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devtmpfs: mount with noexec and nosuid

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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 <kay@vrfy.org>
Cc: Roland Eggner <edvx1@systemanalysen.net>
Co-developed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/YcMfDOyrg647RCmd@debian-BULLSEYE-live-builder-AMD64
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Kees Cook 2021-12-22 17:50:20 +05:00 committed by Greg Kroah-Hartman
parent 67aa58e8d4
commit 28f0c335dd
2 changed files with 19 additions and 2 deletions
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11
drivers/base/Kconfig
View File

@ -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

10
drivers/base/devtmpfs.c
View File

@ -29,6 +29,12 @@
#include <uapi/linux/mount.h>
#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 */