linux-kernel - [PATCH 4.9 48/63] userns: move user access out of the mutex

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Message-Id: <20180907211015.632526501@linuxfoundation.org>
Date:   Fri,  7 Sep 2018 23:10:19 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        Serge Hallyn <serge@...lyn.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 4.9 48/63] userns: move user access out of the mutex

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

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

From: Jann Horn <jannh@...gle.com>

commit 5820f140edef111a9ea2ef414ab2428b8cb805b1 upstream.

The old code would hold the userns_state_mutex indefinitely if
memdup_user_nul stalled due to e.g. a userfault region. Prevent that by
moving the memdup_user_nul in front of the mutex_lock().

Note: This changes the error precedence of invalid buf/count/*ppos vs
map already written / capabilities missing.

Fixes: 22d917d80e84 ("userns: Rework the user_namespace adding uid/gid...")
Cc: stable@...r.kernel.org
Signed-off-by: Jann Horn <jannh@...gle.com>
Acked-by: Christian Brauner <christian@...uner.io>
Acked-by: Serge Hallyn <serge@...lyn.com>
Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/user_namespace.c |   24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -649,7 +649,16 @@ static ssize_t map_write(struct file *fi
 	unsigned idx;
 	struct uid_gid_extent *extent = NULL;
 	char *kbuf = NULL, *pos, *next_line;
-	ssize_t ret = -EINVAL;
+	ssize_t ret;
+
+	/* Only allow < page size writes at the beginning of the file */
+	if ((*ppos != 0) || (count >= PAGE_SIZE))
+		return -EINVAL;
+
+	/* Slurp in the user data */
+	kbuf = memdup_user_nul(buf, count);
+	if (IS_ERR(kbuf))
+		return PTR_ERR(kbuf);
 
 	/*
 	 * The userns_state_mutex serializes all writes to any given map.
@@ -683,19 +692,6 @@ static ssize_t map_write(struct file *fi
 	if (cap_valid(cap_setid) && !file_ns_capable(file, ns, CAP_SYS_ADMIN))
 		goto out;
 
-	/* Only allow < page size writes at the beginning of the file */
-	ret = -EINVAL;
-	if ((*ppos != 0) || (count >= PAGE_SIZE))
-		goto out;
-
-	/* Slurp in the user data */
-	kbuf = memdup_user_nul(buf, count);
-	if (IS_ERR(kbuf)) {
-		ret = PTR_ERR(kbuf);
-		kbuf = NULL;
-		goto out;
-	}
-
 	/* Parse the user data */
 	ret = -EINVAL;
 	pos = kbuf;