Re: [security] Race condition in udev

[Scott James Remnant <scott@canonical.com>]

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On Tue, 2009-08-25 at 16:22 +0200, Florian Zumbiehl wrote:

> > > > > well, in those two cases always rename()ing the new node into place would
> > > > > work, too!? That would be a different strategy than what's in
> > > > > place at the moment, but it wouldn't need a special case!?
> > > > > 
> > > > The rename() will fail.
> > > 
> > > Because?
> > > 
> > POSIX.
> 
> More specifically?
> 
> And anyhow, I thought we were talking about the Linux kernel?!
> 
If you don't know why rename() might fail, you really shouldn't be
mucking around with this kind of code.

> > > > > > Or when racing with devmapper which creates /dev/mapper/foo devices at
> > > > > > basically the same time as udev.
> > > > > 
> > > > > Seriously? How is a piece of code that does the existence check and
> > > > > the subsequent action depending on the result of that check non-atomically
> > > > > supposed to help avoid some race condition resulting from possible
> > > > > concurrent creation of a device node?!
> > > > > 
> > > > Read the code and find out.  It works.
> > > 
> > > Guess how I found out that it can not work.
> > > 
> > I don't know, you haven't given any detail of any problems you've
> > encountered.
> 
> I haven't "encountered any problems", nor have I claimed to have
> "encountered any problems".
> 
> You stated that the codepath in udev-node.c for the case when a
> device node does already exist was somehow there for the case
> when udev races with devmapper. I noted that that codepath is not
> of any use in such a case, and that your argument thus is invalid.
> 
Since I wrote this code, and the code in devmapper, and have not only
strenuously tested it; but have at least 18 million for whom it works
every day, I'd argue that my argument is quite valid ;-)

Scott
-- 
Scott James Remnant
scott@canonical.com

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