userns: move user access out of the mutex

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>

1 files changed, 10 insertions, 14 deletions

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index c3d7583fcd21..e5222b5fb4fe 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -859,7 +859,16 @@ static ssize_t map_write(struct file *file, const char __user *buf,
         unsigned idx;
         struct uid_gid_extent extent;
         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.
@@ -895,19 +904,6 @@ 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;