linux-kernel - [PATCH 1/5] devtmpfs: fix theoretical stale pointer deref in devtmpfsd()

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Message-Id: <20200115184154.3492-2-linux@rasmusvillemoes.dk>
Date:   Wed, 15 Jan 2020 19:41:49 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/5] devtmpfs: fix theoretical stale pointer deref in devtmpfsd()

After complete(&setup_done), devtmpfs_init proceeds and may actually
return, invalidating the *err pointer, before devtmpfsd() proceeds to
reading back *err.

This is of course completely theoretical since the error conditions
never trigger in practice, and even if they did, nobody cares about
the exit value from a kernel thread, so it doesn't matter if we happen
to read back some garbage from some other stack frame. Still, this
isn't a pattern that should be copy-pasted, so fix it.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 drivers/base/devtmpfs.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index 6cdbf1531238..ccb046fe12b7 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -390,12 +390,13 @@ static int handle(const char *name, umode_t mode, kuid_t uid, kgid_t gid,
 
 static int devtmpfsd(void *p)
 {
-	int *err = p;
-	*err = ksys_unshare(CLONE_NEWNS);
-	if (*err)
+	int err;
+
+	err = ksys_unshare(CLONE_NEWNS);
+	if (err)
 		goto out;
-	*err = do_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, NULL);
-	if (*err)
+	err = do_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, NULL);
+	if (err)
 		goto out;
 	ksys_chdir("/.."); /* will traverse into overmounted root */
 	ksys_chroot(".");
@@ -421,8 +422,9 @@ static int devtmpfsd(void *p)
 	}
 	return 0;
 out:
+	*(int *)p = err;
 	complete(&setup_done);
-	return *err;
+	return err;
 }
 
 /*
-- 
2.23.0