Re: [PATCH] GFS2: Fix unsafe dereference in dump_holder()

[Steven Whitehouse]

Hi,

On Tue, 2013-12-24 at 20:51 +0900, Tetsuo Handa wrote:
> >From efee681e4ca6152d9549a50e93ca932b00f03014 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Tue, 24 Dec 2013 18:04:43 +0900
> Subject: [PATCH] GFS2: Fix unsafe dereference in dump_holder()
> 
> Since we are not in RCU nor tasklist_lock held, we need to grab a reference on
> the returned task_struct. Use get_pid_task()/get_task_comm() to safely read the
> target task's comm name.
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> ---

Well this is not entirely true... for 99.9% of calls to this function we
do have the rcu read lock held (when called from the seq file code). So
the only issue is when called from the GLOCK_BUG_ON() macro.

Since it appears that rcu_read_lock() will nest, then we could
presumably just add that around dump_holder or even around
gfs2_dump_glock ?

I think that is a neater solution than taking extra ref counts and
copying the string around,

Steve.

>  fs/gfs2/glock.c |   11 +++++++++--
>  1 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
> index c8420f7..7bda965 100644
> --- a/fs/gfs2/glock.c
> +++ b/fs/gfs2/glock.c
> @@ -1654,15 +1654,22 @@ static int dump_holder(struct seq_file *seq, const struct gfs2_holder *gh)
>  {
>  	struct task_struct *gh_owner = NULL;
>  	char flags_buf[32];
> +	char name[TASK_COMM_LEN];
>  
>  	if (gh->gh_owner_pid)
> -		gh_owner = pid_task(gh->gh_owner_pid, PIDTYPE_PID);
> +		gh_owner = get_pid_task(gh->gh_owner_pid, PIDTYPE_PID);
> +	if (gh_owner) {
> +		get_task_comm(name, gh_owner);
> +		put_task_struct(gh_owner);
> +	} else {
> +		strlcpy(name, "(ended)", sizeof(name));
> +	}
>  	gfs2_print_dbg(seq, " H: s:%s f:%s e:%d p:%ld [%s] %pS\n",
>  		       state2str(gh->gh_state),
>  		       hflags2str(flags_buf, gh->gh_flags, gh->gh_iflags),
>  		       gh->gh_error,
>  		       gh->gh_owner_pid ? (long)pid_nr(gh->gh_owner_pid) : -1,
> -		       gh_owner ? gh_owner->comm : "(ended)",
> +		       name,
>  		       (void *)gh->gh_ip);
>  	return 0;
>  }


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