Documentation,selinux: deprecate setting checkreqprot to 1

Deprecate setting the SELinux checkreqprot tunable to 1 via kernel
parameter or /sys/fs/selinux/checkreqprot. Setting it to 0 is left
intact for compatibility since Android and some Linux distributions
do so for security and treat an inability to set it as a fatal error.
Eventually setting it to 0 will become a no-op and the kernel will
stop using checkreqprot's value internally altogether.

checkreqprot was originally introduced as a compatibility mechanism
for legacy userspace and the READ_IMPLIES_EXEC personality flag.
However, if set to 1, it weakens security by allowing mappings to be
made executable without authorization by policy. The default value
for the SECURITY_SELINUX_CHECKREQPROT_VALUE config option was changed
from 1 to 0 in commit 2a35d196c160e3 ("selinux: change
CONFIG_SECURITY_SELINUX_CHECKREQPROT_VALUE default") and both Android
and Linux distributions began explicitly setting
/sys/fs/selinux/checkreqprot to 0 some time ago.

Signed-off-by: Stephen Smalley <[e-mail]>
Signed-off-by: Paul Moore <[e-mail]>

Documentation/ABI/obsolete/sysfs-selinux-checkreqprot

···
+What:		/sys/fs/selinux/checkreqprot
+Date:		April 2005 (predates git)
+KernelVersion:	2.6.12-rc2 (predates git)
+Contact:	[e-mail]
+Description:
+
+	The selinuxfs "checkreqprot" node allows SELinux to be configured
+	to check the protection requested by userspace for mmap/mprotect
+	calls instead of the actual protection applied by the kernel.
+	This was a compatibility mechanism for legacy userspace and
+	for the READ_IMPLIES_EXEC personality flag.  However, if set to
+	1, it weakens security by allowing mappings to be made executable
+	without authorization by policy.  The default value of checkreqprot
+	at boot was changed starting in Linux v4.4 to 0 (i.e. check the
+	actual protection), and Android and Linux distributions have been
+	explicitly writing a "0" to /sys/fs/selinux/checkreqprot during
+	initialization for some time.  Support for setting checkreqprot to 1
+	will be	removed in a future kernel release, at which point the kernel
+	will always cease using checkreqprot internally and will always
+	check the actual protections being applied upon mmap/mprotect calls.
+	The checkreqprot selinuxfs node will remain for backward compatibility
+	but will discard writes of the "0" value and will reject writes of the
+	"1" value when this mechanism is removed.

Documentation/admin-guide/kernel-parameters.txt

···
 			Default value is set via a kernel config option.
 			Value can be changed at runtime via
 				/sys/fs/selinux/checkreqprot.
+			Setting checkreqprot to 1 is deprecated.
 
 	cio_ignore=	[S390]
 			See Documentation/s390/common_io.rst for details.

MAINTAINERS

···
 F:	scripts/selinux/
 F:	Documentation/admin-guide/LSM/SELinux.rst
 F:	Documentation/ABI/obsolete/sysfs-selinux-disable
+F:	Documentation/ABI/obsolete/sysfs-selinux-checkreqprot
 
 SENSABLE PHANTOM
 M:	Jiri Slaby <[e-mail]>

security/selinux/Kconfig

···
 	  'checkreqprot=' boot parameter.  It may also be changed at runtime
 	  via /sys/fs/selinux/checkreqprot if authorized by policy.
 
+	  WARNING: this option is deprecated and will be removed in a future
+	  kernel release.
+
 	  If you are unsure how to answer this question, answer 0.
 
 config SECURITY_SELINUX_SIDTAB_HASH_BITS

security/selinux/hooks.c

···
 {
 	unsigned long checkreqprot;
 
-	if (!kstrtoul(str, 0, &checkreqprot))
+	if (!kstrtoul(str, 0, &checkreqprot)) {
 		selinux_checkreqprot_boot = checkreqprot ? 1 : 0;
+		if (checkreqprot)
+			pr_warn("SELinux: checkreqprot set to 1 via kernel parameter.  This is deprecated and will be rejected in a future kernel release.\n");
+	}
 	return 1;
 }
 __setup("checkreqprot=", checkreqprot_setup);

security/selinux/selinuxfs.c

···
 	if (sscanf(page, "%u", &new_value) != 1)
 		goto out;
 
+	if (new_value) {
+		char comm[sizeof(current->comm)];
+
+		memcpy(comm, current->comm, sizeof(comm));
+		pr_warn_once("SELinux: %s (%d) set checkreqprot to 1. This is deprecated and will be rejected in a future kernel release.\n",
+			     comm, current->pid);
+	}
+
 	fsi->state->checkreqprot = new_value ? 1 : 0;
 	length = count;
 out: