ELF: fix kernel.randomize_va_space double read

ELF loader uses "randomize_va_space" twice. It is sysctl and can change
at any moment, so 2 loads could see 2 different values in theory with
unpredictable consequences.

Issue exactly one load for consistent value across one exec.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Link: https://lore.kernel.org/r/3329905c-7eb8-400a-8f0a-d87cff979b5b@p183
Signed-off-by: Kees Cook <kees@kernel.org>

Author: Alexey Dobriyan

fs/binfmt_elf.c

@@ -1003,7 +1003,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	if (elf_read_implies_exec(*elf_ex, executable_stack))
 		current->personality |= READ_IMPLIES_EXEC;
 
-	if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
+	const int snapshot_randomize_va_space = READ_ONCE(randomize_va_space);
+	if (!(current->personality & ADDR_NO_RANDOMIZE) && snapshot_randomize_va_space)
 		current->flags |= PF_RANDOMIZE;
 
 	setup_new_exec(bprm);
@@ -1285,7 +1286,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	mm->end_data = end_data;
 	mm->start_stack = bprm->p;
 
-	if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1)) {
+	if ((current->flags & PF_RANDOMIZE) && (snapshot_randomize_va_space > 1)) {
 		/*
 		 * For architectures with ELF randomization, when executing
 		 * a loader directly (i.e. no interpreter listed in ELF