binfmt_elf: safely increment argv pointers

When building the argv/envp pointers, the envp is needlessly
pre-incremented instead of just continuing after the argv pointers are
finished.  In some (likely impossible) race where the strings could be
changed from userspace between copy_strings() and here, it might be
possible to confuse the envp position.  Instead, just use sp like
everything else.

Link: http://lkml.kernel.org/r/20170622173838.GA43308@beastSigned-off-by: Kees Cook <keescook@chromium.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Qualys Security Advisory <qsa@qualys.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Grzegorz Andrejczuk <grzegorz.andrejczuk@intel.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_elf.c

@@ -163,8 +163,6 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
 	unsigned long p = bprm->p;
 	int argc = bprm->argc;
 	int envc = bprm->envc;
-	elf_addr_t __user *argv;
-	elf_addr_t __user *envp;
 	elf_addr_t __user *sp;
 	elf_addr_t __user *u_platform;
 	elf_addr_t __user *u_base_platform;
@@ -304,38 +302,38 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
 	/* Now, let's put argc (and argv, envp if appropriate) on the stack */
 	if (__put_user(argc, sp++))
 		return -EFAULT;
-	argv = sp;
-	envp = argv + argc + 1;
 
-	/* Populate argv and envp */
+	/* Populate list of argv pointers back to argv strings. */
 	p = current->mm->arg_end = current->mm->arg_start;
 	while (argc-- > 0) {
 		size_t len;
-		if (__put_user((elf_addr_t)p, argv++))
+		if (__put_user((elf_addr_t)p, sp++))
 			return -EFAULT;
 		len = strnlen_user((void __user *)p, MAX_ARG_STRLEN);
 		if (!len || len > MAX_ARG_STRLEN)
 			return -EINVAL;
 		p += len;
 	}
-	if (__put_user(0, argv))
+	if (__put_user(0, sp++))
 		return -EFAULT;
-	current->mm->arg_end = current->mm->env_start = p;
+	current->mm->arg_end = p;
+
+	/* Populate list of envp pointers back to envp strings. */
+	current->mm->env_end = current->mm->env_start = p;
 	while (envc-- > 0) {
 		size_t len;
-		if (__put_user((elf_addr_t)p, envp++))
+		if (__put_user((elf_addr_t)p, sp++))
 			return -EFAULT;
 		len = strnlen_user((void __user *)p, MAX_ARG_STRLEN);
 		if (!len || len > MAX_ARG_STRLEN)
 			return -EINVAL;
 		p += len;
 	}
-	if (__put_user(0, envp))
+	if (__put_user(0, sp++))
 		return -EFAULT;
 	current->mm->env_end = p;
 
 	/* Put the elf_info on the stack in the right place.  */
-	sp = (elf_addr_t __user *)envp + 1;
 	if (copy_to_user(sp, elf_info, ei_index * sizeof(elf_addr_t)))
 		return -EFAULT;
 	return 0;