linux-kernel - Re: [PATCH] Fix xoring of arch_get_random

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Message-ID: <20190529162720.GC31099@hmswarspite.think-freely.org>
Date:   Wed, 29 May 2019 12:27:20 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Steve Grubb <sgrubb@...hat.com>, Theodore Ts'o <tytso@....edu>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] Fix xoring of arch_get_random_long into crng->state array

On Wed, May 29, 2019 at 03:57:07PM +0000, David Laight wrote:
> From: Neil Horman [mailto:nhorman@...driver.com]
> > Sent: 29 May 2019 16:52
> > On Wed, May 29, 2019 at 01:51:24PM +0000, David Laight wrote:
> > > From: Neil Horman
> > > > Sent: 29 May 2019 14:42
> > > > On Tue, Apr 02, 2019 at 06:00:25PM -0400, Neil Horman wrote:
> > > > > When _crng_extract is called, any arch that has a registered
> > > > > arch_get_random_long method, attempts to mix an unsigned long value into
> > > > > the crng->state buffer, it only mixes in 32 of the 64 bits available,
> > > > > because the state buffer is an array of u32 values, even though 2 u32
> > > > > are expected to be filled (owing to the fact that it expects indexes 14
> > > > > and 15 to be filled).
> > > > >
> > > > > Bring the expected behavior into alignment by casting index 14 to an
> > > > > unsignled long pointer, and xoring that in instead.
> > > ...
> > > > > diff --git a/drivers/char/random.c b/drivers/char/random.c
> > > > > index 38c6d1af6d1c..8178618458ac 100644
> > > > > --- a/drivers/char/random.c
> > > > > +++ b/drivers/char/random.c
> > > > > @@ -975,14 +975,16 @@ static void _extract_crng(struct crng_state *crng,
> > > > >  			  __u8 out[CHACHA_BLOCK_SIZE])
> > > > >  {
> > > > >  	unsigned long v, flags;
> > > > > -
> > > > > +	unsigned long *archrnd;
> > > > >  	if (crng_ready() &&
> > > > >  	    (time_after(crng_global_init_time, crng->init_time) ||
> > > > >  	     time_after(jiffies, crng->init_time + CRNG_RESEED_INTERVAL)))
> > > > >  		crng_reseed(crng, crng == &primary_crng ? &input_pool : NULL);
> > > > >  	spin_lock_irqsave(&crng->lock, flags);
> > > > > -	if (arch_get_random_long(&v))
> > > > > -		crng->state[14] ^= v;
> > > > > +	if (arch_get_random_long(&v)) {
> > > > > +		archrnd = (unsigned long *)&crng->state[14];
> > > > > +		*archrnd ^= v;
> > > > > +	}
> > >
> > > Isn't that likely to generate a misaligned memory access?
> > >
> > I'm not quite sure how it would, crng->state is an array of _u32's, and so every
> > even element should be on a 64 bit boundary.
> 
> Only if the first item is aligned....
> Add a u32 before it and you'll probably flip the alignment.
> 
Sure (assuming no padding by the compiler of leading elements), but thats not
the case here, state is the first element in the array.  I suppose we could add
an __attribute__((aligned,8)) to the element if you think it would help

Neil

> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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