linux-kernel - Re: [PATCH v2 1/2] hwrng: fetch randomness only after device init

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Date:	Mon, 7 Jul 2014 12:09:56 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Virtualization List <virtualization@...ts.linux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jason Cooper <jason@...edaemon.net>,
	"# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] hwrng: fetch randomness only after device init

On (Mon) 07 Jul 2014 [11:34:58], Amit Shah wrote:
> On (Mon) 07 Jul 2014 [11:23:52], Amit Shah wrote:
> > On (Sun) 06 Jul 2014 [21:41:47], Kees Cook wrote:
> > > On Fri, Jul 4, 2014 at 10:34 PM, Amit Shah <amit.shah@...hat.com> wrote:
> > > > Commit d9e7972619334 "hwrng: add randomness to system from rng sources"
> > > > added a call to rng_get_data() from the hwrng_register() function.
> > > > However, some rng devices need initialization before data can be read
> > > > from them.
> > > >
> > > > This commit makes the call to rng_get_data() depend on no init fn
> > > > pointer being registered by the device.  If an init function is
> > > > registered, this call is made after device init.
> > > 
> > > Thanks, this seems pretty reasonable. One side-effect is that cycling
> > > between hwrngs via sysfs (when they have init functions) will cause
> > > them to add more entropy. I don't think this is a problem, but it is
> > > kind of a weird side-effect.
> > 
> > Yes, agreed.  Having a per-device bool that indicates whether the
> > initial randomness is obtained is quite a heavy solution for this
> > side-effect.  But I can put this in the commit log so it's clear..
> 
> Well actually this feels a little weird, so how about making this the
> default behaviour for all devices, whether they have ->init() or not?
> 
> This also means that if a virtio-rng device is switched, it can
> contribute as its probe would have been over.

Something like this - untested:

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 50f5b76..c4419ea 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -81,17 +81,15 @@ static void add_early_randomness(struct hwrng *rng)
 
 static inline int hwrng_init(struct hwrng *rng)
 {
-       int ret;
-
-       if (!rng->init)
-               return 0;
-
-       ret =  rng->init(rng);
-       if (ret)
-               return ret;
+       if (rng->init) {
+               int ret;
 
+               ret =  rng->init(rng);
+               if (ret)
+                       return ret;
+       }
        add_early_randomness(rng);
-       return ret;
+       return 0;
 }
 
 static inline void hwrng_cleanup(struct hwrng *rng)
@@ -372,8 +370,16 @@ int hwrng_register(struct hwrng *rng)
        INIT_LIST_HEAD(&rng->list);
        list_add_tail(&rng->list, &rng_list);
 
-       if (!rng->init)
+       if (old_rng && !rng->init) {
+               /*
+                * Use a new device's input to add some randomness to
+                * the system.  If this rng device isn't going to be
+                * used right away, its init function hasn't been
+                * called yet; so only use the randomness from devices
+                * that don't need an init callback.
+                */
                add_early_randomness(rng);
+       }
 
 out_unlock:
        mutex_unlock(&rng_mutex);


		Amit
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