efi/libstub: Avoid CopyMem/SetMem EFI services after ExitBootServices

Given that memset/memcpy are intrinsics, the compiler might insert calls
to these routines unexpectedly, including in code that executes after
ExitBootServices(). In this case, the respective boot services are no
longer accessible, and calling them will cause a crash.

So fall back to a bytewise copy/store if this happens to occur, even
though no such occurrences are known to exist in the kernel currently.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/intrinsics.c

@@ -15,8 +15,31 @@ void *__memmove(void *__dest, const void *__src, size_t count) __alias(memmove);
 void *__memset(void *s, int c, size_t count) __alias(memset);
 #endif
 
+static void *efistub_memmove(u8 *dst, const u8 *src, size_t len)
+{
+	if (src > dst || dst >= (src + len))
+		for (size_t i = 0; i < len; i++)
+			dst[i] = src[i];
+	else
+		for (ssize_t i = len - 1; i >= 0; i--)
+			dst[i] = src[i];
+
+	return dst;
+}
+
+static void *efistub_memset(void *dst, int c, size_t len)
+{
+	for (u8 *d = dst; len--; d++)
+		*d = c;
+
+	return dst;
+}
+
 void *memcpy(void *dst, const void *src, size_t len)
 {
+	if (efi_table_attr(efi_system_table, boottime) == NULL)
+		return efistub_memmove(dst, src, len);
+
 	efi_bs_call(copy_mem, dst, src, len);
 	return dst;
 }
@@ -25,6 +48,9 @@ extern void *memmove(void *dst, const void *src, size_t len) __alias(memcpy);
 
 void *memset(void *dst, int c, size_t len)
 {
+	if (efi_table_attr(efi_system_table, boottime) == NULL)
+		return efistub_memset(dst, c, len);
+
 	efi_bs_call(set_mem, dst, len, c & U8_MAX);
 	return dst;
 }