prctl: Disable prctl(PR_SET_MDWE) on parisc

systemd-254 tries to use prctl(PR_SET_MDWE) for it's MemoryDenyWriteExecute
functionality, but fails on parisc which still needs executable stacks in
certain combinations of gcc/glibc/kernel.

Disable prctl(PR_SET_MDWE) by returning -EINVAL for now on parisc, until
userspace has catched up.

Signed-off-by: Helge Deller <[email protected]>
Co-developed-by: Linus Torvalds <[email protected]>
Reported-by: Sam James <[email protected]>
Closes: systemd/systemd#29775
Tested-by: Sam James <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Cc: <[email protected]> # v6.3+

Author: hdeller

kernel/sys.c

@@ -2394,6 +2394,10 @@ 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))
+		return -EINVAL;
+
 	current_bits = get_current_mdwe();
 	if (current_bits && current_bits != bits)
 		return -EPERM; /* Cannot unset the flags */