Date: Sat, 28 Sep 2013 15:57:26 +0100 From: Djalal Harouni <tixxdz@...ndz.org> To: Al Viro <viro@...IV.linux.org.uk> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...nel.org>, "Serge E. Hallyn" <serge.hallyn@...ntu.com>, Cyrill Gorcunov <gorcunov@...nvz.org>, LKML <linux-kernel@...r.kernel.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, tixxdz@...il.com Subject: Re: [PATCH 04/12] seq_file: Make seq_file able to access the file's opener cred On Thu, Sep 26, 2013 at 04:02:54AM +0100, Al Viro wrote: > On Wed, Sep 25, 2013 at 05:22:51PM -0700, Linus Torvalds wrote: > > On Wed, Sep 25, 2013 at 1:14 PM, Djalal Harouni <tixxdz@...ndz.org> wrote: > > > > > > Therefor add the f_cred field to the seq_file struct and a helper > > > seq_f_cred() to return it. > > > > I hate how you've split up this patch from the next one that actually > > _initializes_ the new field. > > > > The two patches should have been one. > > > > I think the patch should also remove the 'user_ns' member, since it's > > now available as f_cred->user_ns. > > > > I also suspect that it would be better to just make the the new > > seq_file member point to the 'struct file' instead. Sure, it's an > > extra level of indirection, but the lifetime of f_cred is not as clear > > otherwise. You don't increment the reference count, which is correct > > *only* because 'seq_file' has the same lifetime as 'struct file', and > > thus the reference count from struct file for the f_cred is > > sufficient. > > That's better than f_cred (or user_ns, for that matter), but... I'm > afraid that it'll get abused very soon. And I don't understand the > argument about the lifetime rules - what makes struct file ones > different from struct cred ones in that respect? Except that in this > case it's really obvious that we can't grab a reference, that is... For that reference count lifetime rule. Sorry I was trying to compare it with other implemented solutions. /proc/pid/environ will increment the mm->mm_count inside __mem_open(), thus mm_struct will stay in until mem_release(). That's it. In the proposed solution and as noted by Linus, the reference count from 'struct file' is sufficient for f_cred. Last, I'm happy with /proc/pid/environ as it is, will not touch it unless there is a request for it, but this proposed file->f_cred solution is better: * Not *all* these /proc/<pid>/* files are interested in acquiring a reference to the task's mm and increment its counters. * It can be adapted to all the /proc/<pid>/* files, we already have the file->f_cred * It allow to pass file descriptors if original opener had enough permission. It's explained in [Patch 0/12] * It allow referencing the correct mm of the currently running target task at read(),write()... Where other solutions will just block/restrict this behaviour. Thanks -- Djalal Harouni http://opendz.org
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