userns: move user access out of the mutex

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@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Christian Brauner <christian@brauner.io>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 86b7854..f789bbb 100644 (file)
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -649,7 +649,16 @@ static ssize_t map_write(struct file *file, const char __user *buf,
        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 *file, const char __user *buf,
        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;