This bug affected almost all versions of ext4, f2fs, and ubifs
> encryption, and it was potentially reachable in any kernel configured
> with encryption support (CONFIG_EXT4_ENCRYPTION=y,
> CONFIG_EXT4_FS_ENCRYPTION=y, CONFIG_F2FS_FS_ENCRYPTION=y, or
> CONFIG_UBIFS_FS_ENCRYPTION=y).  Note that older kernels did not use the
> shared fs/crypto/ code, but due to the potential security implications
> of this bug, it may still be worthwhile to backport this fix to them.

Agreed.

> Fixes: b7236e21d55f ("ext4 crypto: reorganize how we store keys in the inode")
> Cc: stable@xxxxxxxxxxxxxxx # v4.2+
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

Acked-by: Michael Halcrow <mhalcrow@xxxxxxxxxx>

> ---
>  fs/crypto/crypto.c          | 10 +--------
>  fs/crypto/fname.c           |  2 +-
>  fs/crypto/fscrypt_private.h |  4 ----
>  fs/crypto/keyinfo.c         | 52 ++++++++-------------------------------------
>  4 files changed, 11 insertions(+), 57 deletions(-)
> 
> diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
> index 02a7a9286449..6d6eca394d4d 100644
> --- a/fs/crypto/crypto.c
> +++ b/fs/crypto/crypto.c
> @@ -327,7 +327,6 @@ EXPORT_SYMBOL(fscrypt_decrypt_page);
>  static int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags)
>  {
>  	struct dentry *dir;
> -	struct fscrypt_info *ci;
>  	int dir_has_key, cached_with_key;
>  
>  	if (flags & LOOKUP_RCU)
> @@ -339,18 +338,11 @@ static int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags)
>  		return 0;
>  	}
>  
> -	ci = d_inode(dir)->i_crypt_info;
> -	if (ci && ci->ci_keyring_key &&
> -	    (ci->ci_keyring_key->flags & ((1 << KEY_FLAG_INVALIDATED) |
> -					  (1 << KEY_FLAG_REVOKED) |
> -					  (1 << KEY_FLAG_DEAD))))
> -		ci = NULL;
> -
>  	/* this should eventually be an flag in d_flags */
>  	spin_lock(&dentry->d_lock);
>  	cached_with_key = dentry->d_flags & DCACHE_ENCRYPTED_WITH_KEY;
>  	spin_unlock(&dentry->d_lock);
> -	dir_has_key = (ci != NULL);
> +	dir_has_key = (d_inode(dir)->i_crypt_info != NULL);
>  	dput(dir);
>  
>  	/*
> diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
> index 13052b85c393..37b49894c762 100644
> --- a/fs/crypto/fname.c
> +++ b/fs/crypto/fname.c
> @@ -350,7 +350,7 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
>  		fname->disk_name.len = iname->len;
>  		return 0;
>  	}
> -	ret = fscrypt_get_crypt_info(dir);
> +	ret = fscrypt_get_encryption_info(dir);
>  	if (ret && ret != -EOPNOTSUPP)
>  		return ret;
>  
> diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
> index fdbb8af32eaf..e39696e64494 100644
> --- a/fs/crypto/fscrypt_private.h
> +++ b/fs/crypto/fscrypt_private.h
> @@ -67,7 +67,6 @@ struct fscrypt_info {
>  	u8 ci_filename_mode;
>  	u8 ci_flags;
>  	struct crypto_skcipher *ci_ctfm;
> -	struct key *ci_keyring_key;
>  	u8 ci_master_key[FS_KEY_DESCRIPTOR_SIZE];
>  };
>  
> @@ -101,7 +100,4 @@ extern int fscrypt_do_page_crypto(const struct inode *inode,
>  extern struct page *fscrypt_alloc_bounce_page(struct fscrypt_ctx *ctx,
>  					      gfp_t gfp_flags);
>  
> -/* keyinfo.c */
> -extern int fscrypt_get_crypt_info(struct inode *);
> -
>  #endif /* _FSCRYPT_PRIVATE_H */
> diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
> index 02eb6b9e4438..cb3e82abf034 100644
> --- a/fs/crypto/keyinfo.c
> +++ b/fs/crypto/keyinfo.c
> @@ -95,6 +95,7 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
>  	kfree(description);
>  	if (IS_ERR(keyring_key))
>  		return PTR_ERR(keyring_key);
> +	down_read(&keyring_key->sem);
>  
>  	if (keyring_key->type != &key_type_logon) {
>  		printk_once(KERN_WARNING
