exec: separate MM_ANONPAGES and RLIMIT_STACK accounting

get_arg_page() checks bprm->rlim_stack.rlim_cur and re-calculates the
"extra" size for argv/envp pointers every time, this is a bit ugly and
even not strictly correct: acct_arg_size() must not account this size.

Remove all the rlimit code in get_arg_page(). Instead, add bprm->argmin
calculated once at the start of __do_execve_file() and change
copy_strings to check bprm->p >= bprm->argmin.

The patch adds the new helper, prepare_arg_pages() which initializes
bprm->argc/envc and bprm->argmin.

[[e-mail]: fix !CONFIG_MMU version of get_arg_page()]
Link: http://lkml.kernel.org/r/[e-mail]
[[e-mail]: use max_t]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Oleg Nesterov <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Tested-by: Guenter Roeck <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/exec.c

···
 	if (ret <= 0)
 		return NULL;
 
-	if (write) {
-		unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
-		unsigned long ptr_size, limit;
-
-		/*
-		 * Since the stack will hold pointers to the strings, we
-		 * must account for them as well.
-		 *
-		 * The size calculation is the entire vma while each arg page is
-		 * built, so each time we get here it's calculating how far it
-		 * is currently (rather than each call being just the newly
-		 * added size from the arg page).  As a result, we need to
-		 * always add the entire size of the pointers, so that on the
-		 * last call to get_arg_page() we'll actually have the entire
-		 * correct size.
-		 */
-		ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
-		if (ptr_size > ULONG_MAX - size)
-			goto fail;
-		size += ptr_size;
-
-		acct_arg_size(bprm, size / PAGE_SIZE);
-
-		/*
-		 * We've historically supported up to 32 pages (ARG_MAX)
-		 * of argument strings even with small stacks
-		 */
-		if (size <= ARG_MAX)
-			return page;
-
-		/*
-		 * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM
-		 * (whichever is smaller) for the argv+env strings.
-		 * This ensures that:
-		 *  - the remaining binfmt code will not run out of stack space,
-		 *  - the program will have a reasonable amount of stack left
-		 *    to work from.
-		 */
-		limit = _STK_LIM / 4 * 3;
-		limit = min(limit, bprm->rlim_stack.rlim_cur / 4);
-		if (size > limit)
-			goto fail;
-	}
+	if (write)
+		acct_arg_size(bprm, vma_pages(bprm->vma));
 
 	return page;
-
-fail:
-	put_page(page);
-	return NULL;
 }
 
 static void put_arg_page(struct page *page)
···
 	return i;
 }
 
+static int prepare_arg_pages(struct linux_binprm *bprm,
+			struct user_arg_ptr argv, struct user_arg_ptr envp)
+{
+	unsigned long limit, ptr_size;
+
+	bprm->argc = count(argv, MAX_ARG_STRINGS);
+	if (bprm->argc < 0)
+		return bprm->argc;
+
+	bprm->envc = count(envp, MAX_ARG_STRINGS);
+	if (bprm->envc < 0)
+		return bprm->envc;
+
+	/*
+	 * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM
+	 * (whichever is smaller) for the argv+env strings.
+	 * This ensures that:
+	 *  - the remaining binfmt code will not run out of stack space,
+	 *  - the program will have a reasonable amount of stack left
+	 *    to work from.
+	 */
+	limit = _STK_LIM / 4 * 3;
+	limit = min(limit, bprm->rlim_stack.rlim_cur / 4);
+	/*
+	 * We've historically supported up to 32 pages (ARG_MAX)
+	 * of argument strings even with small stacks
+	 */
+	limit = max_t(unsigned long, limit, ARG_MAX);
+	/*
+	 * We must account for the size of all the argv and envp pointers to
+	 * the argv and envp strings, since they will also take up space in
+	 * the stack. They aren't stored until much later when we can't
+	 * signal to the parent that the child has run out of stack space.
+	 * Instead, calculate it here so it's possible to fail gracefully.
+	 */
+	ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
+	if (limit <= ptr_size)
+		return -E2BIG;
+	limit -= ptr_size;
+
+	bprm->argmin = bprm->p - limit;
+	return 0;
+}
+
 /*
  * 'copy_strings()' copies argument/environment strings from the old
  * processes's memory to the new process's stack.  The call to get_user_pages()
···
 		pos = bprm->p;
 		str += len;
 		bprm->p -= len;
+#ifdef CONFIG_MMU
+		if (bprm->p < bprm->argmin)
+			goto out;
+#endif
 
 		while (len > 0) {
 			int offset, bytes_to_copy;
···
 	if (retval)
 		goto out_unmark;
 
-	bprm->argc = count(argv, MAX_ARG_STRINGS);
-	if ((retval = bprm->argc) < 0)
-		goto out;
-
-	bprm->envc = count(envp, MAX_ARG_STRINGS);
-	if ((retval = bprm->envc) < 0)
+	retval = prepare_arg_pages(bprm, argv, envp);
+	if (retval < 0)
 		goto out;
 
 	retval = prepare_binprm(bprm);

include/linux/binfmts.h

···
 #endif
 	struct mm_struct *mm;
 	unsigned long p; /* current top of mem */
+	unsigned long argmin; /* rlimit marker for copy_strings() */
 	unsigned int
 		/*
 		 * True after the bprm_set_creds hook has been called once