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List:       linux-security-module
Subject:    Re: [PATCH 07/18] asymmetric keys: explicitly add the leading zero byte to encoded message
From:       Pavel Machek <pavel () ucw ! cz>
Date:       2013-08-25 16:13:13
Message-ID: 20130825161313.GD5171 () amd ! pavel ! ucw ! cz
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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@suse.cz>
> Signed-off-by: Lee, Chun-Yi <jlee@suse.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
-- 
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