> @@ -102,11 +103,9 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
>  		res = -ENOKEY;
>  		goto out;
>  	}
> -	down_read(&keyring_key->sem);
>  	ukp = user_key_payload(keyring_key);
>  	if (ukp->datalen != sizeof(struct fscrypt_key)) {
>  		res = -EINVAL;
> -		up_read(&keyring_key->sem);
>  		goto out;
>  	}
>  	master_key = (struct fscrypt_key *)ukp->data;
> @@ -117,17 +116,11 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
>  				"%s: key size incorrect: %d\n",
>  				__func__, master_key->size);
>  		res = -ENOKEY;
> -		up_read(&keyring_key->sem);
>  		goto out;
>  	}
>  	res = derive_key_aes(ctx->nonce, master_key->raw, raw_key);
> -	up_read(&keyring_key->sem);
> -	if (res)
> -		goto out;
> -
> -	crypt_info->ci_keyring_key = keyring_key;
> -	return 0;
>  out:
> +	up_read(&keyring_key->sem);
>  	key_put(keyring_key);
>  	return res;
>  }
> @@ -169,12 +162,11 @@ static void put_crypt_info(struct fscrypt_info *ci)
>  	if (!ci)
>  		return;
>  
> -	key_put(ci->ci_keyring_key);
>  	crypto_free_skcipher(ci->ci_ctfm);
>  	kmem_cache_free(fscrypt_info_cachep, ci);
>  }
>  
> -int fscrypt_get_crypt_info(struct inode *inode)
> +int fscrypt_get_encryption_info(struct inode *inode)
>  {
>  	struct fscrypt_info *crypt_info;
>  	struct fscrypt_context ctx;
> @@ -184,21 +176,15 @@ int fscrypt_get_crypt_info(struct inode *inode)
>  	u8 *raw_key = NULL;
>  	int res;
>  
> +	if (inode->i_crypt_info)
> +		return 0;
> +
>  	res = fscrypt_initialize(inode->i_sb->s_cop->flags);
>  	if (res)
>  		return res;
>  
>  	if (!inode->i_sb->s_cop->get_context)
>  		return -EOPNOTSUPP;
> -retry:
> -	crypt_info = ACCESS_ONCE(inode->i_crypt_info);
> -	if (crypt_info) {
> -		if (!crypt_info->ci_keyring_key ||
> -				key_validate(crypt_info->ci_keyring_key) == 0)
> -			return 0;
> -		fscrypt_put_encryption_info(inode, crypt_info);
> -		goto retry;
> -	}
>  
>  	res = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx));
>  	if (res < 0) {
> @@ -229,7 +215,6 @@ int fscrypt_get_crypt_info(struct inode *inode)
>  	crypt_info->ci_data_mode = ctx.contents_encryption_mode;
>  	crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;
>  	crypt_info->ci_ctfm = NULL;
> -	crypt_info->ci_keyring_key = NULL;
>  	memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
>  				sizeof(crypt_info->ci_master_key));
>  
> @@ -273,14 +258,8 @@ int fscrypt_get_crypt_info(struct inode *inode)
>  	if (res)
>  		goto out;
>  
> -	kzfree(raw_key);
> -	raw_key = NULL;
> -	if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) != NULL) {
> -		put_crypt_info(crypt_info);
> -		goto retry;
> -	}
> -	return 0;
> -
> +	if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) == NULL)
> +		crypt_info = NULL;
>  out:
>  	if (res == -ENOKEY)
>  		res = 0;
> @@ -288,6 +267,7 @@ int fscrypt_get_crypt_info(struct inode *inode)
>  	kzfree(raw_key);
>  	return res;
>  }
> +EXPORT_SYMBOL(fscrypt_get_encryption_info);
>  
>  void fscrypt_put_encryption_info(struct inode *inode, struct fscrypt_info *ci)
>  {
> @@ -305,17 +285,3 @@ void fscrypt_put_encryption_info(struct inode *inode, struct fscrypt_info *ci)
>  	put_crypt_info(ci);
>  }
>  EXPORT_SYMBOL(fscrypt_put_encryption_info);
> -
> -int fscrypt_get_encryption_info(struct inode *inode)
> -{
> -	struct fscrypt_info *ci = inode->i_crypt_info;
> -
> -	if (!ci ||
> -		(ci->ci_keyring_key &&
> -		 (ci->ci_keyring_key->flags & ((1 << KEY_FLAG_INVALIDATED) |
> -					       (1 << KEY_FLAG_REVOKED) |
> -					       (1 << KEY_FLAG_DEAD)))))
> -		return fscrypt_get_crypt_info(inode);
> -	return 0;
> -}
> -EXPORT_SYMBOL(fscrypt_get_encryption_info);
> -- 
> 2.11.0.483.g087da7b7c-goog
>