linux-kernel - Re: [RFC 2/2] fs,proc: Respect FMODE

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Message-ID: <CALCETrUJNv48UXPNSXcdnsFZSLwhRtA0s03bJHamXQmr6vs3fQ@mail.gmail.com>
Date:	Mon, 21 Apr 2014 10:00:55 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Pavel Machek <pavel@....cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	David Herrmann <dh.herrmann@...il.com>
Subject: Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

On Mon, Apr 21, 2014 at 9:30 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Mon, Apr 21, 2014 at 09:22:48AM -0700, Andy Lutomirski wrote:
>
>> +static int proc_may_follow(struct nameidata *nd, struct file *f)
>> +{
>> +     if (!nd)
>> +             return 0;  /* This is readlink, */
>> +
>> +     if ((nd->flags & LOOKUP_WRITE) && !(f->f_mode & FMODE_WRITE))
>> +             return -EACCES;
>> +
>> +     return 0;
>> +}
>
> And this is just plain wrong.  WTF are you making the traversal of symlink
> in the middle of pathname dependent on the open flags?

Can you give me a hint?  There are three cases that I need to
distinguish, I think:

1. readlink.  Currently handled by nd == NULL.  It's ugly, and I'll clean it up.

2. Traversal in the middle of a path.  This can be either literally in
the middle (e.g. "/proc/self/fd/3/something_else") or in a symlink
that's the last component of the literal path (e.g. "fd3null" where
"fd3null" is a symlink to "/proc/self/fd/3/null" and "null" is either
a file or a symlink to /dev/null).  I have the latter type wrong in
this patch.

3. Actually opening /proc/self/fd/N.  This can be direct or by opening
a symlink to /proc/self/fd/N.  I think I have this case correct.

What's the best way to fix this?  Should I be checking nd->depth?

--Andy

>
> NAK.

-- 
Andy Lutomirski
AMA Capital Management, LLC
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