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Subject[PATCH 4.4 34/47] userns: move user access out of the mutex
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Jann Horn <jannh@google.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@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>

---
kernel/user_namespace.c | 22 ++++++++++------------
1 file changed, 10 insertions(+), 12 deletions(-)

--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -604,7 +604,16 @@ static ssize_t map_write(struct file *fi
struct uid_gid_extent *extent = NULL;
unsigned long page = 0;
char *kbuf, *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 */
+ if (copy_from_user(kbuf, buf, count))
+ return -EFAULT;
+ kbuf[count] = '\0';

/*
* The userns_state_mutex serializes all writes to any given map.
@@ -645,17 +654,6 @@ static ssize_t map_write(struct file *fi
if (!page)
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 */
- ret = -EFAULT;
- if (copy_from_user(kbuf, buf, count))
- goto out;
- kbuf[count] = '\0';
-
/* Parse the user data */
ret = -EINVAL;
pos = kbuf;

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