x86/boot: Warn on future overlapping memcpy() use

If an overlapping memcpy() is ever attempted, we should at least report
it, in case it might lead to problems, so it could be changed to a
memmove() call instead.
Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Baoquan He <bhe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Lasse Collin <lasse.collin@tukaani.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1462229461-3370-3-git-send-email-keescook@chromium.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/compressed/string.c

@@ -10,7 +10,7 @@
 #include "../string.c"
 
 #ifdef CONFIG_X86_32
-void *memcpy(void *dest, const void *src, size_t n)
+static void *__memcpy(void *dest, const void *src, size_t n)
 {
 	int d0, d1, d2;
 	asm volatile(
@@ -24,7 +24,7 @@ void *memcpy(void *dest, const void *src, size_t n)
 	return dest;
 }
 #else
-void *memcpy(void *dest, const void *src, size_t n)
+static void *__memcpy(void *dest, const void *src, size_t n)
 {
 	long d0, d1, d2;
 	asm volatile(
@@ -55,10 +55,20 @@ void *memmove(void *dest, const void *src, size_t n)
 	const unsigned char *s = src;
 
 	if (d <= s || d - s >= n)
-		return memcpy(dest, src, n);
+		return __memcpy(dest, src, n);
 
 	while (n-- > 0)
 		d[n] = s[n];
 
 	return dest;
 }
+
+/* Detect and warn about potential overlaps, but handle them with memmove. */
+void *memcpy(void *dest, const void *src, size_t n)
+{
+	if (dest > src && dest - src < n) {
+		warn("Avoiding potentially unsafe overlapping memcpy()!");
+		return memmove(dest, src, n);
+	}
+	return __memcpy(dest, src, n);
+}