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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Johannes Berg <johannes.berg@intel.com>,
        Richard Weinberger <richard@nod.at>,
        Nathan Chancellor <nathan@kernel.org>
Subject: [PATCH 4.9 094/167] um: chan_user: Fix winch_tramp() return value
Date:   Mon, 13 Jun 2022 12:09:28 +0200
Message-Id: <20220613094902.980862392@linuxfoundation.org>
In-Reply-To: <20220613094840.720778945@linuxfoundation.org>
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From: Johannes Berg <johannes.berg@intel.com>

commit 57ae0b67b747031bc41fb44643aa5344ab58607e upstream.

The previous fix here was only partially correct, it did
result in returning a proper error value in case of error,
but it also clobbered the pid that we need to return from
this function (not just zero for success).

As a result, it returned 0 here, but later this is treated
as a pid and used to kill the process, but since it's now
0 we kill(0, SIGKILL), which makes UML kill itself rather
than just the helper thread.

Fix that and make it more obvious by using a separate
variable for the pid.

Fixes: ccf1236ecac4 ("um: fix error return code in winch_tramp()")
Reported-and-tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/um/drivers/chan_user.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

--- a/arch/um/drivers/chan_user.c
+++ b/arch/um/drivers/chan_user.c
@@ -220,7 +220,7 @@ static int winch_tramp(int fd, struct tt
 		       unsigned long *stack_out)
 {
 	struct winch_data data;
-	int fds[2], n, err;
+	int fds[2], n, err, pid;
 	char c;
 
 	err = os_pipe(fds, 1, 1);
@@ -238,8 +238,9 @@ static int winch_tramp(int fd, struct tt
 	 * problem with /dev/net/tun, which if held open by this
 	 * thread, prevents the TUN/TAP device from being reused.
 	 */
-	err = run_helper_thread(winch_thread, &data, CLONE_FILES, stack_out);
-	if (err < 0) {
+	pid = run_helper_thread(winch_thread, &data, CLONE_FILES, stack_out);
+	if (pid < 0) {
+		err = pid;
 		printk(UM_KERN_ERR "fork of winch_thread failed - errno = %d\n",
 		       -err);
 		goto out_close;
@@ -263,7 +264,7 @@ static int winch_tramp(int fd, struct tt
 		goto out_close;
 	}
 
-	return err;
+	return pid;
 
  out_close:
 	close(fds[1]);