linux-kernel - Re: [PATCH 07/18] asymmetric keys: explicitly add the leading zero byte to encoded message

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Date:	Sun, 25 Aug 2013 18:13:13 +0200
From:	Pavel Machek <pavel@....cz>
To:	"Lee, Chun-Yi" <joeyli.kernel@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-efi@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-crypto@...r.kernel.org,
	opensuse-kernel@...nsuse.org, David Howells <dhowells@...hat.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Len Brown <len.brown@...el.com>,
	Josh Boyer <jwboyer@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Matt Fleming <matt.fleming@...el.com>,
	James Bottomley <james.bottomley@...senpartnership.com>,
	Greg KH <gregkh@...uxfoundation.org>, JKosina@...e.com,
	Rusty Russell <rusty@...tcorp.com.au>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Gary Lin <GLin@...e.com>, Vivek Goyal <vgoyal@...hat.com>,
	"Lee, Chun-Yi" <jlee@...e.com>
Subject: Re: [PATCH 07/18] asymmetric keys: explicitly add the leading zero
 byte to encoded message

On Thu 2013-08-22 19:01:46, Lee, Chun-Yi wrote:
> Per PKCS1 spec, the EMSA-PKCS1-v1_5 encoded message is leading by 0x00 0x01 in
> its first 2 bytes. The leading zero byte is suppressed by MPI so we pass a
> pointer to the _preceding_ byte to RSA_verify() in original code, but it has
> risk for the byte is not zero because it's not in EM buffer's scope, neither
> RSA_verify() nor mpi_get_buffer() didn't take care the leading byte.
> 
> To avoid the risk, that's better we explicitly add the leading zero byte to EM
> for pass to RSA_verify(). This patch allocate a _EM buffer to capture the
> result from RSA_I2OSP(), then set the first byte to zero in EM and copy the
> remaining bytes from _EM.
> 
> Reviewed-by: Jiri Kosina <jkosina@...e.cz>
> Signed-off-by: Lee, Chun-Yi <jlee@...e.com>

> -	ret = RSA_verify(H, EM - 1, k, sig->digest_size,
> +	EM = kmalloc(k, GFP_KERNEL);
> +	memset(EM, 0, 1);
> +	memcpy(EM + 1, _EM, k-1);
> +	kfree(_EM);

Spot a crash waiting to happen.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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