Re: [PATCH v2] ext4: fix NULL pointer dereference when journal restart fails

["Lukáš Czerner" <lczerner@redhat.com>]

On Thu, 14 May 2015, Jan Kara wrote:

> Date: Thu, 14 May 2015 11:24:57 +0200
> From: Jan Kara <jack@suse.cz>
> To: Lukas Czerner <lczerner@redhat.com>
> Cc: linux-ext4@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>,
>     Jan Kara <jack@suse.cz>, stable@vger.kernel.org
> Subject: Re: [PATCH v2] ext4: fix NULL pointer dereference when journal
>     restart fails
> 
> On Thu 14-05-15 11:03:06, Lukas Czerner wrote:
> > Currently when journal restart fails, we'll have the h_transaction of
> > the handle set to NULL to indicate that the handle has been effectively
> > aborted. We handle this situation quietly in the jbd2_journal_stop() and just
> > free the handle and exit because everything else has been done before we
> > attempted (and failed) to restart the journal.
> > 
> > Unfortunately there are a number of problems with that approach
> > introduced with commit
> > 
> > 41a5b913197c "jbd2: invalidate handle if jbd2_journal_restart()
> > fails"
> > 
> > First of all in ext4 jbd2_journal_stop() will be called through
> > __ext4_journal_stop() where we would try to get a hold of the superblock
> > by dereferencing h_transaction which in this case would lead to NULL
> > pointer dereference and crash.
> > 
> > In addition we're going to free the handle regardless of the refcount
> > which is bad as well, because others up the call chain will still
> > reference the handle so we might potentially reference already freed
> > memory.
> > 
> > Moreover it's expected that we'll get aborted handle as well as detached
> > handle in some of the journalling function as the error propagates up
> > the stack, so it's unnecessary to call WARN_ON every time we get
> > detached handle.
> > 
> > And finally we might leak some memory by forgetting to free reserved
> > handle in jbd2_journal_stop() in the case where handle was detached from
> > the transaction (h_transaction is NULL).
> > 
> > Fix the NULL pointer dereference in __ext4_journal_stop() by just
> > calling jbd2_journal_stop() quietly as suggested by Jan Kara. Also fix
> > the potential memory leak in jbd2_journal_stop() and use proper
> > handle refcounting before we attempt to free it to avoid use-after-free
> > issues.
> > 
> > And finally remove all WARN_ON(!transaction) from the code so that we do
> > not get random traces when something goes wrong because when journal
> > restart fails we will get to some of those functions.
> > 
> > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> > Cc: stable@vger.kernel.org
> > ---
> > v2: As Jan Kara pointed out setting h_transaction to NULL is actually
> > desirable because the handle was detached from that transaction.
> 
> The patch looks good, you can add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
> 
>   Just one small nit below:
> 
> ...
> > @@ -1530,8 +1524,22 @@ int jbd2_journal_stop(handle_t *handle)
> >  	tid_t tid;
> >  	pid_t pid;
> >  
> > -	if (!transaction)
> > -		goto free_and_exit;
> > +	if (!transaction) {
> > +		/*
> > +		 * Handle is already detached from the transaction so
> > +		 * there is nothing to do other than decrease a refcount,
> > +		 * or free the handle if refcount drops to zero
> > +		 */
> > +		if (--handle->h_ref > 0) {
> > +			jbd_debug(4, "h_ref %d -> %d\n", handle->h_ref + 1,
> > +							 handle->h_ref);
> > +			return err;
> > +		} else {
> > +			if (handle->h_rsv_handle)
> > +				jbd2_free_handle(handle->h_rsv_handle);
> > +			goto free_and_exit;
> 
> It would we nicer if you just moved free_and_exit label before freeing of
> the reserved handle instead of duplicating the code here. Also it is more
> future proof if we add more places jumping to the label...

The problem is that this a special case when we have detached handle
so we only need to free the structure. In normal case we're calling
jbd2_journal_free_reserved() instead and I did not wanted to add
(!transaction) condition to multiple places.

jbd2_journal_free_reserved() is different in that it does
sub_reserved_credits() for the journal, but in this case it has
already been done in jbd2__journal_restart().

Thanks!
-Lukas
> 
> 								Honza
>