[PATCH] userns: move user access out of the mutex

[Jann Horn]

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: sta...@vger.kernel.org
Signed-off-by: Jann Horn <ja...@google.com>
---
 kernel/user_namespace.c | 24 ++++++++++--------------
 1 file 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;
-- 
2.18.0.rc2.346.g013aa6912e-goog