[PATCH] Clear subdir_stations when stations directory is removed (was Re: Null pointer dereference when station associates [introduced by 4.0.5?])

[Tom Hughes <tom@compton.nu>]

On 29/06/15 11:28, Tom Hughes wrote:
> On 29/06/15 11:24, Tom Hughes wrote:
> 
>> So I think this happens when hostapd switches the interface
>> to AP mode, which causes the netdev to be torn down and then
>> recreated, and the debugfs directory along with it.
>>
>> Except that if the netlink message to change the mode was
>> sent from a daemon whose selinux context prevents searching
>> debugfs the recreation somehow fails and leaves an invalid
>> state that later causes the null pointer deref.
> 
> Think I have it...
> 
> The teardown runs ieee80211_debugfs_remove_netdev
> which clears sdata->vif.debugfs_dir but does not clear 
> sdata->debugfs.subdir_stations so that when ieee80211_debugfs_add_netdev 
> later fails to create the top level
> netdev directory we are left with a bogus pointer for the stations 
> directory.
> 
> Then when we try and add an entry to the stations directory things blow up.

Here's a proposed patch. I have booted 4.0.6 with this applied and so far
it hasn't failed even with selinux in enforcing mode.

commit 30624496e9f411081d7ea1a407deabe0e32d0c62
Author: Tom Hughes <tom@compton.nu>
Date:   Mon Jun 29 11:31:04 2015 +0100

    Clear subdir_stations when stations directory is removed
    
    If we don't do this, and we then fail to recreate the debugfs
    directory during a mode change, then we will fail later trying
    to add stations to this now bogus directory:
    
    BUG: unable to handle kernel NULL pointer dereference at 0000006c
    IP: [<c0a92202>] mutex_lock+0x12/0x30
    Call Trace:
    [<c0678ab4>] start_creating+0x44/0xc0
    [<c0679203>] debugfs_create_dir+0x13/0xf0
    [<f8a938ae>] ieee80211_sta_debugfs_add+0x6e/0x490 [mac80211]
    
    Signed-off-by: Tom Hughes <tom@compton.nu>

diff --git a/net/mac80211/debugfs_netdev.c b/net/mac80211/debugfs_netdev.c
index 29236e8..c09c013 100644
--- a/net/mac80211/debugfs_netdev.c
+++ b/net/mac80211/debugfs_netdev.c
@@ -723,6 +723,7 @@ void ieee80211_debugfs_remove_netdev(struct ieee80211_sub_if_data *sdata)
 
        debugfs_remove_recursive(sdata->vif.debugfs_dir);
        sdata->vif.debugfs_dir = NULL;
+       sdata->debugfs.subdir_stations = NULL;
 }
 
 void ieee80211_debugfs_rename_netdev(struct ieee80211_sub_if_data *sdata)

Tom

-- 
Tom Hughes (tom@compton.nu)
http://compton.nu/