[FOR STABLE 4.1] ext4 crypto: don't regenerate the per-inode encryption key unnecessarily

Theodore Ts'o
2017-10-08T18:09:55Z

This isn't a backport of the upstream commit; rather, it's a complete
rewrite. An explanation of why is included in the patch description.

>From e421d2269a66540cddddfd8d85902e7ecf4605df Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso@mit.edu>
Date: Sun, 8 Oct 2017 13:32:21 -0400
Subject: [PATCH] ext4 crypto: don't regenerate the per-inode encryption key unnecessarily

[ Relevant upstream commit: 1b53cf9815bb4744958d41f3795d5d5a1d365e2d ]

This fixes the same problem as upstream commit 1b53cf9815bb: "fscrypt:
remove broken support for detecting keyring key revocation".
Specifically, key revocations racing with readpage operations will
cause the kernel to crash and burn with a BUG_ON or a NULL pointer
dereference in a block I/O callback stemming from an ext4_readpage()
operation.

This fix is needed to fix prevent xfstests test runs from crashing
while running the generic/421 test.

The root cause is different in the 4.1 kernel, however, since the
4.1's encryption handling is so _primitive_ compared to later kernels.
The code isn't actually explicitly checking for revoked keys.
Instead, the code is neededly regenerating the per-file encryption key
on every mmap() or open() or directory operation (in the case of a
directory inode). Yelch!

If the file is already opened and actively being read, and there is a
racing open() after the user's master key has been revoked, there will
be the same net effect as the problem fixed by upstream commit
1b53cf9815bb --- the per-file key will be marked as invalid and this
will cause a BUG_ON.

In the AOSP 3.18 and 4.4 android-common kernels, the more modern
version of ext4 encryption have been backported, including a backport
of upstream commit 1b53cf9815bb. This is a dozen plus commits, and
isn't really suitable for the Upstream LTS kernel. So instead, this
is the simplest bug which fixes the same high-level issue as the
upstream commit, without dragging in all of the other non-bug fixes
improvements to the ext4 encryption code found in newer kernels.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
fs/ext4/crypto_key.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/crypto_key.c b/fs/ext4/crypto_key.c
index 52170d0b7c40..c8c3bf0286be 100644
--- a/fs/ext4/crypto_key.c
+++ b/fs/ext4/crypto_key.c
@@ -99,9 +99,14 @@ int ext4_generate_encryption_key(struct inode *inode)
struct ext4_encryption_context ctx;
struct user_key_payload *ukp;
struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
- int res = ext4_xattr_get(inode, EXT4_XATTR_INDEX_ENCRYPTION,
- EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
- &ctx, sizeof(ctx));
+ int res;
+
+ if (ext4_has_encryption_key(inode))
+ return 0;
+
+ res = ext4_xattr_get(inode, EXT4_XATTR_INDEX_ENCRYPTION,
+ EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
+ &ctx, sizeof(ctx));

if (res != sizeof(ctx)) {
if (res > 0)
--
2.11.0.rc0.7.gbe5a750