hwrng: core - treat default_quality as a maximum and default to 1024

Most hw_random devices return entropy which is assumed to be of full
quality, but driver authors don't bother setting the quality knob. Some
hw_random devices return less than full quality entropy, and then driver
authors set the quality knob. Therefore, the entropy crediting should be
opt-out rather than opt-in per-driver, to reflect the actual reality on
the ground.

For example, the two Raspberry Pi RNG drivers produce full entropy
randomness, and both EDK2 and U-Boot's drivers for these treat them as
such. The result is that EFI then uses these numbers and passes the to
Linux, and Linux credits them as boot, thereby initializing the RNG.
Yet, in Linux, the quality knob was never set to anything, and so on the
chance that Linux is booted without EFI, nothing is ever credited.
That's annoying.

The same pattern appears to repeat itself throughout various drivers. In
fact, very very few drivers have bothered setting quality=1024.

Looking at the git history of existing drivers and corresponding mailing
list discussion, this conclusion tracks. There's been a decent amount of
discussion about drivers that set quality < 1024 -- somebody read and
interepreted a datasheet, or made some back of the envelope calculation
somehow. But there's been very little, if any, discussion about most
drivers where the quality is just set to 1024 or unset (or set to 1000
when the authors misunderstood the API and assumed it was base-10 rather
than base-2); in both cases the intent was fairly clear of, "this is a
hardware random device; it's fine."

So let's invert this logic. A hw_random struct's quality knob now
controls the maximum quality a driver can produce, or 0 to specify 1024.
Then, the module-wide switch called "default_quality" is changed to
represent the maximum quality of any driver. By default it's 1024, and
the quality of any particular driver is then given by:

    min(default_quality, rng->quality ?: 1024);

This way, the user can still turn this off for weird reasons (and we can
replace whatever driver-specific disabling hacks existed in the past),
yet we get proper crediting for relevant RNGs.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

8 files changed, 3 insertions, 14 deletions

diff --git a/drivers/char/hw_random/cavium-rng-vf.c b/drivers/char/hw_random/cavium-rng-vf.c
index 7c55f4cf4a8b..c99c54cd99c6 100644
--- a/drivers/char/hw_random/cavium-rng-vf.c
+++ b/drivers/char/hw_random/cavium-rng-vf.c
@@ -225,7 +225,6 @@ static int cavium_rng_probe_vf(struct	pci_dev		*pdev,
 		return -ENOMEM;
 
 	rng->ops.read    = cavium_rng_read;
-	rng->ops.quality = 1000;
 
 	pci_set_drvdata(pdev, rng);
 
diff --git a/drivers/char/hw_random/cn10k-rng.c b/drivers/char/hw_random/cn10k-rng.c
index a01e9307737c..c1193f85982c 100644
--- a/drivers/char/hw_random/cn10k-rng.c
+++ b/drivers/char/hw_random/cn10k-rng.c
@@ -145,7 +145,6 @@ static int cn10k_rng_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 		return -ENOMEM;
 
 	rng->ops.read    = cn10k_rng_read;
-	rng->ops.quality = 1000;
 	rng->ops.priv = (unsigned long)rng;
 
 	reset_rng_health_state(rng);
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index cc002b0c2f0c..afde685f5e0a 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -41,14 +41,14 @@ static DEFINE_MUTEX(reading_mutex);
 static int data_avail;
 static u8 *rng_buffer, *rng_fillbuf;
 static unsigned short current_quality;
-static unsigned short default_quality; /* = 0; default to "off" */
+static unsigned short default_quality = 1024; /* default to maximum */
 
 module_param(current_quality, ushort, 0644);
 MODULE_PARM_DESC(current_quality,
 		 "current hwrng entropy estimation per 1024 bits of input -- obsolete, use rng_quality instead");
 module_param(default_quality, ushort, 0644);
 MODULE_PARM_DESC(default_quality,
-		 "default entropy content of hwrng per 1024 bits of input");
+		 "default maximum entropy content of hwrng per 1024 bits of input");
 
 static void drop_current_rng(void);
 static int hwrng_init(struct hwrng *rng);
@@ -170,10 +170,7 @@ static int hwrng_init(struct hwrng *rng)
 	reinit_completion(&rng->cleanup_done);
 
 skip_init:
-	if (!rng->quality)
-		rng->quality = default_quality;
-	if (rng->quality > 1024)
-		rng->quality = 1024;
+	rng->quality = min_t(u16, min_t(u16, default_quality, 1024), rng->quality ?: 1024);
 	current_quality = rng->quality; /* obsolete */
 
 	return 0;
diff --git a/drivers/char/hw_random/mpfs-rng.c b/drivers/char/hw_random/mpfs-rng.c
index 5813da617a48..c6972734ae62 100644
--- a/drivers/char/hw_random/mpfs-rng.c
+++ b/drivers/char/hw_random/mpfs-rng.c
@@ -78,7 +78,6 @@ static int mpfs_rng_probe(struct platform_device *pdev)
 
 	rng_priv->rng.read = mpfs_rng_read;
 	rng_priv->rng.name = pdev->name;
-	rng_priv->rng.quality = 1024;
 
 	platform_set_drvdata(pdev, rng_priv);
 
diff --git a/drivers/char/hw_random/npcm-rng.c b/drivers/char/hw_random/npcm-rng.c
index 5bf7f370f985..9903d0357e06 100644
--- a/drivers/char/hw_random/npcm-rng.c
+++ b/drivers/char/hw_random/npcm-rng.c
@@ -111,7 +111,6 @@ static int npcm_rng_probe(struct platform_device *pdev)
 	priv->rng.name = pdev->name;
 	priv->rng.read = npcm_rng_read;
 	priv->rng.priv = (unsigned long)&pdev->dev;
-	priv->rng.quality = 1000;
 	priv->clkp = (u32)(uintptr_t)of_device_get_match_data(&pdev->dev);
 
 	writel(NPCM_RNG_M1ROSEL, priv->base + NPCM_RNGMODE_REG);
diff --git a/drivers/char/hw_random/s390-trng.c b/drivers/char/hw_random/s390-trng.c
index 795853dfc46b..cffa326ddc8d 100644
--- a/drivers/char/hw_random/s390-trng.c
+++ b/drivers/char/hw_random/s390-trng.c
@@ -191,7 +191,6 @@ static struct hwrng trng_hwrng_dev = {
 	.name		= "s390-trng",
 	.data_read	= trng_hwrng_data_read,
 	.read		= trng_hwrng_read,
-	.quality	= 1024,
 };
 
 
diff --git a/drivers/char/hw_random/timeriomem-rng.c b/drivers/char/hw_random/timeriomem-rng.c
index 8ea1fc831eb7..26f322d19a88 100644
--- a/drivers/char/hw_random/timeriomem-rng.c
+++ b/drivers/char/hw_random/timeriomem-rng.c
@@ -145,8 +145,6 @@ static int timeriomem_rng_probe(struct platform_device *pdev)
 		if (!of_property_read_u32(pdev->dev.of_node,
 						"quality", &i))
 			priv->rng_ops.quality = i;
-		else
-			priv->rng_ops.quality = 0;
 	} else {
 		period = pdata->period;
 		priv->rng_ops.quality = pdata->quality;
diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index a6f3a8a2aca6..f7690e0f92ed 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -148,7 +148,6 @@ static int probe_common(struct virtio_device *vdev)
 		.cleanup = virtio_cleanup,
 		.priv = (unsigned long)vi,
 		.name = vi->name,
-		.quality = 1000,
 	};
 	vdev->priv = vi;