linux-kernel - Re: [PATCH] fs: make sure we do not read beyond allocation

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Message-ID: <20131003175824.GH13318@ZenIV.linux.org.uk>
Date:	Thu, 3 Oct 2013 18:58:24 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] fs: make sure we do not read beyond allocation

On Thu, Oct 03, 2013 at 09:34:11AM -0700, Kees Cook wrote:
> In dentry_string_cmp (via__d_lookup_rcu), when CONFIG_DCACHE_WORD_ACCESS
> is set, word-width memory reads are performed. However, the string
> allocation size may not be a multiple of the word size. To avoid reading
> past the end of such an allocation, we must allocate in multiples of
> the word size.

grep ^kmalloc /proc/slabinfo.  Observe the suffix after "kmalloc-"...

IOW, kmalloc() does round its argument up.  Seeing that we allocate an
external name only when allocation has to be longer than 32 bytes, the
sucker is guaranteed to be at least a multiple of 32 by the time we
pick the fitting cache (the worst case is when length is between 65
and 96; then we use kmalloc-96).

When you start a port to a 512-bit architecture, you'll have much nastier
problems than this one...
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