[PATCH v2] hwrng: core - Allow runtime disabling of the HW RNG

[Jonathan McDowell <noodles@earth.li>]

From: Jonathan McDowell <noodles@meta.com>

The HW RNG core allows for manual selection of which RNG device to use,
but does not allow for no device to be enabled. It may be desirable to
do this on systems with only a single suitable hardware RNG, where we
need exclusive access to other functionality on this device. In
particular when performing TPM firmware upgrades this lets us ensure the
kernel does not try to access the device.

Before:

root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_*
/sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0
/sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0
/sys/devices/virtual/misc/hw_random/rng_quality:1024
/sys/devices/virtual/misc/hw_random/rng_selected:0

After:

root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_*
/sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none
/sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0
/sys/devices/virtual/misc/hw_random/rng_quality:1024
/sys/devices/virtual/misc/hw_random/rng_selected:0

root@debian-qemu-efi:~# echo none > /sys/devices/virtual/misc/hw_random/rng_current
root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_*
/sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none
/sys/devices/virtual/misc/hw_random/rng_current:none
grep: /sys/devices/virtual/misc/hw_random/rng_quality: No such device
/sys/devices/virtual/misc/hw_random/rng_selected:1

(Observe using bpftrace no calls to TPM being made)

root@debian-qemu-efi:~# echo "" > /sys/devices/virtual/misc/hw_random/rng_current
root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_*
/sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none
/sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0
/sys/devices/virtual/misc/hw_random/rng_quality:1024
/sys/devices/virtual/misc/hw_random/rng_selected:0

(Observe using bpftrace that calls to the TPM resume)

Signed-off-by: Jonathan McDowell <noodles@meta.com>
---
v2: If the user manually forces the HWRNG to none do not override this
     when a new driver is loaded. Pointed out by Herbert Xu.

  drivers/char/hw_random/core.c | 10 +++++++---
  1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 018316f54621..1682a9f1b28c 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -341,6 +341,10 @@ static ssize_t rng_current_store(struct device *dev,
  
  	if (sysfs_streq(buf, "")) {
  		err = enable_best_rng();
+	} else if (sysfs_streq(buf, "none")) {
+		if (current_rng)
+			cur_rng_set_by_user = 1;
+		drop_current_rng();
  	} else {
  		list_for_each_entry(rng, &rng_list, list) {
  			if (sysfs_streq(rng->name, buf)) {
@@ -392,7 +396,7 @@ static ssize_t rng_available_show(struct device *dev,
  		strlcat(buf, rng->name, PAGE_SIZE);
  		strlcat(buf, " ", PAGE_SIZE);
  	}
-	strlcat(buf, "\n", PAGE_SIZE);
+	strlcat(buf, "none\n", PAGE_SIZE);
  	mutex_unlock(&rng_mutex);
  
  	return strlen(buf);
@@ -544,8 +548,8 @@ int hwrng_register(struct hwrng *rng)
  	/* Adjust quality field to always have a proper value */
  	rng->quality = min_t(u16, min_t(u16, default_quality, 1024), rng->quality ?: 1024);
  
-	if (!current_rng ||
-	    (!cur_rng_set_by_user && rng->quality > current_rng->quality)) {
+	if (!cur_rng_set_by_user &&
+	    (!current_rng || rng->quality > current_rng->quality)) {
  		/*
  		 * Set new rng as current as the new rng source
  		 * provides better entropy quality and was not
-- 
2.51.0