From 28f0c335dd4a1a4b44b3e6c6402825a93132e1a4 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Wed, 22 Dec 2021 17:50:20 +0500 Subject: [PATCH] devtmpfs: mount with noexec and nosuid devtmpfs is writable. Add the noexec and nosuid as default mount flags to prevent code execution from /dev. The systems who don't use systemd and who rely on CONFIG_DEVTMPFS_MOUNT=y are the ones to be protected by this patch. Other systems are fine with the udev solution. No sane program should be relying on executing from /dev. So this patch reduces the attack surface. It doesn't prevent any specific attack, but it reduces the possibility that someone can use /dev as a place to put executable code. Chrome OS has been carrying this patch for several years. It seems trivial and simple solution to improve the protection of /dev when CONFIG_DEVTMPFS_MOUNT=y. Original patch: https://lore.kernel.org/lkml/20121120215059.GA1859@www.outflux.net/ Cc: ellyjones@chromium.org Cc: Kay Sievers Cc: Roland Eggner Co-developed-by: Muhammad Usama Anjum Signed-off-by: Kees Cook Signed-off-by: Muhammad Usama Anjum Link: https://lore.kernel.org/r/YcMfDOyrg647RCmd@debian-BULLSEYE-live-builder-AMD64 Signed-off-by: Greg Kroah-Hartman --- drivers/base/Kconfig | 11 +++++++++++ drivers/base/devtmpfs.c | 10 ++++++++-- 2 files changed, 19 insertions(+), 2 deletions(-) diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig index ffcbe2bc460e..6f04b831a5c0 100644 --- a/drivers/base/Kconfig +++ b/drivers/base/Kconfig @@ -62,6 +62,17 @@ config DEVTMPFS_MOUNT rescue mode with init=/bin/sh, even when the /dev directory on the rootfs is completely empty. +config DEVTMPFS_SAFE + bool "Use nosuid,noexec mount options on devtmpfs" + depends on DEVTMPFS + help + This instructs the kernel to include the MS_NOEXEC and MS_NOSUID mount + flags when mounting devtmpfs. + + Notice: If enabled, things like /dev/mem cannot be mmapped + with the PROT_EXEC flag. This can break, for example, non-KMS + video drivers. + config STANDALONE bool "Select only drivers that don't need compile-time external firmware" default y diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c index 8be352ab4ddb..1e2c2d3882e2 100644 --- a/drivers/base/devtmpfs.c +++ b/drivers/base/devtmpfs.c @@ -29,6 +29,12 @@ #include #include "base.h" +#ifdef CONFIG_DEVTMPFS_SAFE +#define DEVTMPFS_MFLAGS (MS_SILENT | MS_NOEXEC | MS_NOSUID) +#else +#define DEVTMPFS_MFLAGS (MS_SILENT) +#endif + static struct task_struct *thread; static int __initdata mount_dev = IS_ENABLED(CONFIG_DEVTMPFS_MOUNT); @@ -363,7 +369,7 @@ int __init devtmpfs_mount(void) if (!thread) return 0; - err = init_mount("devtmpfs", "dev", "devtmpfs", MS_SILENT, NULL); + err = init_mount("devtmpfs", "dev", "devtmpfs", DEVTMPFS_MFLAGS, NULL); if (err) printk(KERN_INFO "devtmpfs: error mounting %i\n", err); else @@ -412,7 +418,7 @@ static noinline int __init devtmpfs_setup(void *p) err = ksys_unshare(CLONE_NEWNS); if (err) goto out; - err = init_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, NULL); + err = init_mount("devtmpfs", "/", "devtmpfs", DEVTMPFS_MFLAGS, NULL); if (err) goto out; init_chdir("/.."); /* will traverse into overmounted root */