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

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Date:	Fri, 11 Dec 2015 14:12:27 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	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 Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin <hpa@...or.com> wrote:
> 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.

Sure, but I'm not talking about stat.  I'm saying that we could add a
new ioctl that works on any ptmx fd (/dev/ptmx or /dev/pts/ptmx) that
answers the question "does this ptmx logically belong to the given
devpts filesystem".

Since it's not stat, we can make it do whatever we want, including
following a link to the devpts instance that isn't f_path or f_inode.

--Andy
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