fs/exec.c: account for argv/envp pointers

When limiting the argv/envp strings during exec to 1/4 of the stack limit,
the storage of the pointers to the strings was not included.  This means
that an exec with huge numbers of tiny strings could eat 1/4 of the stack
limit in strings and then additional space would be later used by the
pointers to the strings.

For example, on 32-bit with a 8MB stack rlimit, an exec with 1677721
single-byte strings would consume less than 2MB of stack, the max (8MB /
4) amount allowed, but the pointers to the strings would consume the
remaining additional stack space (1677721 * 4 == 6710884).

The result (1677721 + 6710884 == 8388605) would exhaust stack space
entirely.  Controlling this stack exhaustion could result in
pathological behavior in setuid binaries (CVE-2017-1000365).

[akpm@linux-foundation.org: additional commenting from Kees]
Fixes: b6a2fea39318 ("mm: variable length argument support")
Link: http://lkml.kernel.org/r/20170622001720.GA32173@beast
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Qualys Security Advisory <qsa@qualys.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
