[PATCH 5.10 22/42] tomoyo: fix UAF write bug in tomoyo_write_control()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

5.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

commit 2f03fc340cac9ea1dc63cbf8c93dd2eb0f227815 upstream.

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 security/tomoyo/common.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/security/tomoyo/common.c
+++ b/security/tomoyo/common.c
@@ -2657,13 +2657,14 @@ ssize_t tomoyo_write_control(struct tomo
 {
 	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. */