linux-kernel - RE: races in ipathfs

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Date:	Fri, 20 Jan 2012 05:55:21 -0800
From:	Mike Marciniszyn <mike.marciniszyn@...gic.com>
To:	Al Viro <viro@...IV.linux.org.uk>,
	Dept_Infinipath <Dept_Infinipath@...gic.com>
CC:	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: RE: races in ipathfs

We are currently investigating this.

Thanks for the review on this issue!

Mike

> -----Original Message-----
> From: linux-rdma-owner@...r.kernel.org [mailto:linux-rdma-
> owner@...r.kernel.org] On Behalf Of Al Viro
> Sent: Thursday, January 19, 2012 3:20 PM
> To: Dept_Infinipath
> Cc: linux-rdma@...r.kernel.org; linux-kernel
> Subject: races in ipathfs
>
>       Use of qib_super is seriously racy.  qibfs_add() (and worse,
> qibfs_remove()) can happen during qibfs_mount() and qibfs_kill_super().
>
>       1) CPU1: qib_init_one().  The sucker is allocated and placed
> on the list.  CPU2: ipathfs is mounted, directory created.  CPU1:
> finally
> gets around to qibfs_add(); by now qib_super is non-NULL and off we go,
> trying to create it again.  The worst part is, that code doesn't even
> notice that dentry is there and positive; you silently leak the old
> inode.
>
>       2) CPU1: qib_init_one().  Allocated the sucker.  CPU2: ipathfs
> is getting mounted.  Picked the first device off the list, creating
> directory for it.  CPU1: inserted new device into the head of the list,
> continued working.  Got around to qibfs_add(); qib_super is NULL, so
> we do nothing.  CPU2: walked the rest of the list, creating directories
> for all devices.  Our device is missed, since we are past that point in
> the list.  Worse, shift the timing a bit and it doesn't matter whether
> you add to the head or to the tail of the list - if qibfs_add() happens
> just before we set qib_super, we are screwed again.
>
>       3) CPU1: qib_remove_one().  CPU2: mount ipathfs is walking that
> list and decides to try and create a directory for the device that is
> being freed.  Oops...
>
>       4) CPU1: qib_init_one() or qib_remove_one(), doesn't matter
> which.
> CPU2: final umount of ipathfs already got through setting sb->s_root to
> NULL but still hadn't set qib_super to the same.  Oops...  And no,
> moving that qib_super = NULL; up prior to kill_litter_super() won't
> fix the race either, of course.
>
> AFAICS, the older driver (in hw/ipath) has the same problems.
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