This is a note to let you know that I've just added the patch titled
fscrypt: lock mutex before checking for bounce page pool
to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
fscrypt-lock-mutex-before-checking-for-bounce-page-pool.patch
and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From a0b3bc855374c50b5ea85273553485af48caf2f7 Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@xxxxxxxxxx>
Date: Sun, 29 Oct 2017 06:30:19 -0400
Subject: fscrypt: lock mutex before checking for bounce page pool
From: Eric Biggers <ebiggers@xxxxxxxxxx>
commit a0b3bc855374c50b5ea85273553485af48caf2f7 upstream.
fscrypt_initialize(), which allocates the global bounce page pool when
an encrypted file is first accessed, uses "double-checked locking" to
try to avoid locking fscrypt_init_mutex. However, it doesn't use any
memory barriers, so it's theoretically possible for a thread to observe
a bounce page pool which has not been fully initialized. This is a
classic bug with "double-checked locking".
While "only a theoretical issue" in the latest kernel, in pre-4.8
kernels the pointer that was checked was not even the last to be
initialized, so it was easily possible for a crash (NULL pointer
dereference) to happen. This was changed only incidentally by the large
refactor to use fs/crypto/.
Solve both problems in a trivial way that can easily be backported: just
always take the mutex. It's theoretically less efficient, but it
shouldn't be noticeable in practice as the mutex is only acquired very
briefly once per encrypted file.
Later I'd like to make this use a helper macro like DO_ONCE(). However,
DO_ONCE() runs in atomic context, so we'd need to add a new macro that
allows blocking.
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/crypto/crypto.c | 3 ---
1 file changed, 3 deletions(-)
--- a/fs/crypto/crypto.c
+++ b/fs/crypto/crypto.c
@@ -484,9 +484,6 @@ int fscrypt_initialize(void)
{
int i, res = -ENOMEM;
- if (fscrypt_bounce_page_pool)
- return 0;
-
mutex_lock(&fscrypt_init_mutex);
if (fscrypt_bounce_page_pool)
goto already_initialized;
Patches currently in stable-queue which might be from ebiggers@xxxxxxxxxx are
queue-4.9/lib-mpi-call-cond_resched-from-mpi_powm-loop.patch
queue-4.9/fscrypt-lock-mutex-before-checking-for-bounce-page-pool.patch
queue-4.9/dm-bufio-fix-integer-overflow-when-limiting-maximum-cache-size.patch
queue-4.9/libceph-don-t-warn-if-user-tries-to-add-invalid-key.patch