prctl: generalize PR_SET_MDWE support check to be per-arch

Patch series "ARM: prctl: Reject PR_SET_MDWE where not supported".

I noticed after a recent kernel update that my ARM926 system started
segfaulting on any execve() after calling prctl(PR_SET_MDWE).  After some
investigation it appears that ARMv5 is incapable of providing the
appropriate protections for MDWE, since any readable memory is also
implicitly executable.

The prctl_set_mdwe() function already had some special-case logic added
disabling it on PARISC (commit 7938381, "prctl: Disable
prctl(PR_SET_MDWE) on parisc"); this patch series (1) generalizes that
check to use an arch_*() function, and (2) adds a corresponding override
for ARM to disable MDWE on pre-ARMv6 CPUs.

With the series applied, prctl(PR_SET_MDWE) is rejected on ARMv5 and
subsequent execve() calls (as well as mmap(PROT_READ|PROT_WRITE)) can
succeed instead of unconditionally failing; on ARMv6 the prctl works as it
did previously.

[0] https://lore.kernel.org/all/2023112456-linked-nape-bf19@gregkh/


This patch (of 2):

There exist systems other than PARISC where MDWE may not be feasible to
support; rather than cluttering up the generic code with additional
arch-specific logic let's add a generic function for checking MDWE support
and allow each arch to override it as needed.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Zev Weiss <[email protected]>
Acked-by: Helge Deller <[email protected]>	[parisc]
Cc: Borislav Petkov <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Florent Revest <[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]>
Cc: <[email protected]>	[6.3+]
Signed-off-by: Andrew Morton <[email protected]>

Author: zevweiss

arch/parisc/include/asm/mman.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_MMAN_H__
+#define __ASM_MMAN_H__
+
+#include <uapi/asm/mman.h>
+
+/* PARISC cannot allow mdwe as it needs writable stacks */
+static inline bool arch_memory_deny_write_exec_supported(void)
+{
+	return false;
+}
+#define arch_memory_deny_write_exec_supported arch_memory_deny_write_exec_supported
+
+#endif /* __ASM_MMAN_H__ */

include/linux/mman.h

@@ -162,6 +162,14 @@ calc_vm_flag_bits(unsigned long flags)
 
 unsigned long vm_commit_limit(void);
 
+#ifndef arch_memory_deny_write_exec_supported
+static inline bool arch_memory_deny_write_exec_supported(void)
+{
+	return true;
+}
+#define arch_memory_deny_write_exec_supported arch_memory_deny_write_exec_supported
+#endif
+
 /*
  * Denies creating a writable executable mapping or gaining executable permissions.
  *

kernel/sys.c

@@ -2408,8 +2408,11 @@ static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
 	if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
 		return -EINVAL;
 
-	/* PARISC cannot allow mdwe as it needs writable stacks */
-	if (IS_ENABLED(CONFIG_PARISC))
+	/*
+	 * EOPNOTSUPP might be more appropriate here in principle, but
+	 * existing userspace depends on EINVAL specifically.
+	 */
+	if (!arch_memory_deny_write_exec_supported())
 		return -EINVAL;
 
 	current_bits = get_current_mdwe();