linux-kernel - Re: [PATCH 002 of 6] md: Fix use-after-free bug when dropping an rdev from an md array.

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Date:	Mon, 14 Jan 2008 03:43:23 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Neil Brown <neilb@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 002 of 6] md: Fix use-after-free bug when dropping an rdev from an md array.

On Mon, Jan 14, 2008 at 02:21:45PM +1100, Neil Brown wrote:

> Maybe it isn't there any more....
> 
> Once upon a time, when I 
>    echo remove > /sys/block/mdX/md/dev-YYY/state

Egads.  And just what will protect you from parallel callers
of state_store()?  buffer->mutex does *not* do that - it only
gives you exclusion on given struct file.  Run the command
above from several shells and you've got independent open
from each redirect => different struct file *and* different
buffer for each => no exclusion whatsoever.

And _that_ is present right in the mainline tree - it's unrelated
to -mm kobject changes.

BTW, yes, you do have a deadlock there - kobject_del() will try to evict
children, which will include waiting for currently running ->store()
to finish, which will include the caller since .../state *is* a child of
that sucker.

The real problem is the lack of any kind of exclusion considerations in
md.c itself, AFAICS.  Fun with ordering is secondary (BTW, yes, it is
a problem - will sysfs ->store() to attribute between export_rdev() and
kobject_del() work correctly?)

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