linux-kernel - Re: [PATCH 2/2 v2] pty: make sure super_block is still valid in final /dev/tty close

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Date:	Sat, 16 Jan 2016 13:09:43 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Herton R. Krzesinski" <herton@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 2/2 v2] pty: make sure super_block is still valid in final
 /dev/tty close

On 01/14/2016 11:56 AM, Herton R. Krzesinski wrote:
> Considering current pty code and multiple devpts instances, it's possible
> to umount a devpts file system while a program still has /dev/tty opened
> pointing to a previosuly closed pty pair in that instance. In the case all
> ptmx and pts/N files are closed, umount can be done. If the program closes
> /dev/tty after umount is done, devpts_kill_index will use now an invalid
> super_block, which was already destroyed in the umount operation after
> running ->kill_sb. This is another "use after free" type of issue, but now
> related to the allocated super_block instance.
> 
> To avoid the problem (warning at ida_remove and potential crashes) for
> this specific case, I added two functions in devpts which grabs additional
> references to the super_block, which pty code now uses so it makes sure
> the super block structure is still valid until pty shutdown is done.
> I also moved the additional inode references to the same functions, which
> also covered similar case with inode being freed before /dev/tty final
> close/shutdown.

Thanks again.

Reviewed-by: Peter Hurley <peter@...leysoftware.com>