Subject: Re: [PATCH AUTOSEL 5.10 41/46] net/rds: Avoid potential use after free in rds_send_remove_from_sock

From: Pavel Machek <[email-masked]>
Date: Mon, 19 Apr 2021 10:49:53 +0200

Hi!

> From: Aditya Pakki <[email-masked]>
> 
> [ Upstream commit 0c85a7e87465f2d4cbc768e245f4f45b2f299b05 ]
> 
> In case of rs failure in rds_send_remove_from_sock(), the 'rm' resource
> is freed and later under spinlock, causing potential use-after-free.
> Set the free pointer to NULL to avoid undefined behavior.

This patch is crazy. It adds dead code.

> +++ b/net/rds/message.c
> @@ -180,6 +180,7 @@ void rds_message_put(struct rds_message *rm)
>  		rds_message_purge(rm);
>  
>  		kfree(rm);
> +		rm = NULL;
>  	}
>  }

We are already exiting function, changing local variable has no
effect.

> +++ b/net/rds/send.c
> @@ -665,7 +665,7 @@ static void rds_send_remove_from_sock(struct list_head *messages, int status)
>  unlock_and_drop:
>  		spin_unlock_irqrestore(&rm->m_rs_lock, flags);
>  		rds_message_put(rm);
> -		if (was_on_sock)
> +		if (was_on_sock && rm)
>  			rds_message_put(rm);
>  	}

If rm was non-NULL calling first rds_message_put (and it was,
otherwise we oopsed), it is still non-NULL in second test.

Best regards,
								Pavel
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