[PATCH for 6.8] tomoyo: fix UAF write bug in tomoyo_write_control()

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Since tomoyo_write_control() updates head->write_buf when write() of long
lines is requested, we need to fetch head->write_buf after head->io_sem is
held. Otherwise, concurrent write() requests can cause use-after-free-write
and double-free problems.

Reported-by: Sam Sun <samsun1006219@gmail.com>
Closes: https://lkml.kernel.org/r/CAEkJfYNDspuGxYx5kym8Lvp--D36CMDUErg4rxfWFJuPbbji8g@mail.gmail.com
Fixes: bd03a3e4c9a9 ("TOMOYO: Add policy namespace support.")
Cc: stable@vger.kernel.org # Linux 3.1+
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
I couldn't reproduce this problem in my environment, but I believe
this does fix a bug. Linus, can you directly apply to linux.git ?
If Linus wants a GIT PULL request, can Paul send this patch via LSM tree
because TOMOYO's git tree is not working?

 security/tomoyo/common.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/security/tomoyo/common.c b/security/tomoyo/common.c
index 57ee70ae50f2..ea3140d510ec 100644
--- a/security/tomoyo/common.c
+++ b/security/tomoyo/common.c
@@ -2649,13 +2649,14 @@ ssize_t tomoyo_write_control(struct tomoyo_io_buffer *head,
 {
 	int error = buffer_len;
 	size_t avail_len = buffer_len;
-	char *cp0 = head->write_buf;
+	char *cp0;
 	int idx;
 
 	if (!head->write)
 		return -EINVAL;
 	if (mutex_lock_interruptible(&head->io_sem))
 		return -EINTR;
+	cp0 = head->write_buf;
 	head->read_user_buf_avail = 0;
 	idx = tomoyo_read_lock();
 	/* Read a line and dispatch it to the policy handler. */
-- 
2.34.1