linux-kernel - Re: [PATCH 16/23] pefile: Parse a PE binary to find a key and a signature contained therein

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Date:	Tue, 30 Oct 2012 18:06:48 -0700
From:	Kees Cook <keescook@...omium.org>
To:	David Howells <dhowells@...hat.com>
Cc:	rusty@...tcorp.com.au, pjones@...hat.com, jwboyer@...hat.com,
	mjg@...hat.com, dmitry.kasatkin@...el.com,
	zohar@...ux.vnet.ibm.com, keyrings@...ux-nfs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/23] pefile: Parse a PE binary to find a key and a
 signature contained therein

On Tue, Oct 30, 2012 at 5:59 PM, David Howells <dhowells@...hat.com> wrote:
> Kees Cook <keescook@...omium.org> wrote:
>
>> This multiplication can push the cursor out of bounds. (n_data_dirents
>> is unverified).
>> ...
>> Both of these cases of n_sections multiplications can wrap.
>> Ultimately, you can end up with cursor close to zero, but n_sections
>> being giant.
>
> Good points.  I wonder if I should limit these to some low number, or just
> check that they don't exceed header_size, which also needs checking as you
> said.

I think header_size can be bounded by datalen? I didn't investigate
that one very deeply. For the multiplications, I think you can just do
a check before chkaddr:

ctx->n_sections = pe->sections;
if (datalen / ctx->n_sections > sizeof(*sec))
    return -ELIBBAD;
chkaddr(0, cursor, sizeof(*sec) * ctx->n_sections);

Or extend chkaddr to do the check for you:

chkaddr(0, cursor, sizeof(*sec), ctx->n_sections);

with other callers just using "1" for the final argument.

Either way, you won't have to choose arbitrary limits.

-Kees

-- 
Kees Cook
Chrome OS Security
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