linux-kernel - Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

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Date:	Fri, 11 Dec 2015 14:07:45 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>, Greg KH <greg@...ah.com>,
	Jiri Slaby <jslaby@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Aurelien Jarno <aurelien@...el32.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

On 12/11/15 13:48, Andy Lutomirski wrote:
> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Al Viro <viro@...IV.linux.org.uk> writes:
>>
>>> On Fri, Dec 11, 2015 at 01:40:40PM -0600, Eric W. Biederman wrote:
>>>
>>>> +    inode = path.dentry->d_inode;
>>>> +    filp->f_path = path;
>>>> +    filp->f_inode = inode;
>>>> +    filp->f_mapping = inode->i_mapping;
>>>> +    path_put(&old);
>>>
>>> Don't.  You are creating a fairly subtle constraint on what the code in
>>> fs/open.c and fs/namei.c can do, for no good reason.  You can bloody
>>> well maintain the information you need without that.
>>
>> There is a good reason.  We can not write a race free version of ptsname
>> without it.
> 
> As long as this is for new userspace code, would it make sense to just
> add a new ioctl to ask "does this ptmx fd match this /dev/pts fd?"
> 

For the newinstance case st_dev should match between the master and the
slave.  Unfortunately this is not the case for a legacy ptmx, as a
stat() on the master descriptor still returns the st_dev, st_rdev, and
st_ino for the ptmx device node.

	-hpa

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