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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Eric Biggers <ebiggers@google.com>, Theodore Ts'o <tytso@mit.edu>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.9 140/293] ext4: require key for truncate(2) of
 encrypted file
Thread-Topic: [PATCH AUTOSEL for 4.9 140/293] ext4: require key for
 truncate(2) of encrypted file
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Date:   Mon, 9 Apr 2018 00:24:42 +0000
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From: Eric Biggers <ebiggers@google.com>

[ Upstream commit 63136858aecbe86dbd3c3289a3f46ba1b5f92239 ]

Currently, filesystems allow truncate(2) on an encrypted file without
the encryption key.  However, it's impossible to correctly handle the
case where the size being truncated to is not a multiple of the
filesystem block size, because that would require decrypting the final
block, zeroing the part beyond i_size, then encrypting the block.

As other modifications to encrypted file contents are prohibited without
the key, just prohibit truncate(2) as well, making it fail with ENOKEY.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 fs/ext4/inode.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5cccec68a0a5..58d57c56ec62 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5166,6 +5166,14 @@ int ext4_setattr(struct dentry *dentry, struct iattr=
 *attr)
 		loff_t oldsize =3D inode->i_size;
 		int shrink =3D (attr->ia_size <=3D inode->i_size);
=20
+		if (ext4_encrypted_inode(inode)) {
+			error =3D fscrypt_get_encryption_info(inode);
+			if (error)
+				return error;
+			if (!fscrypt_has_encryption_key(inode))
+				return -ENOKEY;
+		}
+
 		if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))) {
 			struct ext4_sb_info *sbi =3D EXT4_SB(inode->i_sb);
=20
--=20
2.15.1