linux-kernel - Re: [PATCH] mm: mmap_lock: fix use-after-free race and css ref leak in tracepoints

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Message-ID: <X8gjliMAsPYZFrU1@mtj.duckdns.org>
Date:   Wed, 2 Dec 2020 18:30:30 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Michel Lespinasse <walken@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yafang Shao <laoar.shao@...il.com>,
        "David S . Miller" <davem@...emloft.net>, dsahern@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jakub Kicinski <kuba@...nel.org>, liuhangbin@...il.com,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: mmap_lock: fix use-after-free race and css ref leak
 in tracepoints

Hello,

On Wed, Dec 02, 2020 at 03:23:57PM -0800, Shakeel Butt wrote:
> > There've been some changes to cgroup ids recently and now cgroup id, ino and
> > its file_handle are all compatible. On 64bit ino machines, they're all the
> > same and won't be reused. On 32bit ino machines, the lower 32bit of full id
> > is used as ino. ino may be reused but not the full 64bit id.
> 
> __kernfs_new_node() is using idr_alloc_cyclic() which will return
> 32bit ID. If I am understanding this correctly the full ID is
> generated similarly for 32bit and 64bit machines but for 64bit
> machines the whole ID is inode number while on 32bit machines the
> lower 32bits contains the inode number. Is that correct?

Yes, that's correct.

Thanks.

-- 
tejun