linux-ext4 - Re: [PATCH] jbd2: Fix use after free in kjournald2()

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Date:   Tue, 31 Jan 2017 16:51:55 +0100
From:   Jan Kara <jack@...e.cz>
To:     Sahitya Tummala <stummala@...eaurora.org>
Cc:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] jbd2: Fix use after free in kjournald2()

On Tue 31-01-17 20:08:57, Sahitya Tummala wrote:
> Below is the synchronization issue between unmount and kjournald2
> contexts, which results into use after free issue in kjournald2().
> Fix this issue by using journal->j_state_lock to synchronize the
> wait_event() done in journal_kill_thread() and the wake_up() done
> in kjournald2().
> 
> TASK 1:
> umount cmd:
>    |--jbd2_journal_destroy() {
>        |--journal_kill_thread() {
>             write_lock(&journal->j_state_lock);
> 	    journal->j_flags |= JBD2_UNMOUNT;
> 	    ...
> 	    write_unlock(&journal->j_state_lock);
> 	    wake_up(&journal->j_wait_commit);	   TASK 2 wakes up here:
> 	    					   kjournald2() {
> 						     ...
> 						     checks JBD2_UNMOUNT flag and calls goto end-loop;
> 						     ...
> 						     end_loop:
> 						       write_unlock(&journal->j_state_lock);
> 						       journal->j_task = NULL; --> If this thread gets
> 						       pre-empted here, then TASK 1 wait_event will
> 						       exit even before this thread is completely
> 						       done.
> 	    wait_event(journal->j_wait_done_commit, journal->j_task == NULL);
> 	    ...
> 	    write_lock(&journal->j_state_lock);
> 	    write_unlock(&journal->j_state_lock);
> 	  }
>        |--kfree(journal);
>      }
> }
> 						       wake_up(&journal->j_wait_done_commit); --> this step
> 						       now results into use after free issue.
> 						   }
> 
> Signed-off-by: Sahitya Tummala <stummala@...eaurora.org>

Yeah, what you write looks possible (although rather unlikely). Thanks for
catching this. One small nit below:

> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index a097048..f5cd3c0 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -278,9 +278,11 @@ static int kjournald2(void *arg)
>  end_loop:
>  	write_unlock(&journal->j_state_lock);
>  	del_timer_sync(&journal->j_commit_timer);
> +	write_lock(&journal->j_state_lock);

There's no good reason to do del_timer_sync() outside of j_state_lock. This
is not performance critical code and commit_timeout is trivial and cannot
block on anything. So just keep j_state_lock locked upto the place where
you unlock it now...

								Honza
>  	journal->j_task = NULL;
>  	wake_up(&journal->j_wait_done_commit);
>  	jbd_debug(1, "Journal thread exiting.\n");
> +	write_unlock(&journal->j_state_lock);
>  	return 0;
>  }
>  
> -- 
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR