linux-kernel - Re: Is kernel optimized with dead store removal?

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Message-ID: <19334.40943.479593.304961@pilspetsen.it.uu.se>
Date:	Thu, 25 Feb 2010 17:06:07 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Roel Kluin <roel.kluin@...il.com>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	lkml <linux-kernel@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	linux-crypto@...r.kernel.org
Subject: Re: Is kernel optimized with dead store removal?

Roel Kluin writes:
 > 
 > >  >  Does this optimization also occur during compilation of the Linux
 > >  >  kernel?
 > 
 > > Any such dead store removal is up to the compiler and the lifetime
 > > of the object being clobbered. For 'auto' objects the optimization
 > > is certainly likely.
 > > 
 > > This is only a problem if the memory (a thread stack, say) is recycled
 > > and leaked uninitialized to user-space, but such bugs are squashed
 > > fairly quickly upon discovery.
 > 
 > Thanks for comments,
 > 
 > In the sha1_update() case I don't know whether the stack is recycled and
 > leaked - it may be dependent on the calling function, but isn't it
 > vulnerable?

It's only vulnerable if the data leaks to a less trusted domain.
There is no domain crossing in your user-space example.
In the kernel case, the domain crossing would be as I wrote: leaking
recycled and uninitialized memory to user-space (and those leaks of
uninitialized memory are frowned upon and quickly fixed when discovered).
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