ARM: prctl: reject PR_SET_MDWE on pre-ARMv6

On v5 and lower CPUs we can't provide MDWE protection, so ensure we fail
any attempt to enable it via prctl(PR_SET_MDWE).

Previously such an attempt would misleadingly succeed, leading to any
subsequent mmap(PROT_READ|PROT_WRITE) or execve() failing unconditionally
(the latter somewhat violently via force_fatal_sig(SIGSEGV) due to
READ_IMPLIES_EXEC).

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Zev Weiss <[email protected]>
Cc: <[email protected]>	[6.3+]
Cc: Borislav Petkov <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Florent Revest <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: "James E.J. Bottomley" <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Miguel Ojeda <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Ondrej Mosnacek <[email protected]>
Cc: Rick Edgecombe <[email protected]>
Cc: Russell King (Oracle) <[email protected]>
Cc: Sam James <[email protected]>
Cc: Stefan Roesch <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yin Fengwei <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: zevweiss

arch/arm/include/asm/mman.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_MMAN_H__
+#define __ASM_MMAN_H__
+
+#include <asm/system_info.h>
+#include <uapi/asm/mman.h>
+
+static inline bool arch_memory_deny_write_exec_supported(void)
+{
+	return cpu_architecture() >= CPU_ARCH_ARMv6;
+}
+#define arch_memory_deny_write_exec_supported arch_memory_deny_write_exec_supported
+
+#endif /* __ASM_MMAN_H__ */