linux-kernel - RE: [PATCH] tpm/tpm_crb: Avoid unaligned reads in crb

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Date:   Mon, 4 Feb 2019 12:17:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jarkko Sakkinen' <jarkko.sakkinen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        James Morris <jmorris@...ei.org>,
        Tomas Winkler <tomas.winkler@...el.com>,
        Jerry Snitselaar <jsnitsel@...hat.com>
Subject: RE: [PATCH] tpm/tpm_crb: Avoid unaligned reads in crb_recv():

From: Jarkko Sakkinen
> Sent: 01 February 2019 11:20
> The current approach to read first 6 bytes from the response and then tail
> of the response, can cause the 2nd memcpy_fromio() to do an unaligned read
> (e.g. read 32-bit word from address aligned to a 16-bits), depending on how
> memcpy_fromio() is implemented. If this happens, the read will fail and the
> memory controller will fill the read with 1's.

To my mind memcpy_to/fromio() should only be used on IO addresses that are
adequately like memory, and should be implemented in a way that that won't
generate invalid bus cycles.
Also memcpy_fromio() should also be allowed to do 'aligned' accesses that
go beyond the ends of the required memory area.

...
> 
> -	memcpy_fromio(buf, priv->rsp, 6);
> +	memcpy_fromio(buf, priv->rsp, 8);
>  	expected = be32_to_cpup((__be32 *) &buf[2]);
> -	if (expected > count || expected < 6)
> +	if (expected > count || expected < 8)
>  		return -EIO;
> 
> -	memcpy_fromio(&buf[6], &priv->rsp[6], expected - 6);
> +	memcpy_fromio(&buf[8], &priv->rsp[8], expected - 8);

Why not just use readl() or readq() ?

Bound to generate better code.

	David

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