From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D546725A642; Wed, 25 Feb 2026 01:44:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771983843; cv=none; b=ZorA6bkRjtb2X3lLqjjDleIubJd5I+exTVmDqeTD+LUUm6lHXhnXdwTykaxvCBzjwuVeesotXHsBcdQgnQw+JniLI2YEaXrPosrUT1SV6IRrncg4wcC6LA7t/9ZXfq1IUSDr8tHyPC45DYmHed55CMERHgLG8SkjAyG7wgASy1Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771983843; c=relaxed/simple; bh=PMO7C00k1MkQZ//mUkDriTarzSqjUiOHaJecc6eN54s=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=KHsv+FddnGAey4qhtWuddKCJyPMuDtkGclUnlsWT5LlQ8HB06kvoCLsvHF95LUvIPGPhqHQ8Qc8OzNMDEglLijTZmUFqG9Vikt0z+DICUDiYzgD7jrjfM6gTd4HwMQPQSoNhj9YwwoptuDvHzgFuncixtbtylup2KShSUqs8CWc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=Fc1vo3rC; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="Fc1vo3rC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8F1C5C116D0; Wed, 25 Feb 2026 01:44:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1771983843; bh=PMO7C00k1MkQZ//mUkDriTarzSqjUiOHaJecc6eN54s=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Fc1vo3rCZE67ly7oVD3ODCcawI3wj0iSP0iQi230Vr1pKtM3W2UvN7MJM0wKU+jmy hRYgu4lLcc13h/iMKUIOx0oIA1gSBN0k4oEoJjmaHhbij3faFJ1qSK07nMp+VEBVt2 fkuWwLP69Fu5YYA3QnRkACUwcP2bbcQMl6RvL73M= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jonathan McDowell , Herbert Xu , Sasha Levin Subject: [PATCH 6.18 121/641] hwrng: core - Allow runtime disabling of the HW RNG Date: Tue, 24 Feb 2026 17:17:27 -0800 Message-ID: <20260225012352.068008800@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260225012348.915798704@linuxfoundation.org> References: <20260225012348.915798704@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jonathan McDowell [ Upstream commit e74b96d77da9eb5ee1b603c937c2adab5134a04b ] 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 Signed-off-by: Herbert Xu Stable-dep-of: cc2f39d6ac48 ("hwrng: core - use RCU and work_struct to fix race condition") Signed-off-by: Sasha Levin --- drivers/char/hw_random/core.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c index 018316f546215..56d888bebe0c0 100644 --- a/drivers/char/hw_random/core.c +++ b/drivers/char/hw_random/core.c @@ -341,6 +341,9 @@ static ssize_t rng_current_store(struct device *dev, if (sysfs_streq(buf, "")) { err = enable_best_rng(); + } else if (sysfs_streq(buf, "none")) { + 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 +395,7 @@ 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 +547,8 @@ 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