[PATCH v4] MIPS: Add noexec=on|off kernel parameter

[Aleksandar Markovic]

From: Miodrag Dinic <miodrag.dinic@xxxxxxxx>

Add a new kernel parameter to override the default behavior related
to the decision whether to set up stack as non-executable in function
mips_elf_read_implies_exec().

The new parameter is used to control non executable stack and heap,
regardless of PT_GNU_STACK entry or CPU RIXI support.

Allowed values:

noexec=on	Force non-exec stack & heap
noexec=off	Force executable stack & heap

If this parameter is omitted, kernel behavior remains the same as it
was before this patch is applied.

This functionality is convenient during debugging and is especially
useful for Android development where non-exec stack is required.

Signed-off-by: Miodrag Dinic <miodrag.dinic@xxxxxxxx>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@xxxxxxxx>

---
Parameter name was changed from "nonxstack" to "noexec" in v4, in
order to achieve consistency with similar parameter naming for Intel
platform.
---
 Documentation/admin-guide/kernel-parameters.txt | 10 +++++++
 arch/mips/kernel/elf.c                          | 38 +++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6571fbf..6dff711 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2596,6 +2596,16 @@
 			noexec=on: enable non-executable mappings (default)
 			noexec=off: disable non-executable mappings
 
+	noexec	[MIPS]
+			Force setting up stack and heap as non-executable or
+			executable regardless of PT_GNU_STACK entry or CPU XI
+			support. Valid arguments: on, off.
+			noexec=on:	Force non-executable stack and heap
+			noexec=off:	Force executable stack and heap
+			If omitted, stack and heap will or will not be set
+			up as non-executable depending on PT_GNU_STACK
+			entry and possibly other factors (like CPU XI support).
+
 	nosmap		[X86]
 			Disable SMAP (Supervisor Mode Access Prevention)
 			even if it is supported by processor.
diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
index 731325a..a0235d1 100644
--- a/arch/mips/kernel/elf.c
+++ b/arch/mips/kernel/elf.c
@@ -326,8 +326,46 @@ void mips_set_personality_nan(struct arch_elf_state *state)
 	}
 }
 
+static int noexec = EXSTACK_DEFAULT;
+
+/*
+ * kernel parameter: noexec=on|off
+ *
+ * Force setting up stack and heap as non-executable or
+ * executable regardless of PT_GNU_STACK entry or CPU RIXI
+ * support. Valid arguments: on, off.
+ *
+ *     noexec=on:	Force non-executable stack and heap
+ *     noexec=off:	Force executable stack and heap
+ *
+ * If omitted, stack and heap will or will not be set
+ * up as non-executable depending on PT_GNU_STACK
+ * entry and possibly other factors (CPU RIXI support).
+ */
+static int __init noexec_setup(char *str)
+{
+	if (!strcmp(str, "on"))
+		noexec = EXSTACK_DISABLE_X;
+	else if (!strcmp(str, "off"))
+		noexec = EXSTACK_ENABLE_X;
+	else
+		pr_err("Malformed noexec format! noexec=on|off\n");
+
+	return 1;
+}
+__setup("noexec=", noexec_setup);
+
 int mips_elf_read_implies_exec(void *elf_ex, int exstack)
 {
+	switch (noexec) {
+	case EXSTACK_DISABLE_X:
+		return 0;
+	case EXSTACK_ENABLE_X:
+		return 1;
+	default:
+		break;
+	}
+
 	if (exstack != EXSTACK_DISABLE_X) {
 		/* The binary doesn't request a non-executable stack */
 		return 1;
-- 
2.7.4