Re: [PATCH] NFS: fix BUG() crash in notify_change() with patch to chown_common()

[Andrew W Elble <aweits@rit.edu>]

Any update/feedback?

"J. Bruce Fields" <bfields@fieldses.org> writes:

> Looks OK to me.  I think it would go through Al.--b.
>
> Acked-by: J. Bruce Fields <bfields@redhat.com>
>
> On Mon, Feb 23, 2015 at 08:51:24AM -0500, Andrew Elble wrote:
>> We have observed a BUG() crash in fs/attr.c:notify_change(). The crash
>> occurs during an rsync into a filesystem that is exported via NFS.
>> 
>> 1.) fs/attr.c:notify_change() modifies the caller's version of attr.
>> 2.) 6de0ec00ba8d ("VFS: make notify_change pass ATTR_KILL_S*ID to
>>     setattr operations") introduced a BUG() restriction such that "no
>>     function will ever call notify_change() with both ATTR_MODE and
>>     ATTR_KILL_S*ID set". Under some circumstances though, it will have
>>     assisted in setting the caller's version of attr to this very
>>     combination.
>> 3.) 27ac0ffeac80 ("locks: break delegations on any attribute
>>     modification") introduced code to handle breaking
>>     delegations. This can result in notify_change() being re-called. attr
>>     _must_ be explicitly reset to avoid triggering the BUG() established
>>     in #2.
>> 4.) The path that that triggers this is via fs/open.c:chmod_common().
>>     The combination of attr flags set here and in the first call to
>>     notify_change() along with a later failed break_deleg_wait()
>>     results in notify_change() being called again via retry_deleg
>>     without resetting attr.
>> 
>> Solution is to move retry_deleg in chmod_common() a bit further up to
>> ensure attr is completely reset.
>> 
>> There are other places where this seemingly could occur, such as
>> fs/utimes.c:utimes_common(), but the attr flags are not initially
>> set in such a way to trigger this.
>> 
>> Fixes: 27ac0ffeac80 ("locks: break delegations on any attribute modification")
>> Reported-by: Eric Meddaugh <etmsys@rit.edu>
>> Tested-by: Eric Meddaugh <etmsys@rit.edu>
>> Signed-off-by: Andrew Elble <aweits@rit.edu>
>> ---
>>  fs/open.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/fs/open.c b/fs/open.c
>> index 33f9cbf2610b..44a3be145bfe 100644
>> --- a/fs/open.c
>> +++ b/fs/open.c
>> @@ -570,6 +570,7 @@ static int chown_common(struct path *path, uid_t user, gid_t group)
>>  	uid = make_kuid(current_user_ns(), user);
>>  	gid = make_kgid(current_user_ns(), group);
>>  
>> +retry_deleg:
>>  	newattrs.ia_valid =  ATTR_CTIME;
>>  	if (user != (uid_t) -1) {
>>  		if (!uid_valid(uid))
>> @@ -586,7 +587,6 @@ static int chown_common(struct path *path, uid_t user, gid_t group)
>>  	if (!S_ISDIR(inode->i_mode))
>>  		newattrs.ia_valid |=
>>  			ATTR_KILL_SUID | ATTR_KILL_SGID | ATTR_KILL_PRIV;
>> -retry_deleg:
>>  	mutex_lock(&inode->i_mutex);
>>  	error = security_path_chown(path, uid, gid);
>>  	if (!error)
>> -- 
>> 1.9.2
>> 
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>

-- 
Andrew W. Elble
aweits@discipline.rit.edu
Infrastructure Engineer, Communications Technical Lead
Rochester Institute of Technology
PGP: BFAD 8461 4CCF DC95 DA2C B0EB 965B 082E 863E C912