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

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Date:   Tue, 5 Feb 2019 12:49:10 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "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():

On Tue, Feb 05, 2019 at 12:47:47PM +0200, Jarkko Sakkinen wrote:
> On Tue, Feb 05, 2019 at 12:44:06PM +0200, Jarkko Sakkinen wrote:
> > On Mon, Feb 04, 2019 at 12:17:43PM +0000, David Laight wrote:
> > > 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.
> > 
> > For the first read can be done. The second read is of variable
> > length.
> 
> Neither can be done to the first one, because readq() does
> le64_to_cpu(). Shoud not do any conversions, only raw read.
> So I'll just stick it to what we have.

ATM tmp_crb is only used in LE architectures (x86 and ARM64), but
I still hate to have hidden extra byte order conversions, even if
they get compiled as nil ops. It is more wrong than to use
memcpy_fromio().

/Jarkko