s390: add no-execute support

Bit 0x100 of a page table, segment table of region table entry
can be used to disallow code execution for the virtual addresses
associated with the entry.

There is one tricky bit, the system call to return from a signal
is part of the signal frame written to the user stack. With a
non-executable stack this would stop working. To avoid breaking
things the protection fault handler checks the opcode that caused
the fault for 0x0a77 (sys_sigreturn) and 0x0aad (sys_rt_sigreturn)
and injects a system call. This is preferable to the alternative
solution with a stub function in the vdso because it works for
vdso=off and statically linked binaries as well.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/s390/kernel/early.c b/arch/s390/kernel/early.c
index d756315b7985..4e65c79cc5f2 100644
--- a/arch/s390/kernel/early.c
+++ b/arch/s390/kernel/early.c
@@ -354,6 +354,10 @@ static __init void detect_machine_facilities(void)
 		S390_lowcore.machine_flags |= MACHINE_FLAG_VX;
 		__ctl_set_bit(0, 17);
 	}
+	if (test_facility(130)) {
+		S390_lowcore.machine_flags |= MACHINE_FLAG_NX;
+		__ctl_set_bit(0, 20);
+	}
 }
 
 static inline void save_vector_registers(void)
@@ -384,6 +388,21 @@ static int __init disable_vector_extension(char *str)
 }
 early_param("novx", disable_vector_extension);
 
+static int __init noexec_setup(char *str)
+{
+	bool enabled;
+	int rc;
+
+	rc = kstrtobool(str, &enabled);
+	if (!rc && !enabled) {
+		/* Disable no-execute support */
+		S390_lowcore.machine_flags &= ~MACHINE_FLAG_NX;
+		__ctl_clear_bit(0, 20);
+	}
+	return rc;
+}
+early_param("noexec", noexec_setup);
+
 static int __init cad_setup(char *str)
 {
 	int val;