lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite for Android: free password hash cracker in your pocket
[<prev] [next>] [day] [month] [year] [list]
Message-ID: <aLWltVMmuYQn8Pwa@earth.li>
Date: Mon, 1 Sep 2025 14:55:01 +0100
From: Jonathan McDowell <noodles@...th.li>
To: Olivia Mackall <olivia@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Cc: linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-integrity@...r.kernel.org
Subject: [PATCH] hwrng: core - Allow runtime disabling of the HW RNG

From: Jonathan McDowell <noodles@...a.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@...ian-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@...ian-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@...ian-qemu-efi:~# echo none > /sys/devices/virtual/misc/hw_random/rng_current
root@...ian-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@...ian-qemu-efi:~# echo "" > /sys/devices/virtual/misc/hw_random/rng_current
root@...ian-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@...a.com>
---
 drivers/char/hw_random/core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 018316f54621..11c8077b792b 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);
-- 
2.51.0


Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